Bertha Armyworm

Bertha Armyworm, (Mamestra configurata) The adult moth is gray with a black-green background color. The green fades after it dies. The orbicular wing spot is gray or gray-brown with a black border and the reniform spot is white.

Mature caterpillars are about 30 mm long, dark on top, and have a yellow-orange stripe on the side and mottled gray or green on the bottom. Young larvae may be green, brown, or black with a light brown or tan head capsule. There is no white “Y” on the front of the head. Adults are brown “miller” moths. Larvae feed on leaves and defoliate plants. The larvae are often found feeding on crops and weeds in late summer or early fall during the daytime. Host plants: rape, legumes, sugarbeets, hops, cabbage, corn, peas, beans.

The adult moth is about 1 1/2 inch and mainly gray-black with a silvery/whitish kidney shaped spot and silvery/whitish fringe on each forewing. Moths emerge from the overwintering pupae in early-mid June and emergence continues through early August. These night fliers are particularly attracted to blooming canola fields for their nectar and egg laying. Eggs are laid on the lower side of leaves in cluster of 50-500 eggs in a typical honeycomb pattern, and hatch in about one week.

The emerging larvae 1/10th of a inch are usually green in color and hide underneath leaf litter and clumps of soil during the day. Making them difficult to find! Mature larvae are about 1 1/2 inch long and vary in color from brown or velvety black. Larvae develop for six weeks and then drop to the ground in mid-late August to pupate. If the autumn is unusually warm, some adult moths may emerge from the puparium only to perish when winter arrives.

Life Cycle: Bertha armyworm has one generation per year and overwinters as pupae in the soil. Adults emerge in mid to late June and emergence continues through early August. Moths are strong fliers and are active at night. Adults usually mate within the first five days of emergence and are particularly attracted to blooming canola fields for nectar and egg-laying sites. Eggs are laid as clusters on the lower surface of canola leaves. Each cluster contains 50 to 500 eggs laid in a honeycomb pattern. Eggs hatch after four to seven days, depending on daily temperatures.

Larvae feed at night and often hide underneath leaf litter and clumps of soil during the day, which makes them difficult to see. Young larvae feed on the undersides of leaves, chewing irregular-shaped holes. The wind can disperse larvae by ballooning them on threads of silk to other plants. Larvae of the diamondback moth also exhibit a similar ballooning behavior with silk threads. When disturbed, mature larvae curl into a ball, a defensive behavior of cutworms and armyworms. Larvae develop for six weeks and pass through six larval instars before dropping to the ground in mid to late August to pupate.

Pupation usually begins in mid to late August and continues through early September. Pupae overwinter in the ground at depths of 2 to 6 inches (5 to 16 cm).

Damage: Economic damage is the result of significant larval feeding on foliage and developing seedpods of canola. First instar larvae feed on the underside of canola leaves, chewing irregular-shaped holes. They usually cause little damage, even when the population density is high. However, more mature larvae, greater than ½ inch (1.25 cm) long, can cause substantial crop damage. Mature larvae eat approximately 85 percent of the plant materials consumed during their larval development.

As canola drops its leaves, mature larvae begin to feed directly on the pods, which results in economically important yield losses and premature pod shattering. Larvae chew holes in the pods and eat the seeds. Mature larvae even will continue to feed on pods in the swath. The entire seedpod can be consumed under high population densities.

Controls: Natural parasites and virus diseases are important factors that regulate the population of this armyworm in British Columbia. Little is known about the influence of these natural enemies on the population in the northwest, although they could be contributing to the sporadic occurrence of this pest. Insecticides are registered to control larvae of this pest, but should be used carefully to protect natural enemies. See the Pacific Northwest Insect Control Handbook for a list of registered insecticides and recommendations on particular crops.

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