Chapter 10b - Insects

Insect Pests of Canola

Insect Management

Canola crops can be attacked by a number of insect pests during the growing season. When making any insect management decisions, take the following steps:

• Monitoring and Identification
  Positively identify the insect and its damage. Examine fields often during the growing season for the first signs of insect damage. Table 1 shows approximate times during which major insect pests are active in canola. Consult the local crop specialist if insects are found but cannot be identified.
• Economic Threshold
  Determine the insect's economic threshold. Once the pest has been identified, determine if the population density is above or below its economic threshold. Basically, the economic threshold for an insect is the number of insects above which damage will be greater than the cost of control. This threshold will vary with the weather, soil fertility, crop price, chemical costs and crop growth stage.
• Management
  Decide on the type of management and timing for the best results. If chemical control is necessary, select a product registered for the purpose and apply it at a stage when a benefit is ensured, not too soon or too late in the life cycle. The goal is to use cultural or chemical control where required and reduce unnecessary pesticide applications.

Field Scouting

Field scouting is the regular examination of fields to accurately assess the kind and number of insects present, and the amount of damage being done. Scouting should be done weekly during the growing season and daily when infestations approach economic threshold levels or when weather conditions favour the rapid development of specific pests.

To properly scout for insect pests, know when they occur, where they live, what they look like and how to find and count them. Generally, in fields of less than 100 acres, check a minimum of five locations. In fields greater than 100 acres, check a minimum of 10 locations. There are several possible scouting patterns that can be used when checking fields. These options are based on pest distribution and field configuration

The first pattern is used when pests are uniformly distributed (Figure 1).

Figure 1. Scouting for Uniformly Distributed Pests

This scouting pattern typically looks like an X, Z or W, excluding field edges. Pests that fit this pattern include aphids, bertha armyworm, diamondback moth and lygus bugs.

The second pattern is used when pests are at the edges of fields (Figure 2).

Figure 2. Scouting for Pests at Field Edges

Scout by walking along field edges, fence lines or ditches. Some examples of pests appearing at the edges of fields include flea beetles and grasshoppers.

Table 1 outlines the major insect pests of canola, including appearance, damage and crop stage.

Table 1. Major Insect Pests of Canola

Insect Pest	Appearance at Most Damaging Insect Stage	Damage	Time and Crop Stage
Alfalfa Looper	Looping caterpillar Older larvae are light green to olive green and about 25 mm (1") long Move by looping	Chewed leaves Flowers and small seedpods cut off by larger larvae	Bud to ripening Generations overlap Mid-June to September
Beet Webworm	Older larvae are active, black caterpillars 25 to 32 mm (1 to 1.3") long Two parallel light stripes down the back, flanked by circular figures	Green tissue stripped from stems and pods by larger larvae	Flowering to ripening July to mid-August
Bertha Armyworm	Older larvae: smooth velvety black to brown caterpillars with light brown heads, 40 to 50 mm (1.6" to 2") long Younger larvae are smaller and green	Green tissue stripped from leaves Stems and pods consumed by larvae	Late flowering to ripening in Manitoba and Saskatchewan August 10 to 30 in Alberta
Cabbage Seedpod Weevil	Grey to black beetles with curved snout Larvae legless, white with brown head capsules found within canola pods	Buds destroyed by adults Seeds consumed by larvae within developing pods Seeds damaged by adult feeding	Bud to pod ripening Early June to late August
Clover Cutworm	Climbing cutworms Older larvae are green to pale brown similar in size to the bertha armyworm	Leaves, young pods and stems fed on by larger larvae Plants often chewed to the ground	First generation larvae at bud to late flowering Second generation larvae at ripening-August 10 to 30
Cutworms	Pale western cutworm: older larvae are greenish or slaty-grey with a brown head, about 30 to 36 mm long (1.2 to 1.4") Red-backed cutworm: older larvae are dark-grey with dull reddish stripe down back	Seedlings destroyed by being cut off at, or just below soil surface so plants fall over and die Usually in patches on hilltops and south facing slopes in field	Late May to third week in June
Diamondback Moth	Older larvae are pale yellowish green about 8 mm (0.3") long and very active Wriggle rapidly backward with a whipping motion when disturbed, or drop quickly on a spun silken thread	Feeding damage on flowers, young pods and surface tissue of stems and pods Flowering to ripening Three generations overlap Mid-July to early August	Flowering to ripening Three generations overlap Mid-July to early August
Flea Beetles	Shiny beetles about 2 to 3 mm (0.08" to 0.1") long, black or black with yellow stripes Jump off plants quickly when disturbed	Shot holes chewed in cotyledons and early leaves by adults Heavy infestations can severely damage or destroy crops	Seedling to early rosette Mid-May to mid-June
Lygus Bug	Adults about 5 mm (0.2") long and 2.5 mm (0.1") wide Pale green to reddish brown with a "V" mark in front of wings Small nymphs green Larger nymphs have black dots on thorax and back abdomen	Bud blasting by adults Seeds punctured within pods leaving them shrunken and shrivelled	Bud to pod ripening June to late August
Red Turnip Beetle	Dark red beetles 10 mm (0.4") long Three black stripes running down back	Plant defoliated by adult beetles moving into fields from edge Occasionally larvae can also cause damage	Early rosette to bud Early to mid-June
Root Maggot	Larvae 6 to 10 mm (0.2" to 0.4") long White and legless without head capsules	Tunnelling and girdling of taproots In severe infestations roots are infested with secondary root rot and may be completely severed	Bud to pod ripening June to August

Alfalfa Looper (Autographia Californica Speyer)

Identification and Life Cycle (Figure 3)

Figure 3. Alfalfa Looper Life Cycle

There are usually two overlapping generations of alfalfa looper. The alfalfa looper adult, like the diamondback moth, is blown in from the U.S., although some may overwinter as pupae in the soil.

The adult moth's forewings are grey with a distinct yellow, sickle-shaped spot near the middle of each wing, while the body and hind wings are dull grey or brown. These moths appear all summer long due to overlapping generations. They feed on flower nectar at dusk and fly during daylight hours. Adults lay up to 150 to 200 yellow, hemispherical eggs, singly or in small groups on host plants prior to bloom, often near floral parts if present.

Larvae hatch in about a week and climb to the flowers and cut them off. As a result, patches of fields that showed bloom suddenly do not have any flowers. After four weeks of feeding, the larvae are mature and are about 25 mm (1") long. They are light green to olive green in colour, with a paler head, a light stripe down each side, and two light stripes along the back (Figure 4). The mature larvae appear to have a swollen abdomen. Mature larvae attach to plants and spin cocoons in which to pupate.

Figure 4. Alfalfa Looper Larva

Photo by Roy Ellis

Damage

Infestations are not common but have occurred in northern and southern Alberta and parts of Manitoba. Damage is characterized by defoliation and clipping of flowers and small pods. Canola plants can recover from light damage. Alfalfa looper larvae are very susceptible to virus diseases that frequently and rapidly destroy populations of late-instar larvae. Unfortunately, in some cases, feeding damage is usually done before viral control can be effective.

Monitoring

Scout the field for larvae at the start of flowering. Look for characteristic clipping of flowers.

Economic Threshold

There is no economic threshold for alfalfa looper.

Management

Insecticides: If an infestation occurs, assess the damage and delay spraying as long as possible to allow diseases an opportunity to control the pest. Check provincial crop protection guides for registered insecticides.

Aphids (Brevicoryne Brassicae L. and Other Species)

Identification and Life Cycle

Occasionally aphids (Figure 5) become abundant in canola crops. Noticeable numbers usually appear at the tops of plants in late July to early August. The aphids frequently cover the entire top 10 to 15 cm (4 to 6") of plants (Figure 6). In most cases, individual or small groups of plants are infested.

Figure 5. Aphids

Photo by Roy Ellis

Figure 6. Aphid Infestation on a Canola Plant

Photo by Phil Thomas

Damage

The damage is rarely significant since the bulk of pod formation has been completed, and the damaged top few small pods contribute little to the overall yield.

Economic Threshold

There is no economic threshold for aphids in canola.

Management

Spraying is not economical. Several beneficial insects, like the ladybird beetle and lacewing feed primarily on aphids. Their populations increase as aphid populations increase, usually in numbers sufficient to control the aphids.

Beet Webworm (Loxostege Sticticalis L.)

Identification and Life Cycle (Figure 7)

Figure 7. Beet Webworm Life Cycle

There are two generations of beet webworm per year. Beet webworms overwinter either as pupae or larvae within cocoons. The first generation moths emerge from the pupal stage in late May or early June. The moths are small, about 25 mm (1") long and pale brown. They are often seen flying during the day in swarms, especially if disturbed from favoured patches of stinkweed in bloom. The hard-to-find, tiny white eggs are laid in rows on the undersides of leaves of preferred plants. Lamb's-quarters are favourite egg-laying sites and food plants for the beet webworm.

The larvae first appear in late June and July. The 25 to 32 mm (1 to 1.3") larvae or caterpillars are slender and active, and dark green in the early instar stages, becoming black as they mature. There are two white or cream-coloured stripes on either side of the centre line of the back, plus two rows of paired circular figures down either side of the back (Figure 8). The larvae spin silk which appears as webbing at the tops of plants. The caterpillars will often migrate in "armies" to nearby crops when weed hosts are destroyed by defoliation, drought or herbicides. When the larvae of the first generation mature in mid August, they burrow into the soil and spin long tubular silken cocoons just below the soil surface. Soil particles stick to the outside of the silky cocoons.

Figure 8. Beet Webworm Larva

Photo by Roy Ellis

Damage

The larvae start feeding on the leaves of canola, then on the stems and pods, stripping surface tissue and giving the crop a whitish appearance, usually in localized areas within the field. Damage often results from invasion by a migrating army of beet webworms that has developed elsewhere in an adjacent weedy field. Such an invasion can almost completely destroy the crop. Light infestations may cause reduced yields from pod peeling, which leads to incomplete formation and filling of pods. This insect was a serious pest in 1971 and 1972. Its importance has since declined in recent years, possibly because of improved control of lamb's-quarters and pigweed.

Monitoring

Scout the field for larvae late June through mid-August.

Economic Threshold

An economic threshold for beet webworm has not been firmly established, but is thought to be similar to that of the bertha armyworm.

Management

Cultural: Good weed control practices, especially for lamb'squarters, can prevent high larval densities from developing.

