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Bug of the Month – Bertha Armyworm

Bertha Armyworm

Some bugs are cute, some are helpful, and some are creepy little pests. The Bertha Armyworm fits in the last category. This worm may be tiny, but the damage they can do to a crop can be gigantic.

This little worm lives throughout Manitoba, Saskatchewan, the interior of British Columbia, and our very own Alberta. It belongs to a group of insects referred to as climbing cutworms. These worms develop through four distinct stages: adult, egg, larva and pupa. The adult stage is a moth, but not the biggest concern you should have with this critter.

Bertha armyworm pupa Mamestra configurata

Larvae are the only stage of the bertha armyworm that causes crop damage. They feed on a variety of crops and weeds such as canola, rapeseed, mustard, alfalfa and related plants that are the preferred host plants. These bugs also enjoy feeding on secondary hosts including flax, peas and potato. The degree of damage changes with the crop, the plant’s growth stage, the growth stage of the larvae and how many larvae are present. Significant crop damage commonly occurs within a three-week period between late July and late August, depending on the crop location and the season.

The small larvae feed on the underside of the leaves, leaving irregularly shaped holes in the leaves. They usually cause little damage at this stage, even when populations are high. Crop damage occurs more rapidly once the larvae molt to the second-last stage. Larvae in the last two larval stages are the problem since they eat about 80% to 90% of the plant material consumed during its larvae life. These little guys measure about 1.3 cm (1/2 inch) in length and are pale green with a pale yellowish stripe along both of its sides. Because they are this size and colour it can be difficult to spot these guys on the underside of leaves. It takes approximately six weeks for them to complete their development, depending on the temperature. During this time, they will molt five times and pass through six growth stages, and as they mature their colour will change. Some remain green, but many turn brown or velvety black. When they reach maturity, they will be 4 cm (1.5-inch) long with a light brown head and a broad yellowish-orange strip along their sides. Larvae that are part of the velvety black variety have three narrow, broken white lines on their back.

The presence of bertha armyworm larvae can fluctuate widely year to year meaning one year you may have a high population then the next year it could be low. When you have a high population of these pests there are ways to control both naturally and with insecticide. Crop and field management may reduce crop loss from bertha armyworms through planting alternative crops, early swathing and fall cultivation. The biological control can be parasitic insects such as the tachinid fly, predators such as birds, and pathogens known to infect and kill such as the nuclear polyhedrosis virus.

Chemical control is the producer’s last line of defence against this pest. A single, well-timed application of any registered insecticide applied with aerial or high clearance ground equipment is usually effective. Apply the insecticide in the morning or late evening when the larvae are actively feeding. Do not apply during warm afternoons, use enough water to ensure adequate coverage and use high water volumes in crops with dense canopies such as canola.

That is about all you need to know about these little pests, but for more information, check out this government page for more information.