Bertha Armyworm I

Green phase of larvae	Brown phase of larvae
Black phase of larvae	Trap for collecting adult moths

Heavy infestations may reduce yields because of defoliation and seed pod consumption. Crop losses due to pod feeding will be most severe if there are few leaves remaining on the crop.
 

Host Crops

Bertha armyworms prefer canola but also feed on rapeseed, mustard, alfalfa, flax, field peas and potatoes. They are most damaging to canola and flax.
 

Biology

Bertha armyworms overwinter as pupae in the soil. The moths emerge from mid-June to mid-July. After mating, the female moths lay clusters of pinhead-sized eggs on the undersides of canola leaves when the plants are in bloom. Larvae usually emerge about seven days later.

As the larvae mature, they usually darken in colour. Newly-emerged larvae are small, 3 mm (⅛ inch) long, pale green with a yellowish stripe along each side. The green larvae usually become pale brown caterpillars. Most older larvae become large, velvety black caterpillars but others remain green or pale. Regardless of colour, the mature larvae are 4 to 5 cm (1.5 to 2 inches) long, with a light brown head and a broad, pale-orange or yellowish stripe along each side. The head is light brown in colour with a yellowish-orange stripe along each side.

During hot, dry weather, the young larvae may be found feeding on the lower leaves or hiding under debris at the soil surface. The older larvae tend to feed both during the day and night. Larvae migrate from an infested field only when the food supply is short or when the crop is over-ripe.

After feeding is complete (usually by late August), the mature larvae burrow five to 15 cm (2 to 6 inches) into the soil where they pupate and overwinter.

May	June	July	August	September
Pupae	Moths emerge from pupae	Egg laying; larval feeding starts	Larval feeding continues; pupation starts	Pupae begin to overwinter

 

Scouting Techniques

Regional counts of adult moths in pheromone-baited traps indicates the importance of later monitoring for larvae. Generally, when the accumulated trap numbers exceed 900, there is a moderate chance that larvae may reach economic levels, and when cumulative counts exceed 1,200 there is a high risk.

From a distance, infested canola fields look pale-white because the larvae eat the outer green layer of the stems and pods exposing white tissue. Checking for larvae can be done from mid-June through late July. Because their distribution within a field may be spotty, several spots should be sampled.

Shake the canola plants to dislodge the larvae and then count all the larvae on the ground over a one quarter of a square metre (50cm X 50cm) area. It is important to check under leaf litter, lumps of soil, and debris when counting the larvae. Multiply the number counted by 4 to get the number per square metre, and use the average number of larvae per square metre within each field to determine if the economic threshold has been exceeded.

In other crops, examine any cruciferous weeds present. Larvae may not attack the crop until all of the weeds are consumed.
 

Economic Thresholds

Control is usually recommended when larval population densities exceed 20-30 larvae per square metre in canola. The threshold level ranges from ten to 35 per square metre, depending on the value of the crop and the cost of control. For detailed information on economic injury levels for bertha armyworm in canola, consult the Guide to Crop Protection. Economic thresholds for this pest in flax have not been determined.
 

Control Tips

Fall tillage and low winter snowfall levels may reduce the number of pupae surviving to spring. Seeding early and choosing early maturing varieties may help minimize damage.

Application of insecticide, if required, should be made in the evening when the larvae are actively feeding on the upper portions of the plant.

For further information, contact your GO representative.