Insecticides: Beet webworms are readily controlled with registered foliar insecticide sprays. Check provincial crop protection guides for registered insecticides.

Bertha Armyworm (Mamestra Configurata Wlk.)

Bertha armyworm is one of the most significant insect pests of canola in Canada. It occurs throughout Manitoba, Saskatchewan, Alberta and the interior of British Columbia. Severe infestations can occur throughout most of this area but are usually limited to the parkland area of the prairies and the Peace River region of British Columbia and Alberta.

Bertha armyworm is native to North America and belongs to a group of insects called "climbing cutworms." The true armyworm and variegated cutworm are also in this group.

In most years, populations are kept low by unfavourable weather conditions such as cold winters and cool wet weather, and by parasites, predators and diseases. But when these natural regulators fail, populations can increase dramatically, creating the potential for widespread damage to a variety of broadleaf crops. In extreme situations, infestations of more than 1,000 larvae/m² (1,200/yd²) have been reported while densities of 50 to 200 larvae/m² (60 to 240/yd²) may be common. Infestations may be localized or spread over millions of acres.

Identification and Life Cycle (Figure 9)

Figure 9. Bertha Armyworm Life Cycle

There is one generation of bertha armyworm per year.

Bertha armyworm feed on a variety of crops and weeds. Canola, rapeseed, mustard, lamb's quarters and related plants are preferred host plants. Bertha will also feed on a range of secondary hosts including flax, peas and potato.

Adult moths emerge from overwintering pupae in mid-June and emergence continues until late July. The moth has a wingspan of about 4 cm (1.6") and is active only at night. The forewing is predominantly gray, and flecked with patches of black, brown, olive and white scales (Figure 10). Near the middle of the forewing, toward the leading wing margin (front), there is a prominent, white, kidney-shaped marking defined with a ring of whitish scales. Near the tip of the forewing, there is a conspicuous white and olivecoloured, irregular transverse marking that is characteristic of the species. It is suspected that moths are strongly attracted to canola fields that are in bloom and secreting nectar. Adult moths mate within five days of emergence and lay their eggs on the undersides of canola leaves soon after emergence. The eggs are laid in single-layer clusters of 50 to 500 in a honeycomb arrangement (Figure 11). Each female moth will lay about 2,150 eggs but as many as 3,500 eggs per female have been recorded. The eggs are sculptured, ridged and pinhead in size. When first laid, they are white but become darker as they develop. At average temperatures, the eggs hatch within a week.

Figure 10. Bertha Armyworm

Photo by Roy Ellis

Figure 11. Bertha Armyworm Eggs

Photo by Roy Ellis

Newly hatched larvae are about 0.3 cm (0.1") long (Figure 12). They are pale green with a pale yellowish stripe along each side. Due to their size and colour, they are difficult to see on the undersides of leaves. When disturbed, small larvae may drop off the leaves by a fine silken thread. This behaviour makes it difficult to distinguish small bertha armyworm larvae from those of the diamondback moth, which display a similar behaviour. Large larvae drop off the plants and curl up when disturbed, a defensive behaviour typical of cutworms and armyworms.

Figure 12. Newly Hatched Bertha Armyworm Larvae

Photo by Roy Ellis

Depending on the temperature, larvae take approximately six weeks to complete their development. During this period, they moult five times and pass through six growth stages. As they mature their colour becomes variable. Some remain green, but may become brown or velvety black (Figure 13). At maturity, the larvae are about 4 cm (1.6") long, with a light brown head and a broad, yellowishorange stripe along each side. The velvety black larvae have three narrow, broken white lines on their backs. At maturity in late summer or early fall, larvae burrow into the ground and form pupae.

Figure 13. Mature Bertha Armyworm Larva

Photo by Roy Ellis

Pupation usually begins in mid- to late August and continues through to early to mid-September. If autumn is unusually warm, some pupae may continue their development and emerge as moths in late August or September, only to perish when winter arrives. Bertha armyworm overwinters as pupae in the ground at depths of 5 to 16 cm (2 to 6.3"). The pupa is a reddish brown pod-like structure about 0.5 to 1.8 cm (0.2 to 0.7") in size and tapered with flexible, terminal abdominal segments (Figure 14). The pupa protects bertha armyworm while it transforms from the larval stage to the adult moth. Bertha armyworm pupae are indistinguishable from other cutworm pupae.

Figure 14. Bertha Armyworm Pupae

Damage

Larvae are the only development stage of bertha armyworm to cause crop damage.

The degree of damage varies with the crop, the plant's growth stage, the growth stage of the larvae and the number of larvae present. Depending on the season and crop location, significant crop damage usually occurs within a three-week period between late July and late August. A larval population of 200/m² (187/yd²) can reduce yields by 50%. The larvae are not highly mobile and migrate from an infested field only when the food supply is short, or the field is over-ripe.

Small larvae feed on the underside of leaves, chewing irregularly shaped holes in the leaves. They usually cause little damage at this stage, even when population levels are high. Substantial crop damage can occur after the larvae moult to the second-last instar. These larvae are about 1.3 cm (0.5") in length. Larvae in the last two larval stages eat about 80 to 90% of the plant material consumed during the life of the larvae.

If the plants drop their leaves before the larvae are mature, the developing larvae will feed directly on stems and pods. From a distance, infested canola fields look pale white because larvae eat the outer green layer of the stems and pods exposing underlying white tissue. Pods may be "debarked," but more commonly, the larvae chew holes in the pods and eat the seeds. At high numbers, the entire pod may be consumed. Even if the pods are only stripped of their outer green layer and not eaten entirely, crop losses still may occur because of premature shattering. At swathing larvae can continue to feed for a few days until the crop dries in the swath. However, by swathing most larvae have dropped to the ground to pupate. In some years, early-seeded canola can be swathed prior to damage.

Monitoring

The number of bertha armyworm larvae in a crop one year is not a reliable indicator of what to expect the following year. Bertha armyworm populations fluctuate widely from year to year. Monitoring programs established by provincial governments assist growers with making crop protection decisions. The monitoring programs use accumulated degree-days to monitor the development of overwintering pupae into moths. Pheromone-baited traps that attract the male moths monitor the flight of bertha armyworm moths. The number of moths collected by these traps gives an indication of the risk of bertha armyworm larval infestations. Generally, higher moth numbers during the flight period (around mid-June through July) indicate greater risk of larval damage (in July and August). Risk assessment maps are available from crop specialists and on provincial Web sites.

Check for larvae in each field regularly to minimize crop losses. Begin larval monitoring after peak flowering or about two weeks after peak trap catches and continue until either the mean number of larvae per m2 exceeds the economic threshold (at which point the crop is sprayed) or the crop is swathed. It is important to monitor larval numbers in each field. Adjacent fields may have very different larval densities, depending on how attractive the crop was when the moths were laying their eggs. Adjacent fields may also have different-sized larvae, depending on when the eggs were laid.

For accurate larval estimates in a crop, sample at least three locations, a minimum of 50 m (164') apart. Do not sample headlands (20 m or 66' wide) and areas within the crop that are not representative of the field. At each location, mark out an area of 1 m2 and beat the plants growing within that area to dislodge the larvae. Push the plants aside or remove them and count the number of larvae in the m2. It is important to take your time when counting larvae. Carefully search the soil and leaf litter. The larvae are difficult to see and may be hidden underneath clumps of soil, in cracks in the soil or within curled leaves. Use the average number of larvae at the sites surveyed within each field to determine if the economic threshold has been exceeded and an insecticide application is necessary.

Economic Threshold

The economic threshold is the number of larvae present when the value of the crop they consume is greater than the cost of controlling them. The economic threshold for bertha armyworm varies with the cost of the insecticide, the method of application and the crop's value. Using crop values and application costs, the following table indicates the larval density (larvae/m²) at which an insecticide treatment in canola would be warranted (Table 2).

Table 2. Economic Injury Levels for Bertha Armyworm in B. napus

		$ Value of Crop to Grower
		$/bu	6	7	8	9	10	11	12	13	14	15	16
		$/tonne	265	309	353	397	441	485	529	573	617	662	706
Cost of Spraying
$/ac	$/ha		# Larvae/m2
7	17		20	17	15	13	12	11	10	9	9	8	8
8	20		23	20	17	15	14	13	11	11	10	9	9
9	22		26	22	19	17	16	14	13	12	11	10	10
10	25		29	25	22	19	17	16	14	13	12	11	11
11	27		32	27	24	21	19	17	16	15	14	13	12
12	30		34	30	26	23	21	19	17	16	15	14	13
13	32		37	32	28	25	22	20	19	17	16	15	14
14	35		40	35	31	27	24	22	20	19	17	16	15
15	37		43	37	32	29	26	23	22	20	19	17	16

For example, spraying would not be economical if larval counts were less than 19 per m² given a crop price of $353/t ($8.00/bu) and a spray cost of $22/ha ($9.00/ac). Note: Table has been updated in 2008 to include relevant pricing (John Gavloski, MAFRI).

Management

Effect of environment: Environmental conditions have a significant impact on bertha armyworm populations, especially on the overwintering pupae. During harsh winters in snow-free fields, most bertha armyworm pupae die. Bertha armyworm outbreaks appear to be favoured by snow accumulation, which protects pupae from prolonged exposure to temperatures below -10°C. Newly hatched larvae are especially vulnerable to inclement weather and diseases. The increase in reduced tillage and stubble conservation results in more snow accumulation on infested fields and could favour bertha armyworm survival, especially in years with early snowfall.

Cultural control: Canola losses from bertha armyworm populations can be minimized by planting alternative crops, effective weed control, early swathing and fall cultivation.

Early seeding of an early maturing variety (or fall seeding any variety) can help avoid a significant bertha armyworm infestation unless the moth flight is exceptionally early. Canola harvested before August 15 is unlikely to suffer yield loss from bertha armyworm. Mated female moths prefer to lay eggs on canola in the early flowering stage. Fields in this stage during the egg-laying period tend to be hardest hit. Infestations in early maturing B. rapa varieties tend to be lower than in B. napus varieties.

Fall cultivation can kill many bertha armyworm pupae by mechanical damage. Tillage can also reduce the amount of snow trapped on a field by removing or flattening stubble and exposing pupae to sub-zero temperatures over the winter. This practice may be effective for individual fields but is not likely effective unless all growers in an area adopt the practice. Adult moths are strong flyers and can easily move to adjacent fields. Do not use fall cultivation on lighttextured soils susceptible to erosion.

