Beet Armyworm

Beet armyworm adults are 0.5 to 0.75 inch, nondescript brown moths. They typically lay masses of 20 to 120 eggs and cover them with light-colored scales, which gives the egg masses a distinctive cottony appearance. Up to 400 or more eggs can be laid by each female. The early instar larvae are a pale green and often feed gregariously for the first few days. Larger larvae range from green to black, and often have a broad stripe along each side of the body. The larger larvae feed singly. The larval stages last from 2.5 to 3 weeks at 70° to 80°F. Large larvae generally hide in the center of the plant or underground during most daylight hours. After five to six instars, the larvae burrow into the soil to pupate. The pupal stage persists 7 to 10 days but may be much longer at cooler temperatures. Although newly emerged adults can be found throughout the year in central and southern California, major emergences and migration flights are usually recorded in spring through late summer.

Young worms feed on celery leaves but rarely cause substantial damage. However, larger larvae feed on petioles and cause significant crop loss. Large larvae are quite mobile and have been observed to travel over 10 feet per night, feeding on several plants. If petioles near the outside of the plants are damaged, they can be removed. If damage occurs in a centrally located petiole the plant must often be discarded.

Many natural enemies attack beet armyworms. Among the most common parasites are the wasps, Hyposoter exiguae and Chelonus insularis, and the tachinid fly, Lespesia archippivora. Viral diseases also kill significant numbers.

Disc fields immediately following harvest to remove the food source for any remaining larvae. Some pupae may be affected by discing as well.

Cultural and biological controls and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for use on organically grown produce.

Beet armyworm eggs and larvae are often easier to find on weeds in and near the field than in the celery plant. Weeds can be pulled and readily examined whereas celery is often brittle and easily damaged during the early morning and late evening hours when larvae are active. Chenopodium species (e.g., lambsquarters, goosefoot) appear to be particularly attractive to beet armyworm larvae. Populations can build rapidly, so check fields twice a week. Monitor adults with pheromone traps placed along the edges of fields. This is a particularly good technique for detecting large emergences or migrations occurring on weather fronts. Reliable treatment levels have not been established, but the presence of more than one large larva or five small larvae on 25 plants would be of concern. Make treatments when larvae are small; large larvae are more difficult to kill with compounds such as Bacillus thuringiensis. Because larvae become active at dusk, and sunlight degrades many pesticides, the best time for insecticide treatment is in the twilight evening hours.