Bertha Armyworm

Bertha armyworm (Mamestra configurata) is one of the most significant insect pests of canola in Canada. The pest occurs throughout Manitoba, Saskatchewan, Alberta and the interior of British Columbia.

Severe infestations can occur throughout most of this area, but are usually (but not always) limited to the Parkland area of the Prairies and the Peace River region of British Columbia and Alberta.

Background

Bertha armyworm is native to North America and belongs to a group of insects referred to as “climbing cutworms.” Also included in this group are the true armyworm and variegated cutworm.

In most years, populations are kept low by unfavorable weather conditions such as cold winters and cool, wet weather as well as by parasites, predators and diseases. However, when these natural regulators fail, populations can increase dramatically, creating the potential for widespread damage to a variety of broad-leaved crops.

In extreme situations, infestations of more than 1,000 larvae per square metre have been reported while densities of 50 to 200 larvae per square metre may be common.

Infestations may be localized or spread over millions of acres. 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 the chemicals before severe damage is done. Also, there may be temporary insecticide shortages if suppliers are not aware of the potential outbreak.

Monitoring programs are now in place to help forecast outbreaks and to act as an early warning signal for farmers, crop scouts and industry.

Life Cycle

Bertha armyworms develop through four distinct stages: adult, egg, larva and pupa. In Canada, there is one complete generation per year.
Pupae
Bertha armyworms survive the winter as pupae in the ground at depths of 5 to 16 cm. 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, about 0.5 to 1.8 cm in size and tapered with flexible, terminal abdominal segments. Bertha armyworm pupae are indistinguishable from other cutworm pupae.

Adults
Adult moths begin emerging from the overwintering pupae in early to mid-June and continue until early August. Moths appear to be strongly attracted to canola fields that are in bloom and secreting nectar.

The moth has a wing span of about 4 cm and is active only at night. The forewing is predominantly grey 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-coloured, irregular transverse marking that is characteristic of the species.

Adult moths mate within five days of emergence, and the females 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.

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.

Larvae
Newly hatched bertha armyworm larvae are about 0.3 cm long. They are pale green with a pale yellowish stripe along each side. Because of their size and colour, 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 behaviour makes it difficult to distinguish small bertha armyworm larvae from those of the diamondback moth, which display a similar behaviour. Large larvae usually drop off the plants and curl up when disturbed, a defensive behaviour typical of cutworms and armyworms.

Larvae take approximately six weeks to complete their development, depending on temperature. During this period, they moult five times and pass through six growth stages. As they mature, their colour becomes variable. Some remain green, but many become brown or velvety black.

At maturity, the larvae are about 4 cm 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.

When mature in late summer or early fall, larvae burrow into the ground and form pupae.

Host Plants and Damage

Larvae are the only development stage of the bertha armyworm to cause crop damage. They feed on a variety of crops and weeds. Canola, rapeseed, mustard, alfalfa, lamb’s quarters and related plants are preferred host plants. Bertha armyworm will also feed on a range of secondary hosts including flax, peas and potato.

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. 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.

Monitoring

The number of bertha armyworm larvae in a crop is not a reliable indicator of what to expect the following year. Bertha armyworm populations fluctuate widely from year to year. To assist producers with making crop protection decisions, a monitoring program has been implemented for the Canadian Prairies.

Alberta Insect Pest Monitoring Program
The emergence of adult moths from overwintering pupae is monitored using accumulated degree-days. After emergence, the flight of bertha armyworm moths is monitored using a network of pheromone-baited traps, which attract the male moths.

This network of approximately 200 pheromone traps is organized by Alberta Agriculture and Rural Development, and individual traps are managed throughout Alberta by a wide range of co-operators. The monitoring program has been conducted in Alberta since 1995.

The number of moths collected by these traps gives an indication of the risk of bertha armyworm larval infestations within a region, although not specifically for the field the trap is in (Table 1). Generally, higher numbers of moths during the flight period (around mid-June through July) indicate a greater risk of larval damage (in July and August).

Table 2. Economic threshold for Bertha armyworm larvae
	Expected seed value ($/bushel)
Spraying cost ($/acre)	6	7	8	9	10	11	12	13	14	15	16
Number of larvae per square metre
7	20	17	15	13	12	11	10	9	9	8	8
8	23	20	17	15	14	13	11	11	10	9	9
9	26	22	19	17	16	14	13	12	11	10	10
10	29	25	22	19	17	16	14	13	12	11	11
11	32	27	24	21	19	17	16	15	14	13	12
12	34	30	26	23	21	19	17	16	15	14	13
13	37	32	28	25	22	20	19	17	16	15	14
14	40	35	31	27	24	22	20	19	17	16	15
15	43	37	32	29	26	23	22	20	19	17	16

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

• The larvae should be at least 1.3 cm long.
• Apply the insecticide early in the morning or late in the evening when the larvae are actively feeding. Do not apply during warm afternoons.
• Use enough water to ensure adequate coverage.
• Use high water volumes in crops with dense canopies, such as canola.
• Use the higher label rates of application when a range is indicated.

• Use risk assessment maps as a regional early warning sign to scout fields for larvae. These maps are available on the Alberta Insect Pest Monitoring Network website at: http://agriculture.alberta.ca/bugs-pest.
• Monitor larval numbers in the early pod stage (stages 5.1-5.2) in each field as adjacent fields may have very different larval densities laid.
• Twenty larvae per square metre in canola can reduce yields by 1.16 bushels per acre. Use economic threshold levels to determine if spraying is warranted.
• Under drought conditions, dividing the economic thresholds by 1.48 may give more appropriate economic thresholds under drought conditions.
• Allow beneficial insects to prey on bertha armyworm by only spraying when economic thresholds are reached.
• Observe pre-harvest intervals.

• Manitoba Agriculture
• Saskatchewan Agriculture and Food
• Alberta Agriculture, Food and Rural Development
• British Columbia Ministry of Agriculture Fisheries and Food
• Agriculture and Agri-Food Canada