Effective control of weeds such as lamb's-quarters and wild mustard can reduce bertha armyworm infestations in flax, peas, lentils and sugar beets. Larvae will first feed upon these weeds and then move onto these crops after the weeds have been destroyed.

Biological control: Various diseases and parasites attack bertha armyworm including:

• a nuclear polyhedrosis virus
• an ichneumonid wasp (Banchus flavescens)
• a tachinid fly (Athrycia cinerea)

However, these natural enemies often do not destroy larvae until after considerable crop damage has occurred. They have their greatest impact on bertha armyworms produced a year or two after the peak of an outbreak. This is probably why severe infestations only last two or three years. Their presence in a crop does not, however, indicate that control measures are unwarranted.

Insecticides: An insecticide is the grower's last line of defence against bertha armyworm.

With severe infestations widespread crop losses can be minimized with insecticides if the infestation is detected early. However, failure to detect infestations early may result in insufficient time to apply insecticides before severe damage is done. Also, there may be temporary insecticide shortages if suppliers are not aware of the potential outbreak.

For best results, apply an insecticide as soon as economic thresholds are reached. A single well-timed application of any registered insecticide is usually effective. Check provincial crop protection guides for registered insecticides.

• Ensure the larvae are at least 1.3 cm (0.5") long (before the fifth instar larvae appear).
• Apply the insecticide early in the morning or late evening when the larvae are actively feeding. Do not apply during warm afternoons. A single, welltimed application of any registered insecticide applied with aerial or high clearance ground equipment is usually effective.
• Use enough water to ensure adequate coverage.
• Use high water volumes in crops with dense canopies.
• Use the higher label rates of application when a range is indicated.

To protect foraging honeybees, delay insecticide applications until after the crop has finished blooming. If this is not possible, select the safest insecticide to control the bertha armyworm larvae and apply during the evening.

Cabbage Seedpod Weevil (Ceutorhynchus Obstrictis (Marsham))

The cabbage seedpod weevil was introduced to North America from Europe about 70 years ago. The weevil was discovered in British Columbia in 1931, and from there it dispersed south and eastward. It now occurs throughout most of the U.S.

It was first found infesting canola in southern Alberta in 1995, and since then the weevil has spread to central Alberta and southwestern Saskatchewan. In 2000, it was found in Quebec for the first time.

Identification and Life Cycle (Figure 15)

Figure 15. Cabbage Seedpod Weevil Life Cycle

Host plants of the cabbage seedpod weevil belong to the mustard family (Brassicaceae), and include canola, brown mustard, cole crops (such as cabbage, broccoli and cauliflower) and cruciferous weeds (such as wild mustard, flixweed and stinkweed).

Host plants are either true hosts or food hosts. Both hosts can provide food, especially pollen, for adult feeding, but only those with large seedpods that can sustain larval development are true hosts. Examples of true hosts are canola, brown mustard and wild mustard while examples of food hosts are flixweed, stinkweed and hoary cress.

Adult weevils are ash-grey and approximately 3 to 4 mm (0.1 to 0.2") long. They have a prominent curved snout that is typical of most weevils (Figure 16).

Figure 16. Cabbage Seedpod Weevil

Photo by Roy Ellis

The cabbage seedpod weevil takes about eight weeks to develop from egg to adult. Development time will vary somewhat depending on weather conditions, especially temperature. There is one generation per year.

The adults overwinter beneath leaf litter in tree shelterbelts, roadside ditches and woodlots. Late in the season (September to early November) they select overwintering sites and burrow beneath the soil surface where they are protected from low temperatures. In spring, they emerge from these sites over a period of several weeks and seek out host plants. Adults occur most commonly on the buds and flowers of host plants, but during windy days they move to sheltered areas within the plant canopy. Before canola crops enter the bud stage, adults can be found on wild mustard, flixweed, hoary cress, stinkweed and volunteer canola. When disturbed, the adults often drop to the ground and play dead. After several seconds they resume activity. Mating occurs from spring to early pod development, usually on a host plant. When small pods develop, the females can deposit an egg through the pod wall onto or adjacent to a developing seed.

Eggs are very small, oval and opaque white. Usually only a single egg is deposited per pod. However, two or more eggs can be laid per pod during outbreaks. Eggs hatch in about six or seven days. Females continue to lay eggs until they die later in the season.

Larvae are white and grub-like, without legs or eyes (Figure 17). Soon after hatching the larvae begin feeding within the pods on developing seeds. Larval development takes approximately six weeks During this time a single larva consumes about five canola seeds. There are three larval stages (instars).

Figure 17. Cabbage Seedpod Weevil Larva

Photo by Roy Ellis

Mature larvae chew small, circular exit holes in the pod walls (Figure 18), drop to the ground, burrow in and pupate within earthen cells. New generation adults emerge about 10 days later and feed on immature canola or other green cruciferous plants until late in the season when they enter overwintering sites.

Figure 18. Cabbage Seedpod Weevil Exit Holes on Canola Pods

Photo by Phil Thomas

The distribution and abundance of the cabbage seedpod weevil have been monitored yearly in western Canada since 1997. Predictive models based largely on climate data indicate that this pest will eventually disperse to all regions of canola production in western Canada, including the Peace River region. Weevil distribution maps are available on the Web sites of provincial departments of agriculture.

Damage

Crop damage from cabbage seedpod weevil can occur in several ways. Adults feed on developing canola buds causing bud-blasting and reduced yield potential in dry years when the ability of plants to compensate is limited. Larvae feed within developing pods with each larva consuming about five seeds during its development. Although this amount represents only 15 to 20% of the total yield of a particular pod, these pods are predisposed to premature shattering.

Canola pods harbouring cabbage seedpod weevil larvae often appear distorted. When larvae consume some seeds within pods, the undamaged seeds enlarge and mature, often leaving misshapen pods.

Larvae emerge from pods via exit holes. In humid weather, these exit holes provide an entry point for fungal infections and additional seeds can be damaged.

When new generation adults emerge in late summer, they can invade nearby fields and damage the immature pods of late-seeded canola by feeding directly on the seeds through the pod walls.

Monitoring

The risk of infestation can be predicted based on the adult population of the preceding fall. High numbers of weevil adults in fall will likely mean significant infestation levels in the following spring, although a severely cold winter with little snow cover could reduce the survival of overwintering adults.

Cabbage seedpod weevil adult populations can be monitored with sweep nets in each field. Begin sampling when the crop first enters the bud stage and continue through the flowering period. Select 10 locations within each field, and at each location count the number of weevils from ten 180° sweeps. Sample both the perimeter and interior of the field to obtain an accurate estimate of weevil numbers throughout the field.

A few other weevil species may also be found in canola but do not require control measures. The most common of these is a closely related species, Ceutorhynchus neglectus, about one-half the size of the cabbage seedpod weevil that will feed on canola but prefers flixweed.

Economic Threshold

Insecticide application is warranted when an average of three to four adult weevils are collected per sweep.

Management

Cultural control: At present, trap cropping is the most promising cultural strategy for controlling the cabbage seedpod weevil. This approach takes advantage of the weevil concentration that often occurs at field edges when weevils first invade a canola field. Planting the perimeter of a Brassica napus field with early flowering Brassica rapa , and spraying this perimeter strip with insecticide allows the grower to control cabbage seedpod weevils before they spread throughout the field. Alternatively, a strip of the same variety planted seven to 10 days before the rest of the field can also serve as a trap for adult weevils. Other potential cultural control strategies being investigated by researchers include the effect of altering seeding rates, row spacing and fertility regimes. Although B. napus, B. rapa, and Brassica juncea (brown mustard) are susceptible to infestation by cabbage seedpod weevil, Sinapis alba (yellow mustard) is completely resistant. Yellow mustard crops do not require monitoring or control measures.

Biological control: Research in 2000 and 2001 found some evidence of parasitism. A major focus of work in the future will be to enhance the abundance and dispersal of these parasites, reducing the need for insecticide use.

Insecticides: Check provincial crop protection guides for registered insecticides.

If control is required, the best time to spray is when crops are in 10 to 20% flower to avoid egg laying in newly formed pods. This is the stage when 70% of plants in the field have at least three to 10 open flowers. Spray late in the day to minimize harmful effects to beneficial insects in the crop, especially bees. Check provincial crop protection guides for registered insecticides.

Cabbageworms (Pieris Species)

Two cabbageworm species are commonly found in canola fields-the imported cabbageworm (Figure 19) and the western cabbageworm. Imported cabbageworms are green with a velvety texture, and have a faint yellow stripe down their backs. Imported cabbageworm also attack cabbage. The western cabbageworm has brilliant blue and yellow stripes down its back. The adult stages of both species are white butterflies that fly by day. There are two or three generations per year. The cabbageworms feed on leaves but do not cause economic losses.

Figure 19. Imported Cabbageworm Larva

Photo by Roy Ellis

Clover Cutworms (Dicestra Trifoli (Hufnagel))

Identification and Life Cycle (Figure 20)

Figure 20. Clover Cutworm Life Cycle

Clover cutworms overwinter as pupae in the soil. There are two generations per year. The first moth generation appears in June, the second in late July. The adult moths are uniform or mottled ash grey to pale brown. White or pale yellow eggs are laid singly on the undersides of leaves in late spring and during the summer. The newly hatched light green caterpillars feed on the undersides of lower leaves, gradually moving up the plant as they mature, causing damage during late June through early July and again from mid- August through to September.

Clover cutworm larvae are very similar in appearance and size to bertha armyworm larvae, but fewer velvety black caterpillars are found and the majority are either green or pale brown. The most distinct difference between the two species is the wide stripe along each side of the clover cutworm is yellowish pink (Figure 21) and yellowish orange on bertha armyworm. Mature caterpillars burrow into the topsoil to pupate. The pupa is somewhat smaller than that of bertha armyworm, and has a greenish tinge at one end. Most of the pupae formed in mid-summer emerge to produce the second flight of moths. The second-generation larvae frequently cause damage at the same time as bertha armyworm (August 10 to 30). In years when both are present, clover cutworms may be mistaken for bertha armyworm. This insect has only been an economic problem in the Peace River area of Alberta.

