Agricultural Pests
The following information was compiled from the Government of Alberta's Ropin' The Web site.
Bertha Armyworm
Host: Canola, rapeseed, mustard, alfalfa, 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.
Appearance and Life History: Bertha armyworms develop through four distinct stages: adult, egg, larva and pupa. In Canada, there is one complete generation per year.
Adults
The adult stage is a moth. Moths begin emerging from the over-wintering pupae in early to mid-June and continue until early August.
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 host plants. Each female moth will lay about 2,150 eggs but numbers as high as 3,500 eggs per female have been recorded.
The moth has a wing span of about 4 cm (1.5 in.) and is active only at night. The forewing is predominantly gray, and flecked with patches of black, brown, olive and white scales. Near the middle of the forewing, towards 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 olive-colored, irregular transverse marking that is characteristic of the species.
Adult Moth
Eggs
Bertha armyworm eggs are laid in single-layered clusters of about 50 to 500 eggs on the lower surface of the host plant leaves. 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.
Eggs are white when first laid
Larvae
Newly hatched bertha armyworm larvae are about 0.3 cm (1/10 in.) long. They are pale green with a pale yellowish stripe along each side. Because of their size and color, they are difficult to see on the underside of leaves.
When disturbed, small larvae may drop off the leaves by a fine silken thread. This behavior makes it difficult to distinguish small bertha armyworm larvae from those of the diamondback moth, which display a similar behavior Large larvae may drop off the plants and curl up when disturbed, a defensive behavior typical of cutworms and armyworms.
Larvae take approximately six weeks to complete their development, depending upon temperature. During this period, they moult five times and pass through six growth stages. As they mature, their color becomes variable. Some remain green, but many become brown or velvety black.
At maturity, the larvae are about 4 cm (1.5 in.) long, with a light brown head and a broad, yellowish-orange 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.
The larvae stage is the only stage to cause crop damage
Newly hatched larvae
Larvae colours become variable as they mature
Pupae
Bertha armyworms survive the winter as pupae in the ground at depths of 5 to 16 cm (2 to 6 in.). The pupa is a pod-like structure that protects the bertha armyworm while it transforms from the larval stage to the adult moth.
Pupation usually begins in mid to late August. All larvae will have pupated by early to mid September. If the 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.
Pupae are reddish brown in color, about 0.5 to 1.8 cm (0.2 to 0.7 in.) in size and tapered with flexible, terminal abdominal segments. Bertha armyworm pupae are indistinguishable from other cutworm pupae.
Bertha Armyworm Pupae
Damage: The degree of crop damage varies with the crop, the plant's growth stage, the growth stage of the larvae and the number of larvae present. Significant crop damage usually occurs within a three-week period between late July and late August, depending on the season and crop location.
Small larvae feed on the underside of the leaves, chewing irregularly-shaped holes in the leaves. They usually cause little damage at this stage, even when population levels are high. Crop damage occurs rapidly once the larvae moult to the second-last stage. These larvae are about 1.3 cm (½ in.) in length. Larvae in the last two larval stages eat about 80 to 90 percent of the plant material consumed during the life of the larvae.
If the plants, especially canola, drop their leaves before the larvae are mature, the developing larvae will feed directly on the seed pods. Seed pods may be "debarked," but more commonly, the larvae chew holes in the pods and eat the seeds. At high numbers, the entire seed pod may be consumed. Even if the pods are only stripped of their outer green layer and not eaten entirely, crop losses may still occur because of premature shattering.
In flax, the larvae eat the flowers and developing bolls. Once the flax bolls are full-size and start to ripen, larvae usually feed on the calyx below the boll. Occasionally, larvae will feed on the green stems of ripening bolls, causing them to drop off.
Diamondback Moth
Host: Diamondback moth larvae feed on all plants in the mustard family (canola, mustard), cole crops (broccoli, cabbage) and on several green house plants. In Western Canada, canola and mustard are its primary targets.
Appearance and Life History: Diamondback moth has four life states: egg, larva, pupa and adult. Crop damage is caused by the larval stage. 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 and generations usually overlap. Therefore after the first generation all life stages may be present in the field at the same time. This can cause problems with optimizing control that should be targeted at the destructive larval stage.
Adults
The adult moth is approximately 8 to 9 mm (1/3 in.) long with a wing span of 12-15 mm (1/2 in.). 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. 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 are laid the first night following emergence from the pupa although egg-laying can continue for about 10 days. In the field, moths will flutter up out of the canopy as the crop is disturbed.
Eggs
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.
Larvae
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 (figure 2). The posterior end of the caterpillar is forked. Larvae moult three times during the larval stage which lasts about 10 to 21 days, depending upon temperature and the availability of food. At maturity the larvae are spindle-shaped (broader at the middle and tapering at both ends) and about 12 mm (1/2 in.) long. The diamondback moth larva is easily identified by its peculiar reaction to being disturbed. It will wriggle backwards 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.
Pupae
Larvae pupate in delicate, white, open-mesh cocoons attached to the leaves, stems or seed pods of the host plant. Initially, the pupae are light green but as they mature, they become brown as the adult moth becomes visible through the cocoon (figure 3). The pupal stage lasts from five to 15 days, depending on environmental conditions.
Damage: An infestation of diamondback moths 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 depend on two factors – over-wintering populations to the south and strong south winds that transport the moths north into Manitoba, Saskatchewan and Alberta in the spring.
In years when conditions are right for the moths - that is, when moths arrive on the wind in large numbers in early May and summer temperatures are hot - diamondback moth can go through multiple generations and infestations can cause millions of dollars of damage. In 2001 the moths arrived early, in high numbers and resulted in severe infestations crops across the Prairie Provinces. The early arrival and favourable environmental conditions resulted in four generations in some areas.
Diamondback moth larvae feed on leaves, buds, flowers, seed pods, the green outer layer of the stems, and occasionally, the developing seeds within the older seed pods of canola and mustard. The amount of damage varies greatly, depending on plant growth stage, larval densities and size. For example, the first generation larvae of the 2001 infestations were found feeding on early growth stages of canola in late May and resulted in stunted plant growth.
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 seed pods. Feeding damage during the early flowering stage can be considerable. 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 seed pods. The seeds within a damaged pod will not fill completely and pods may shatter. Larvae may also chew into seed pods and eat the developing seeds.
Crop damage is usually first evident on plants growing on ridges and knolls in the field. Damage can only be prevented by early field monitoring and the application of insecticides, if larval numbers exceed the action threshold.