Beet Armyworm

Beet armyworm is a key insect pest that can cause significant damage to the petioles of celery stalks. The adult is a mottled gray- or dusky-winged moth 0.5 to 0.75-inches (12–19 mm) long. In the beet armyworm life cycle, females typically lay masses of 20 to 120 eggs and cover them with light-colored scales that give the egg masses a distinctive cottony appearance. Each female lays 400 or more eggs. The early instar larvae are 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, late-instar larvae generally hide in the center of the plant or underground during most daylight hours. After 5 to 6 instars, the larvae burrow into the soil to pupate. The pupal stage persists for 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 can 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 armyworm, especially parasitoids. Among the most common are the parasitoid 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. Discing may also destroy pupae of this pest. Because beet armyworm, as a generalist herbivore, can reproduce on many weeds and host plants, it is important to control weeds within and adjacent to field locations.

Use cultural and biological controls, sprays of Bacillus thuringiensis, and the Entrust formulation of spinosad in a certified organic crop.

Beet armyworm eggs and larvae are often easier to find on weeds in and near the field than in the celery plant. Whereas celery is often brittle and easily damaged during the early morning and late evening hours when larvae are active, weeds can be pulled and readily examined. Chenopodium species (e.g., lambsquarters, goosefoot) appear to be particularly attractive to beet armyworm larvae. Numbers can increase 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.

Although reliable treatment levels have not been established, the presence of more than 1 large larva or 5 small larvae on 25 celery plants is concerning. Apply insecticide when larvae are small; large larvae are more difficult to kill with insecticides such as Bacillus thuringiensis. Because larvae become active at dusk and sunlight degrades many pesticides, the best time for insecticide application is during the twilight evening hours.