Figure 21. Clover Cutworm Larva

Photo by Roy Ellis

Damage

Clover cutworms feed anywhere on the canola plant and can consume the entire plant. The economic threshold for this insect is probably similar to that for bertha armyworm. However, bertha armyworms tend to be dispersed throughout the field, while clover cutworm larvae are aggregated in clusters producing more concentrated damage. First generation clover cutworm larvae attack canola earlier than bertha armyworms. However, with light infestations, the plants will probably recover. Infestations in localized areas of northern Alberta have caused severe damage. In most years, diseases control the insect with only an occasional isolated outbreak.

Monitoring

Scout fields mid-June through late August. The clover cutworm is occasionally an economic problem, especially in the Peace River region.

Economic Threshold

The economic threshold for clover cutworm is hard to determine. Many studies show that it may be similar to the economic threshold for bertha armyworm. However, bertha armyworm tends to be fairly evenly distributed in a field, while clover cutworms tend to be in patches across a field. Therefore, while some areas of a field suffer heavy damage, others may be unaffected.

Management

Check provincial crop protection guides for registered insecticides. Spraying for clover cutworms can put honeybees at risk when canola crops are in flower.

Cutworms - Red-Backed (Euxoa Ochrogaster Guen), Pale Western (Agrotis Orthogonia Morr.)

Identification and Life Cycle (Figure 22)

Figure 22. Cutworm Life Cycle

The pale western cutworm is of greatest concern in the southern, more open prairie areas, while the redbacked cutworm is of concern in the parkland belt and northern agricultural areas of the prairies. Both cutworm species feed on practically all field crops, vegetables and home garden plants. They are most destructive when feeding on cereals, flax, sugar beets, canola and mustard.

There is one generation per year. Cutworms overwinter as tiny eggs that are laid in fall. In April or early May, the eggs hatch and the young larvae feed mainly at night on weeds and volunteer plants. The larvae of the pale western cutworm usually remain in the soil unless forced to the surface by rain or hard soil. The red-backed cutworm often comes to the surface in search of food.

The young larvae of both species pass through six stages (instars), each separated by a shedding of skin. Late May and the first three weeks in June are the most likely times for cutworm activity (seedling to rosette stage). The larger cutworms are usually easy to find in the soil beside freshly cut plants. When disturbed, the cutworms will curl up. This is characteristic of all cutworms and armyworm species. At maturity, the pale western cutworm larvae are greenish or slate-grey with a brown head, and vary in length from 30 to 36 mm (1.2 to 1.4"). Mature redbacked cutworm larvae are dark grey with two broad, dull, brick-red stripes along the back (Figure 23). They are about 38 mm (1.5") in length. After the cutworms complete their larval growth, usually in late June, they burrow deeper into the soil where they make a small soil chamber in which to pupate. The reddish brown torpedo-shaped pupae of the redbacked cutworm are similar in size and shape to those of other cutworm and armyworm pupae.

Figure 23. Redbacked Cutworms

Following the pupal stage, adult moths emerge from the soil in August to early September. Redbacked cutworm moths are light fawn to brick red in colour, while pale western cutworm moths are mottled greenish grey with distinct pale lines on the forewings. The moths are night fliers and not usually seen. After mating, the pale western moths lay eggs on or just below the surface of loose, dry soil. Cutworm moths may lay several hundred eggs on their host plants in weedy stubble or fallow fields.

Damage

Young redbacked cutworms chew holes and notches in leaves, while older larvae and the larvae of the pale western cutworm eat into the stems and usually sever them at or just above the soil surface. Cut plants can be found drying up and lying on the soil surface. Patches of bare soil characterize infestations where the crop has started to disappear. These patches gradually enlarge until anywhere from 0.5 to 1 ha (1 to 2 ac) or complete fields are destroyed. First signs of damage usually appear on hilltops, south facing slopes or in areas of light soil that normally warms faster and shows damage early. In many cases, bare hilltops are attributed to poor germination rather than cutworms. Determine which is the case.

Monitoring

Scout the fields and inspect seedlings on a weekly basis from mid-May to mid-June. Determine whether bare areas with no seedlings have resulted from poor germination or cutworm damage. Check the edges of bare areas for cut-off plants and search the top 5 cm (2") of soil around such plants for larvae. The key to control is detection. When notched, wilted, dead or cut-off plants (weed or crop seedlings) are seen, dig around the roots of the plants for cutworms.

To collect cutworm larvae, a garden trowel and a soil sifter are useful tools. Cutworms may be found down to about 5 cm (2") below the soil surface. The small, worm-like larvae curl up or attempt to hide in the debris. Pupae may also be collected in this way. The larvae and pupae can be reared to adult moths if necessary for species determination.

Because cutworm moths, like most moths, are nocturnal and attracted to light, the adult population can be monitored using light traps. Sex attractants also can be used to trap adult cutworm moths in commercial or homemade pheromone traps.

Economic Threshold

Cutworm control may only be necessary in small areas of the field, when bare patches appear and large numbers of cutworms are still actively feeding. Canola is much more susceptible to cutworm damage than cereals because no regeneration and tillering occurs to compensate for loss of plants. Use an insecticide when cutworms exceed three to four cutworms per m² (yd²).

Management

Cultural: Summerfallow fields in areas infested with redbacked cutworms free of weeds in August (egg laying time). Summerfallow fields that have a protective crust through August and the first half of September are much less attractive for egg laying by pale western cutworms. Therefore, work fields in late July and allow them to harden by summer rains. In the spring, a delay of 10 to 14 days between cultivation and seeding can help reduce populations because larvae that have already fed will die if deprived of food for several days. Cold weather after cultivation and seeding will have a similar effect.

Insecticides: Check provincial crop protection guides for registered insecticides. Apply insecticides in the evening since these pests feed during the night

Diamondback Moth (Plutella Xylostella L.)

Diamondback moth was introduced into North America from Europe about 150 years ago. It now occurs throughout North America wherever its host plants are grown. Diamondback moth larvae feed on all plants in the mustard family (canola, mustard), cole crops (broccoli, cabbage) and on greenhouse Brassica plants. In western Canada, canola and mustard are its primary targets.

Although the diamondback moth occurs each year throughout the Canadian prairies and north central U.S., the severity of the infestation varies considerably from year to year and location to location.

Identification and Life Cycle (Figure 24)

Figure 24. Diamondback Moth Life Cycle

Normally, the diamondback moth takes about 32 days to develop from egg to adult. However, the time to complete a generation may vary from 21 to 51 days depending on weather and food conditions. There may be several generations per growing season. Generations usually overlap and all four life stages, egg, larva, pupa and adult may be present in the field at the same time.

The adult moth is approximately 8 to 9 mm (0.3 to 0.4") long with a wingspan of 12 to 15 mm (0.5 to 0.6") (Figure 25). At rest, the moth folds its wings over the abdomen in a tent-like manner. The folded wings flare upwards and outward at the tips. The wing tips are fringed with long hairs. In the male, the forewing margins have a series of yellow wavy markings. When the wings are folded while the moth is at rest, these markings come together to form three yellow diamonds, hence the name diamondback.

Figure 25. Diamondback Moth

Photo by Roy Ellis

Adult females lay an average of 160 eggs during their life span of about 16 days. Egg laying occurs at night. The greatest number of eggs is laid the first night after emergence and egg laying continues for about 10 days. In the main canola growing areas, most of the canola crops will not have emerged by the time the moths arrive and so many eggs are laid on cruciferous weeds and volunteer canola.

Eggs are oval, yellowish-white and tiny. They are glued to the upper and lower leaf surfaces singly or in groups of two or three, usually along the veins or where the leaf surface is uneven. The eggs hatch in about five or six days.

Immediately after hatching from the egg, larvae burrow into the leaf and begin mining the leaf tissue internally. After feeding within the leaf for about a week, the larvae exit from the underside of the leaf and begin feeding externally. The larvae are pale yellowish-green to green caterpillars covered with fine, scattered, erect hairs. The posterior end of the caterpillar is forked.

Larvae moult three times during the larval stage that lasts about 10 to 21 days, depending upon temperature and the availability of food. At maturity the larvae are cigar-shaped and about 12 mm (0.5") long (Figure 26). The diamondback moth larva is easily identified by its peculiar reaction to being disturbed. It will wriggle backward violently and may drop from the plant, suspended by a silken thread. After several seconds, the larva will climb back onto the leaf and continue feeding.

Figure 26. Diamondback Moth Larva

Photo by Roy Ellis

Larvae pupate in delicate, white, open-mesh cocoons attached to the leaves, stems or seedpods of the host plant (Figure 27). Initially, the pupae are light green but as they mature they become brown as the adult moth becomes visible through the cocoon. The pupal stage lasts from five to 15 days depending on environmental conditions, then adult moths of the next generation emerge.

Figure 27. Diamondback Moth Pupa

Photo by Roy Ellis

Damage

An infestation of diamondback moth cannot be predicted based on the previous year's population because very few, if any, pupae survive the long, cold Canadian winters. Instead, the severity of the infestation in any given year depends on two factors-overwintering populations in the U.S. and strong south spring winds to transport the moths north into Manitoba, central Saskatchewan and eastern Alberta.

In years when conditions are right for the moths (when moths arrive on the wind in large numbers in early May and summer temperatures are hot), diamondback moth infestations can cause millions of dollars of damage. Crop damage is caused by the larval stage. Diamondback moth larvae feed on any green tissue of canola and mustard plants but prefer leaves. The amount of damage varies greatly, depending on plant growth stage, larval densities and size.

When larvae are small, damage is evident as small irregular holes or "shot holes" in the leaves. If larvae are numerous, they may eat the entire leaf, leaving only the veins. When plants begin to flower, larger larvae often feed on the flower buds, flowers and young pods. Feeding damage during the early flowering stage can be extensive. Extensive feeding on the flowers will delay plant maturity, cause the crop to develop unevenly and significantly reduce seed yields.

When plants are fully podded and leaves begin to wilt or die in late July or early August, larvae will remove the surface tissue from the stems and pods. The seeds within a damaged pod will not fill completely and pods may shatter, resulting in yield loss. Larvae may also chew into pods and eat the developing seeds.

Crop damage is usually first evident on plants growing on ridges and knolls in the field. In severe cases, damage shows from a distance as abnormal whitening. Early field monitoring and the application of insecticides can prevent damage, if larval numbers exceed the economic threshold. After an infestation is controlled at the podding stage, a new infestation is not likely to become established because of the rapid advance of the crop toward maturity.

Monitoring

Scout fields in July and August. Consult with crop specialists and entomologists for the size and timing of the moth flight. The presence and relative abundance of the diamondback moth can be determined by using pheromone traps. These traps cannot predict the potential for crop damage, but trap counts can provide an early warning of a possible infestation. Environmental conditions will determine how many eggs are laid and whether the larvae emerge and survive.

In the field, moths will flutter up as the canopy is disturbed.

Monitor diamondback moth larvae by removing the plants in an area measuring 0.1 m² (about 12" square), beating them onto a clean surface, and counting the number of larvae dislodged from the plants. To obtain an accurate count, repeat this procedure in at least five locations in the field. Monitor crops at least twice a week during the growing season.

Economic Threshold

The economic threshold for diamondbacks in canola at the advanced pod stage is 20 to 30 larvae/0.1 m² (approximately two to three larvae/plant). Apply an insecticide when larval counts reach this point.

An economic threshold for canola or mustard in the early flowering stage has not been established. However, at this plant growth stage insecticide applications are likely required at larval densities of 10 to 15 larvae/0.1 m² (one or two larvae/plant).

Management

Effect of Environment: Environmental factors can have a profound impact on diamondback moth populations. Cool, windy weather reduces adult activity and females often die before they lay all their eggs. Heavy rainfall can drown small larvae and reduce numbers by more than half. Humid conditions within the crop following a rainfall can promote the spread of fatal fungal diseases throughout the diamondback moth population.

Cultural: Tillage reduces the availability of cruciferous weeds and volunteer canola host plants, preventing the successful establishment of first generation larvae where moths arrive before canola emergence. Rainfall is a natural control agent.

Biological: Diseases, parasites and predators affect diamondback moths. Entomophthorales fungi cause natural disease outbreaks in diamondback populations. These outbreaks usually occur late in the growing season when populations are high. The rate of infection of diamondback moth larvae can be high enough to limit the development of additional generations late in the season.

In western Canada, three species of parasitic wasps attack the diamondback moth. Diadegma insulare (Cresson) and Microplitis plutellae (Muesebeck) attack the larval stages, while the third species, Diadromus subtilicornis (Gravenhorst) attacks the pre-pupal and pupal stages. Flies, wasps, lacewings, plant bugs, pirate bugs, beetles, spiders and birds also prey on the diamondback moth larvae. Despite the abundance of potential biological control agents, the only effective way of controlling a diamondback moth outbreak once the population exceeds the economic threshold is to apply an insecticide.

Insecticides: Check provincial crop protection guides for registered insecticides. A single, well-timed application of an insecticide with either aerial or ground equipment is usually effective in controlling larval populations. Make insecticide applications when larval populations are high because the effectiveness is reduced against adults or pupae. Always apply insecticides with enough water to ensure adequate coverage. Use high water volumes and label rates when the crop canopy is dense. If the leaves are beginning to turn yellow and dry up, damage will become more serious as larvae move to feed on pods. If this is the case, consider control at the lower end of the economic threshold range.

Injury to honeybees and other pollinating insects can be minimized by not spraying flowering crops. When it is necessary to apply an insecticide to a flowering crop, use the safest product available and apply it during the evening.

Flea Beetles (Phyllotreta Cruciferae (Goeze) and Phyllotreta Striolata (F.))

Flea beetles feed on plants belonging to the mustard family (Brassicaceae) grown throughout the Northern Great Plains of North America (North Dakota, South Dakota, Montana, and northwestern Minnesota, Manitoba, Saskatchewan, Alberta, and the Peace River region of British Columbia). Eight flea beetle species are known to attack canola, mustard and rapeseed. Of these, only the crucifer flea beetle, Phyllotreta cruciferae (Goeze) and the striped flea beetle, Phyllotreta striolata (F.), which were both introduced from Eurasia, are significant pests.

The economic impact of flea beetles on crop production varies with population densities. Yield losses of about 10% are common where flea beetles are abundant even when the crop is protected with insecticides. A 1%/ac yield reduction results in a total crop loss of about $25 million to $35 million. Annual crop losses in North America from flea beetles probably exceed $300 million.

Identification and Life Cycle (Figure 28)

Figure 28. Flea Beetle Life Cycle

Flea beetles attacking canola, mustard and rapeseed are small, elliptical or oval-shaped and less than 2.5 mm (0.1") long. When disturbed they use their powerful hind legs to jump away like a flea. Hence the name, flea beetle.

The crucifer flea beetle is the most widely distributed and destructive, attacking canola, mustard and rapeseed (Figure 29). Adult crucifer flea beetles are uniformly black with a metallic bluish sheen. The wing covers (elytra) are randomly punctuated and the large hind legs are a dark amber colour.

Figure 29. Crucifer Flea Beetle

Photo by Roy Ellis

The striped flea beetle is less abundant except near the northern edge of the agricultural region in Canada. Adults are black with distinctive yellow stripes on their elytra (Figure 30).

Figure 30. Striped Flea Beetle

Each species has a single generation per year, although adults appear twice during the growing season. In the spring, overwintered adults emerge and feed on canola seedlings. In the fall, it is the offspring of the overwintering adults that are observed feeding on canola leaves, stems and seed pods.

Flea beetles overwinter as adults near the surface of the leaf litter, grass and debris beneath hedges, shelterbelts, poplar groves and in association with canola stubble and volunteer cruciferous plants. Within the leaf litter in these locations flea beetle densities may be as high as 140 to 250 beetles/m² (131 to 234/yd²).

Five to 11 days after leaf litter begins to thaw in the spring (between late April and early May) the first adult flea beetles become active. Depending on temperature, it may take an additional three weeks before all the overwintering adults emerge. Adult striped flea beetles begin emerging slightly before adult crucifer flea beetles.

Under cool conditions, flea beetles walk or hop into the adjacent cruciferous crops or weeds. They feed on volunteer canola and mustard, or on weeds such as wild mustard, flixweed, lamb's quarters, stinkweed or peppergrass. Then they will move to newly emerged canola seedlings. When temperatures exceed 14°C (early to mid-May) and wind is calm, they may take flight and invade other fields, attacking seedlings as they emerge.

After selecting a host plant and feeding has commenced, beetles mate repeatedly. Egg laying begins in late May and continues until the end of June or for about 30 days. A very small proportion of the population may continue to lay eggs until early August. Females deposit about 100 smooth, yellow, elongated, oval eggs 0.38 to 0.46 mm (0.01 to 0.02") by 0.18 to 0.25 mm (0.006 to 0.009") wide, either singly or in groups of three or four in the soil adjacent to the host plant's roots. Unless the eggs are in contact with moist soil, they desiccate within a few hours. The eggs take about 12 days to hatch. After egg laying the overwintering adults begin to die off.

Flea beetle larvae are grub-like with off-white bodies and a brown head and anal plate. Larvae moult twice during the 25 to 34 days (usually mid-June to late July) it takes them to complete three larval stages. Full-grown larvae are about 3 to 4 mm (0.1 to 0.2") long. Larvae feed on the root hairs and taproots of seedlings. In a few cases, larvae have been observed burrowing into the plant near the juncture of the root and stem. When larval development is complete, larvae pupate in small earthen cells.

Flea beetle pupae are usually present in the field by early to mid-July. They are entirely white except for the eyes, which darken as the pupal stage progresses to completion. The body appendages are free and distinguishable. The pupal stage lasts for about seven to nine days.

Adult emergence begins after mid-July and continues until early September. The beetles feed on the leaves, stems and pods of cruciferous plants. Development from egg to adult takes about seven weeks. In late August and September, adults move into leaf litter and debris to overwinter.

Damage

Flea beetles feed on the cotyledons, leaves, apical bud tissue, petioles, stems, roots and pods of crucifers (canola, mustard and rapeseed). The impact of feeding on crop development depends on where on the plant they feed, crop development, growing conditions and the intensity of the attack. Sunny, warm, dry weather increases feeding activity. Cool, damp weather slows flea beetle activity and promotes plant growth.

During seedling emergence, severe stand loss can occur if flea beetle populations are high and the cotyledons are the only green tissue available. Adult beetles feed on the surfaces of leaves, stems and pods and produce small pits (Figure 30). The tissue underneath the injury eventually withers and dies. On leaves and cotyledons, the damaged tissue breaks up and falls out producing a shot hole appearance. When feeding is extensive, the small feeding pits merge and form larger holes in the leaves.

Heavy infestations may severely damage cotyledons, the first leaves, petioles and stems. Crop thinning and growth rate reduction caused by flea beetle feeding are most severe the first two weeks after seedling emergence. At the three- or four-leaf stage, the plants are generally established and can outgrow the feeding damage. At this time, the number of adult flea beetles often begins to decline. The crop can usually compensate for the destruction of individual plants provided large portions of the crop are not totally destroyed.

Feeding damage is most severe when flea beetles attack the growing point (meristem) because it limits the ability of the plant to compensate. Heavy attacks can destroy the entire crop, forcing growers to reseed or leave the field fallow. Occasionally, seedling loss from plant disease is mistaken for flea beetle damage. Dig damaged plants from the soil and examine carefully for evidence of disease on the shoots and roots.

Early damage to seedlings produces plant stands with uneven height and maturity, reduced seed yield and contributes to seeds with elevated chlorophyll content. Delayed maturity may expose the crop to adverse temperatures during flowering or to frost before the plants have matured. Uneven maturity at harvest reduces seed quality or yield. Delaying harvest to allow immature pods to ripen contributes to yield loss when over-ripe pods shatter during harvest. Harvesting too early produces a crop with many immature seeds containing high chlorophyll levels, affecting seed quality and yield. Most of this damage can be prevented if canola is protected from flea beetle injury during the two to three weeks following emergence.

During summer months the larval stages contribute to yield losses by feeding on plant roots and root hairs. Root damage is estimated to reduce yield by about 5%.

Flea beetles that emerge after mid-July can also affect yield. Their feeding during pod development and filling causes injury to pods, leading to premature pod drying, shrivelled seeds, pod shattering and encourages fungal growth within the pods during damp weather. Injury to the pods is usually concentrated on the youngest pods and on lateseeded crops.

In the fall, the adult population feeds on pods making them prone to shattering and contributes to the production of small seeds and seeds with increased chlorophyll content.

Monitoring

Note flea beetle densities in the fall. This will be the first signal of potential problems next spring. If flea beetles are abundant, seriously consider using insecticides at planting.

Scout fields in the spring, and assess damage to cotyledons and the first true leaves of seedlings daily. Continue scouting for the first 14 days after emergence, especially on sunny, calm days when temperatures exceed 14°C. Collect plants at random as you walk across the field and estimate the foliage damage. Check all field and slough margins where the insects overwinter. This sampling procedure determines the extent and distribution of damage.

Economic Threshold

Use the economic threshold to decide whether a foliar spray for flea beetle control will be an economical investment. Since control decisions are made prior to seeding when seed treatments and in-furrow granules are used, the use of economic thresholds for flea beetle control in canola only apply when foliar sprays are used as a flea beetle control strategy.

Canola seedlings can withstand 50% leaf loss. Flea beetles can damage plants very quickly, however, so the economic threshold for flea beetle feeding on canola is when there is 25% defoliation and flea beetles are present. Applying controls at 25% defoliation will reduce the risk of flea beetles reaching a level where yield loss and plant development are substantially reduced.

When scouting fields for flea beetle damage, it is important to understand that flea beetles generally invade canola fields from the field edges. Flea beetle damage and the number of flea beetles may be higher at the field edge than farther into the field. If this appears to be the situation, a foliar spray around the field edge may provide sufficient protection. On hot and calm days, flea beetles are capable of moving longer distances and may populate the field more uniformly.

When assessing economic thresholds also consider growing conditions. When flea beetle feeding is combined with poor plant growth during hot, dry weather, canola can tolerate less feeding than if plants are growing under more ideal growing conditions.

Flea beetles can locate, attack and quickly injure or destroy seedlings shortly after emergence, making them extremely difficult to control. To manage flea beetles, use a combination of cultural and chemical control strategies.

Management

Biological Control: Predators, parasites and diseases can be important in regulating insect populations. To date the effect of biological control agents seems to be limited but several insects have been observed attacking adult flea beetles.

Lacewing larvae (Chrysopa carnea), big-eyed bugs (Geocoris bullatus), the two-lined collops (Collops vittatus), the western damsel bug (Nabis alternatus), and the northern field cricket (Gryllus pennsylvanicus) are a few of the insects known to prey on flea beetles.

The native braconid wasp (Microctonus vittatae) parasitizes flea beetle adults. However, its overall effect on flea beetle populations is unknown.

Unfortunately, flea beetles emerge in large numbers during a relatively short period of time and tend to overwhelm the parasites and predators.

Cultural Control: The larger the seedling, the more it can withstand injury from flea beetle feeding. To obtain large plants early, use good quality seed and plant as shallow as available moisture will allow. This produces seedlings that germinate and emerge quickly and grow vigorously. Seedlings of vigorously growing varieties are able to tolerate flea beetle feeding more than seedlings of less vigorous varieties. At present no canola varieties are resistant to flea beetle damage.

Do not cultivate summerfallow fields with cruciferous weeds and volunteer canola until canola crops are at the four-leaf stage. Leaving a trap strip of volunteer canola near overwintering sites can be an effective control strategy if the trap strip is sprayed before beetles move into the canola crop. Control cruciferous weeds and volunteer canola in cereal fields to starve out early spring populations.

Flea beetles prefer bright relatively warm conditions. Direct seeding provides a microclimate that is less ideal for flea beetles than tillage before seeding. Planting into stubble may reduce injury due to the cooler microenvironment created by stubble shading on surface soil. Cooler temperatures at the soil surface slow flea beetle activity reducing damage. This seeding method produces large plants early and may reduce the grower's dependence on seed treatments, granular insecticides and foliar insecticides, except under conditions of intense flea beetle pressure.

Fall-seeding canola and using varieties and weed control programs that allow for earlier seeding may also help to minimize flea beetle damage to canola. Early seeding will maximize plant size before flea beetle emergence and the larger plants tolerate more injury.

Increasing seeding rates can help reduce the impact of flea beetle attack. For a given population of flea beetles, having more plants per unit area means that feeding damage per plant is reduced and seedlings can recover more readily from flea beetle injury.

At a given seeding rate, wider row spacing of 20 to 30 cm (8 to 12"), rather than 10 cm (4"), can also result in less flea beetle damage per plant. Although the reasons are not yet clear, it appears that flea beetles are more attracted to the reduced visual contrast between vegetation and soil that occurs at narrow row spacing.

Crop rotation is not an effective means of controlling flea beetles. Adults overwinter inside and outside of the cropped areas and are capable of long-range migration.

When flea beetle populations are very high, no cultural controls will effectively reduce their attack.

Insecticides: Canola, mustard and rapeseed crops can be protected from flea beetle attack through insecticide application as a seed treatment, granules applied with the seed or post-emergence foliar sprays. Check provincial crop protection guides for registered insecticides.

In areas where past damage from flea beetles has been light, no treatment may be required. Daily field scouting for flea beetles is important, particularly on hot, calm days. If plants are being lost, determine if the cause is flea beetles or seedling blight. A post-emergent insecticide application may be required to protect seedlings that are exposed to severe or prolonged periods of intense attack. If heavy flea beetle damage nears threshold levels and high numbers of flea beetles are noticed in the field, apply foliar sprays as soon as possible, since flea beetles can cause substantial damage quickly.

It can be difficult to apply insecticides over a large area quickly when feeding pressure is high. Where damage starts at the field margins, only a small portion of the field may require treatment. Apply sprays when it is sunny and warm, and the beetles are active and exposed on plants and soil. Additional foliar sprays may sometimes be needed since flea beetles may continue to move into fields at the susceptible stage after residual from the first foliar spray has become ineffective. An alternative strategy is to use treated seed or in-furrow granules for the first few passes of the seeder around the field perimeter. Scout fields daily to determine if the insects have moved into the untreated areas and are damaging plants.

In areas where damage in the previous year was moderate, use a seed treatment. Coating seeds with an insecticide in combination with one or more fungicides prior to planting is a widespread practice. With the seed treatments currently available, canola seedlings are protected from flea beetles as soon as the plants emerge. Most seed treatments currently available for control of flea beetles in canola come with the insecticide at either a lower or high rate. The high rate is more expensive than the lower rate, but has an extended period of protection from flea beetles relative to the lower rate. If high flea beetle populations occur and the seed treatments are not protecting the seedlings adequately, use a foliar spray.

In areas where crop damage from flea beetles has been high (parkland regions of Manitoba and Saskatchewan), use the combination of both a seed treatment and in-furrow granule treatment to provide economically superior control. The speed at which flea beetle populations can destroy plant stands makes the reliance on post-emergence foliar insecticides risky, particularly where flea beetles are a perennial problem. In-furrow granules are extremely effective against flea beetles when used in conjunction with an insecticide seed treatment, especially on early-seeded crops. Granular insecticides are taken in by the seedling's roots and translocated to the cotyledons and leaves. The movement of the insecticide from the in-furrow granules to the cotyledons may take two or three days, during which time the seedlings may be unprotected if an insecticide seed treatment is not used in conjunction with the granules. Maximum protection begins to occur about five to six days after emergence and is available for up to 28 days after emergence. If hot, dry conditions occur, a foliar insecticide may also be necessary.

Granular insecticides are most effective when placed adjacent to the seed in moist soil on early-planted crops. Granules are effective with all seeding equipment if seed to granule placement is 7.6 cm (3") or less. Granular insecticides when broadcast with the seed and incorporated with harrows are ineffective against flea beetles and a significant hazard to wildlife.

Lygus Bugs (Several Species)

Lygus bugs are small, oval-shaped insects that feed on a variety of crops and weeds. Several species infest canola and alfalfa. In western Canada, Lygus lineolaris (the tarnished plant bug), L. borealis, L. elisus and L. keltoni have been observed destroying canola flower buds and seeds. All four species are thought to be equally destructive. Lygus bugs feed on the sap of new growth and reproductive tissue. Host plants of lygus include alfalfa, canola, lentils, potato, strawberries, vegetable crops, flax, hemp, fababean, tree fruits, and weeds such as redroot pigweed, stinkweed, wild mustard and lamb's-quarters.

Identification and Life Cycle (Figure 31)

Figure 31. Lygus Bug Life Cycle

Adult lygus bugs overwinter under plant litter at the soil surface in shelterbelts, headlands, uncultivated areas and field margins. They emerge soon after the snow melts in spring. Upon emergence, adults feed on winter annuals and the buds of flowering shrubs. Overwintering adults can be abundant in volunteer canola, fall-seeded and early springseeded canola, especially if these crops are in bud or flower and other hosts are not yet available.

Adult lygus bugs (Figure 32) are about 3 mm (0.1") wide and 6 mm (0.2") long. They have relatively long antennae and legs. They vary from pale green to reddish brown to black and from fairly uniform colour to mottled. Lygus bugs have a distinctive triangle or "V" shaped marking in the upper centre of their backs and membranous wingtips. Adults are active and fly readily when approached.

Figure 32. Adult Lygus Bug

Photo by Roy Ellis

After mating and once the eggs mature, females seek suitable host plants, such as budding alfalfa or canola, on which to lay their eggs. Weed hosts include flixweed, lamb'squarters, wild mustard, stinkweed, redroot pigweed, kochia, Russian knapweed, and Russian thistle. Adults feed and lay eggs when buds and flowers are developing in May through early July. Eggs are laid individually into the stems and leaves of host plants. The first nymphs appear by about the end of May. In the south, the new generation adults first appear by about the end of June.

Immature lygus bugs (nymphs) (Figure 33) are light green and wingless. Young nymphs are often mistaken for aphids, which are similar in size and shape. However, lygus bug nymphs are much more active, are harder bodied and lack the cornicles ("tail pipes") of aphids. Nymphs feed on new growth and reproductive parts of the plant. Several black spots, usually five, become noticeable on the backs of nymphs as they moult or mature through five instars (growth stages) before becoming adults. Wing buds are evident in the fourth and fifth instars. In late summer, the new generation adults disperse from mature canola fields into later maturing hosts, such as alfalfa, and continue feeding until they migrate to overwintering sites.

Figure 33. Lygus Bug Nymph

Photo by Roy Ellis

The lygus pest species produce at least one generation per year in canola on the prairies. Only one generation has been observed north of approximately 53° N latitude (e.g., Camrose, AB). However, south of approximately 50° N latitude (e.g., Vulcan, AB), there can be two generations. In alfalfa, a new generation can mature about the time the first cut of alfalfa is made for forage. The new lygus bug generation leaves alfalfa seeking another crop-often canola- on which to lay its eggs. In canola, the hatch of lygus eggs is complete by the end of July.

Damage

Only recently have lygus bugs been considered serious economic pests of canola. In 1996, 4,047 ha (10,000 ac) were sprayed (the first time canola was treated for a lygus bug infestation) in the area of Vulcan, AB. In 1997, lygus damaged canola in the Foothills, Newell, Vulcan, Willow Creek, Bonnyville and Peace River region of Alberta and in the Meadow Lake area of Saskatchewan. Insecticide was applied to about 16,200 ha (400,000 ac) for lygus bug control that year, yet crop damage was estimated to have exceeded $10 million. In 1998, more than 1,400,000 acres were sprayed to control lygus bugs in Alberta.

All species of lygus bugs feed preferentially on either buds, flowers, developing seeds or on new stem and leaf tissue. Lygus bugs use their piercing-sucking mouthparts to puncture plant tissue and suck plant juices causing visible lesions to surfaces of stems, buds, flowers and pods (Figure 34). As they feed, a toxin in their saliva is injected into the plant tissue that results in further physical injury. In canola and alfalfa, lygus bug feeding is most injurious to the flowers and developing seeds. With the trend to earlier seeding and fall seeding of canola, overwintered adults can damage canola in the bud stage.

Figure 34. Puncture Points from Lygus Bug Feeding on Canola Stem

Photo by Phil Thomas

Feeding on buds, flowers and young pods causes "blasting" (buds turn white and fail to develop), flowers fall without forming pods or pods drop without maturing. Feeding punctures on the outside of pods and stems may ooze droplets of sap, causing an infested crop to become noticeably sticky. These droplets promote the entry of pathogens. Lygus feeding creates small, dark circular patches on the pod surface.

Seeds that have been fed upon will collapse or shrink, darken and lose their quality and viability. Additional loss may occur if flowering is delayed by heavy feeding pressure or drought. In western Canada, lygus bugs typically damage up to 7% of the seed.

Adults and the oldest (fourth and fifth instar) nymphs are responsible for most of the feeding injury. In 1997, damage estimates were as high as 40% in the most heavily infested areas.

The damage caused by lygus bugs in canola is related to weather conditions at the time of the infestation. Weather affects the development of both lygus bugs and the crop. If rainfall is abundant (more than 100 mm or 4") from the time of bud formation to the end of flowering and if growing conditions are adequate for the rest of the season, canola can compensate for much of the bud damage. Hot, dry weather promotes insect development and worsens the damage caused by insect feeding.

Monitoring

Begin monitoring canola when it bolts and continue until seeds within the pods are firm. Since adults can move into canola from alfalfa, check lygus bug numbers in canola when nearby alfalfa crops are cut.

Start scouting fields at the bud stage. Sample the crop for lygus bugs on a sunny day when the temperature is above 20°C and the crop canopy is dry. With a standard insect net of 38 cm (15") diameter, take ten 180° sweeps. Count the number of lygus in the net.

Repeat the sampling in another 14 locations. Samples can be taken along or near the field margins. Calculate the cumulative total number of lygus and then consult the sequential sampling chart (Figure 35). If the total number is below the lower threshold line, no treatment is needed. If the total is below the upper threshold line, take more samples.

Figure 35. Sequential Sampling for Lygus Bug at Late Flowering Stage

If the total is on or above the upper threshold line, calculate the average number of lygus per 10-sweep sample and consult the economic threshold table (Table 3).

Table 3. Lygus Bug Economic Threshold

Application Costs		Number of Lygus Bugs at Different Canola Crop Stages *
$/ha	$/ac	Bud	End of Flowering			Pod Ripening
22	8.90	No economic threshold available	14	12	10	20	17	15
24	9.70		16	13	11	22	18	16
26	10.50		17	14	12	24	20	17
28	11.35		18	15	13	25	22	19
30	12.15		19	16	14	27	23	20
32	12.95		21	17	15	29	25	21
Canola Price
$/tonne			220	260	300	220	260	300
$/bu			5.00	5.90	6.80	5.00	5.90	6.80

*Crop Staging: 'End of Flowering' to early pod development in the upper canopy is stage 69; 'Pod Ripening' is stage 89.

Economic Threshold

The economic threshold for lygus bugs in canola covers the end of the flowering and the early pod ripening stages. Once the seeds have ripened to yellow or brown, the cost of controlling lygus bugs may exceed the damage they will cause prior to harvest, so insecticide application is not warranted.

Consider the estimated cost of spraying and expected return prior to making a decision to treat a crop. For example, if an application will cost $26/ha ($10.50/ac) and the expected return is $260/tonne ($5.90/bu), the threshold level is an average of 14 bugs per 10-sweep sample. An economic threshold for lygus bugs in canola at the bud stage has not been established.

If soil moisture levels and rainfall are high at flowering, plants likely will be able to compensate for damage caused by lygus bug populations well above economic thresholds and control may not be necessary. Since plants under moisture stress during this time usually will be unable to compensate for most of the feeding injury, spray using the economic thresholds above.

Management

Biological Control: Lygus bugs have several natural control agents. A tiny fairy wasp, in the family Mymaridae, parasitizes the eggs of the lygus bug. In western Canada, a parasitic wasp, Peristenus pallipes, attacks lygus nymphs in alfalfa but is less effective in canola. Nabid plant bugs, bigeyed bugs and spiders occasionally prey on young lygus bug nymphs.

A European wasp, Peristenus digoneutis, has been introduced into alfalfa fields in eastern North America where it parasitizes about 40% of the tarnished plant bugs. One of the few parasitoids of lygus adults is a tachinid fly, Alophorella sp.

Insecticides: Check provincial crop protection guides for registered insecticides. Insecticide application is the only option for control of lygus bugs once populations have reached economic threshold levels. Usually, one application of a registered insecticide at the end of flowering (bloom 90% complete or more) (GS 69) or at pod formation (GS 71) will prevent most yield losses. Insecticide applications applied at flowering (GS 60 to 68) often do not control later emerging nymphs. Delaying application may be prudent if nearby alfalfa crops are being cut. However, this may allow extensive feeding by early emerging nymphs. Use enough water to ensure the insecticide penetrates the canopy and provides adequate coverage.

To reduce direct exposure to pollinators, apply insecticides very late in the evening or early morning when bees are not foraging and the crop has completed at least 90% bloom. When treating for lygus, prevent spay drift from moving onto beehives, blooming weeds or surrounding fields. Notify beekeepers of your intention to treat the field 48 hours in advance of spraying. In general, use an insecticide that has a short residual activity to reduce the impact on pollinators.

Painted Lady, Thistle Butterfly (Vanessa Cardui (L.))

Larvae feed primarily on Canada thistle leaves, leaving the stem and midrib. Feeding temporarily inhibits the weeds but new growth usually develops in the fall. They also feed on about 60 other hosts, including sunflowers and canola.

Identification and Life Cycle

Adults migrate into the prairies from overwintering sites in Mexico, arriving in early June. There is no evidence that they can survive our cold winters. This butterfly normally prefers to lay eggs on Canada thistle plants but under some conditions will lay eggs on other plants. Larvae feed on the leaves producing loose webbing and, if numerous, can completely defoliate a plant. The larvae are up to 30 mm (1.2") long and dark purple to black in colour (Figure 36). They have long spines on each segment of the abdomen.

Figure 36. Thistle Caterpillar

Photo by Roy Ellis

Management

Because this occasional pest is usually limited to small, scattered patches within a field, insecticide treatments are rarely justified.

The major concern with this normally beneficial insect arises when fecal pellets webbed on thistles are harvested with the canola seed. Grade standards have a low tolerance to insect excreta.

Red Turnip Beatle (Entomoscelis Americana Brown)

Red turnip beetles are native to North America and can occasionally become pests of canola and mustard across western Canada.

Life Cycle and Appearance (Figure 37)

Figure 37. Red Turnip Beetle Life Cycle

The red turnip beetle overwinters in the soil as reddish brown oval eggs that hatch in early May. The grubs or larvae feed on the foliage of cruciferous plants such as flixweed, shepherd's purse and volunteer canola. Mature grubs are black, about 1 cm (0.4") long, with a roughskinned, segmented body. After feeding they enter the soil to form bright orange pupae, which transform into the adult beetles (Figure 38). The adult beetles appear from early June until early July and are 7 mm (0.3") long with bright red and black patches on their heads and three distinct black stripes running down their backs. After feeding into mid July, the adults burrow into the soil, rest for the summer and then leave the soil in late July or early August to mate and lay eggs. The beetles are often found in groups scattered throughout canola fields, mating near the tops of maturing plants. After mating, the adults do not migrate to the fringes of the field, but lay eggs randomly throughout the field. There is only one generation per year.

Figure 38. Red Turnip Beetle

Photo by Phil Thomas

Damage

The larvae and adult beetles both feed on canola, but the adults are more damaging to the seedling crop. The beetles do not fly in spring. Damage occurs when beetles move into a canola field from a neighbouring field sown to canola the previous year. Unless canola is sown on canola, the beetles remaining in the previous year's canola field will feed on volunteer canola and cruciferous weeds until the food supply is exhausted or the field is cultivated, forcing migration in search of food. The beetles can move considerable distances to reach a canola or mustard crop. They feed only on plants of the mustard family. They may move through a cereal crop, feeding on the cruciferous weeds and volunteer canola as they go. The beetles move slowly, completely devouring canola plants as they move toward the centre from the field's edge, making the damage obvious from a distance. Damage from the red turnip beetle has been sporadic and usually local and minor in northern portions of the prairies.

Monitoring

Scout canola fields daily for the first 14 days after emergence.

Economic Threshold

No economic threshold has been developed for red turnip beetle.

Management

Cultural: Cultivate fields with red turnip beetles in late fall to early spring to bury eggs and reduce larval survival. The beetles have caused severe damage in canola fields situated next to fields where canola was under seeded the previous year to fescue and, therefore, not cultivated. Early spring cultivation removes cruciferous weeds and volunteer canola, destroying the food supply for the larvae.

Insecticides: Check provincial crop protection guides for registered insecticides. Spraying with a recommended insecticide after the insects enter the canola crop can control the adult or larval stages. Since they move en masse, one or two passes with the sprayer along the field margin, over and in front of the invading insects, will provide total control.

Root Maggots (Delia Species)

This insect arrived in North America from Europe in the 19th century and is now common throughout the prairies. Root damage to canola crops in Alberta and Manitoba is caused mainly by the cabbage root maggot, Delia radicum, except in northeastern Alberta where the turnip maggot, D. floralis, is the more numerous species. These insects can be a serious pest of cruciferous crops such as canola, mustard, cabbage, rutabaga, radishes, cauliflower and broccoli.

Identification and Life Cycle (Figure 39)

Figure 39. Cabbage Root Maggot Life Cycle

The cabbage root maggot is capable of completing two generations per year under Manitoba conditions. In Alberta, usually only one generation occurs per year in canola, but two generations occur on cole crops. The adult cabbage root maggot is a fly and looks much like a common house fly, but is smaller (5 mm or 2"), ash grey in colour with a dark stripe along the back of the abdomen, and is covered with many black bristles (Figure 40). It emerges in the spring from overwintered puparia (cocoons) about 2 to 20 cm (0.8 to 8") long that are beneath the soil surface. A second generation may emerge later in the summer as the first generation completes its life cycle. Therefore, adult cabbage root maggots may be found from May to October.

Figure 40. Cabbage Root Maggot Adult

Photo by Roy Ellis

Female flies leave the fields from which they emerge and locate new crops by odour. They begin laying eggs within one week of emerging and continue egg laying for their five to six week life span. Each female will lay 50 to 200 eggs either singly or in small masses. The eggs are small (1 mm or 0.04" long), white and elongated (Figure 39). Eggs are deposited at or near the base of host plants usually just beneath the soil surface and will hatch in three to 10 days. Occasionally eggs are also laid on the lower stems and leaves of canola plants.

The larval stage is a small (6 to 10 mm or 0.2 to 0.4" long), white and legless maggot, similar in appearance to most fly maggots (Figure 41). When the eggs hatch, the maggots move down the root and begin feeding on small roots and root hairs. Eventually, they tunnel into the plant's taproot. Maggot feeding will last for three to four weeks as the maggots mature (6 to 10 mm or 0.2 to 0.4" long), after which they will pupate-either in the root itself or in the soil.

Figure 41. Cabbage Root Maggot

Photo by Roy Ellis

The puparia are elongated and reddish brown, resembling small wheat kernels. Pupariation occurs in the top 5 to 20 cm (2 to 8") of soil. Pupariation lasts about two weeks (unless interrupted by winter), after which the adult fly emerges and the cycle repeats itself (where there are two generations per year). The puparia usually remain dormant over the winter and adult flies emerge the following spring.

The life cycle of the turnip maggot in Alberta is similar, but emergence of adults from overwintering puparia does not begin until about two weeks later than the cabbage maggot. The turnip maggot has only one generation throughout its Alberta range.

Damage

The level of infestation and subsequent yield losses are dependent on environmental conditions and will vary from year to year. Maggots prefer cool, moist environments. Therefore, crops grown under cool, moist conditions are most susceptible. Maggots will tunnel into the root and create channels along the outside (Figure 42). Canola plants are not usually affected by slight root feeding and in some studies have even shown a yield increase. However, more severe tunnelling can cause decreased plant vigour, stunting, yellowing, poor seed set and even death. Infested plants will appear pale green and stunted and may wilt on hot, dry days. Feeding by three or more larvae may girdle the root resulting in breakage at the soil surface. Additionally, fungi often invade maggot-feeding tunnels, especially Fusarium (foot rot) from surrounding soil. Often, infested roots are darker in colour than healthy, uninfested roots. Under wet soil conditions the plants may wilt and finally break off just below ground level from the combined effects of maggot feeding and root rot. Heavy maggot infestations in canola and mustard can halt blooming and cause severe lodging and yield losses.

Figure 42. Cabbage Root Maggot Damage

Photo by Phil Thomas

Historically, infestations have been most severe in north central and northwestern Alberta where 95 to 100% of plants in a field are frequently infested to some degree by root maggots. B. rapa varieties are more susceptible than B. napus. Research at the University of Alberta has shown that yield losses can be as high as 50% for B. rapa varieties and 20% for B. napus.

Adult root maggots are known to move distances of at least a few kilometres in search of host plants. The adults are most abundant in canola during June and early July.

Monitoring

In the spring, use sticky traps or sweep nets to look for the adult fly. Adult flies can be trapped in yellow bowls of water set around the field margins at the rosette stage. Their identification requires expertise because many similar species of flies are attracted to canola fields when the crop is in flower.

When searching for maggot infestations at flowering, carefully pull several plants with the taproot intact. Brush off the soil and inspect the root for scars caused by maggot feeding. Check several plants in at least five or more locations. If the maggot is present, carefully brush off the soil to find it. Cut into the root to assess for maggot damage.

Management

Biological: There are several natural biological control agents present on the prairies. The most important predators of the immature stages of root maggots are larvae and adults of ground beetles. The most abundant species of ground beetles in canola fields in northern Alberta are Bembidion species, which feed on root maggot eggs. Rove beetles are predators of root maggot eggs and are also parasitic on root maggot puparia, their most abundant prey during July. Adult files are infected by two species of parasitic fungi, Entomophthora and Strongwellsea. Entomophthora infection causes quick death of a fly. The Strongwellsea fungi may achieve high rate of infection and while it does not kill flies the females are unable to mature eggs.

Cultural: Cultural practices can have a considerable impact on cabbage maggot populations. There is evidence that a moderate increase in canola seeding rates will reduce damage and yield loss experienced from maggot feeding. Heavier canola plant densities result in smaller basal stems that are less attractive to egg-laying females. Delayed seeding of B. rapa varieties until late in May, rather than in early or mid-May, may avoid the maggot life cycle but is not recommended as yield loss from late seeding outweighs maggot control.

Tillage can reduce the level of emergence of adult flies. Research by the Alberta Research Council in Vegreville, AB showed that the greatest reductions in root fly emergence (55 to 70%) occurred in plots tilled only in the fall or in plots tilled in both the fall and the spring. Tillage can move pupae nearer to the soil surface where they are more susceptible to attack by natural enemies. Fall tillage can expose the pupae to more severe environmental conditions over the winter. However, use caution when tilling light-textured soils susceptible to erosion. Tillage prior to seeding will help warm and dry soil (maggots thrive under cool moist conditions). Direct seeding results in higher root maggot populations but some research indicates the increased yield with direct seeding outweighs root maggot damage.

B. napus varieties are less susceptible than B. rapa. In areas where the growing season is sufficiently long, consider growing B. napus varieties. Another pest management option is to control cruciferous weed species such as shepherd's purse, stinkweed, wild mustard and flixweed that can act as host plants, and result in increased overwintering populations of the maggot.

Insecticides: Controlling the maggot in canola is difficult at best. In-furrow application of granular insecticides with the seed may provide some first generation maggot control. However, currently no insecticides are available for control of maggot infestations in canola or mustard later in the season. Research by the Alberta Research Council at Vegreville found that elemental sulphur granules either drilled in with the seed or top-dressed in the spring at greater than 11 kg/ha (10 lb/ac) resulted in significantly less root damage from root maggots.

In high value cruciferous vegetable crops such as cabbage and broccoli, root damage and crop losses can be reduced by applying a soil insecticide drench at the time of transplanting and at two-week intervals until the end of June, and again from late July to mid-August. However, this is not economical in canola crops.

Insecticides for Control of Canola Insects

Here are some general guidelines for insecticide applications:

• Check provincial crop protection guides for registered insecticides.
• Examine the fields weekly or daily in the seedling stage, and check for signs of feeding damage on various parts of the plant.
• Consult with the local crop specialist to ensure pests have been identified correctly. This also alerts the crop specialist to the possibility that a problem may be forthcoming.
• Examine the entire field, make counts and determine whether the entire field requires treatment. Frequently, insect numbers are high at the periphery of the field and decrease rapidly across the field.
• Canola crops can withstand some insect damage and usually compensate if the damage occurs early in the growing season. Don't panic and spray because a neighbour is spraying. Assess the situation, get outside advice if necessary and make a decision.
• If using a field sprayer for insecticides, make sure that it is thoroughly cleaned of any herbicides.

Safety

Insecticides must be used properly to protect people and the environment. Always follow label directions and wear the appropriate safety equipment to protect yourself, family members and the environment.

Beneficial Insects

When scouting fields with sweep nets it is common to find beneficial insects such as:

• lady beetle adult and larva (Figure 43) and larva (Figure 44)
• hover fly (Figure 45)
• lacewing (Figure 46 and 47)
• parasitic wasp (Figure 48)
• honey bee (Figure 49)

Figure 43. Ladybird Beetle

Photo by Roy Ellis

Figure 44. Ladybird Beetle Larva

Photo by Roy Ellis

Figure 45. Hover Fly

Photo by Roy Ellis

Figure 46. Lacewing Adult

Photo by Roy Ellis

Figure 47. Lacewing Larva

Photo by Roy Ellis

Figure 48. Parasitic Wasp

Photo by Roy Ellis

Figure 49. Honey Bee

Photo by Roy Ellis

These insects serve a positive role in the production of canola. A healthy population of these insects is good for canola health and yield.

Beekeeping is a major industry in the canola growing areas of Canada. Caution must be taken to avoid killing bees or beneficial insects with insecticides. Although it may be difficult to protect native pollinating insects, honeybee kill can be minimized through co-operation with beekeepers. Keep the following information in mind when spraying canola fields to ensure continued good working relationships with commercial beekeepers:

• Carefully sample sites in a field to be absolutely sure that insect population levels are high enough to require control measures.
• Discuss spray plans with beekeepers prior to spraying. Since the hives may have to be moved, give beekeepers at least two days notice prior to spraying.
• If hives cannot be adequately protected (moved or covered) before insecticide spraying begins, alert the spray applicator to the exact location of the beehives within the intended spray area, so beekeepers can avoid direct spraying or spray drift contamination of the hives.
• Give careful attention to wind direction and velocity in relation to bee yard locations.
• Do not spray a crop in flower unless absolutely necessary.
• If spraying a crop in flower is necessary, do the spraying when there will be minimal bee activity in the fields, preferably during the evening hours. During most summer evenings, honeybees leave fields by 8:00 p.m. and do not return until 8:00 a.m. or later. Warm temperatures, however, can "hold" bees in flowering fields for periods longer than normal.
• Whenever possible, choose insecticides with low hazard potential to bees. Consult with the local agricultural representative or district agriculturist.
• Do not spray crops of uneven maturity which are partly in flower or which contain weeds when bees are active.