Pests: Beware the Armyworm

Pest of the Month: Armyworms (Family Noctuidae)

Armyworm

Native sweet corn is popping up everywhere: on farm stands, in local co-ops, at your favorite grocery store, and your town’s farmer's market. Whether you planted a couple of short rows at home, or if you have acres and acres of corn you’re harvesting for market, it’s not unusual to occasionally peel back those tough green husks and reveal that little green worm munching happily away on the sweet kernels. One little green armyworm larvae can ruin a perfectly marketable ear of corn in very little time.

Life Cycle

By definition, an “armyworm” is a caterpillar that has the habit of feeding above ground, can become abundant at times, and occurs in large migrations. In its adult stage as a gray-brown moth with a white dot in the center of the forewing, many species migrate to southern climates such as Mexico and Central America. Some species of the armyworm irregularly survive the winter in the U.S., while other species overwinter as pupa in the soil. Depending on the species, larval development can take from 2-8 weeks. Early stages are smooth pale green, while older larvae reach about 1.5"; turning a greenish brown with white stripes on the sides, dark or light stripes along the back. During the migration from warmer southern climates to northern cooler climates, adult moths lay eggs only at night and on the undersides of leaves. Eggs are greenish white and are laid in huge masses of up to 400 eggs. Younger stages of larvae typically feed only on one side of younger leaves, leaving leaves with a transparent appearance. Older larvae are less discriminating and often consume entire areas of the leaf. Larvae feed only at night and can tunnel into the whorl of young corn. As ears develop, they tunnel into the ears, many times entering from sides and tips. Pupation occurs in a loose cocoon in the soil. Three or four generations are typical in southern climates, while one or two generations are more common for northern U.S. states.

Plants Affected

Vegetables, grasses, corn, legumes, fruit trees, and flowers are occasionally damaged.

Symptoms

Tunneling into whorls, shredded leaves, transparent younger leaves, and in grasses a chewed ragged appearance.

Controls

Attracting natural, native predators such as parasitic wasps and flies. Live releases can be made on a regular schedule based on the army worm’s life cycle. A BTK spray can also be used to kill larvae. Dipel DF is a product we’ve made available to our customers and is a very effective method of control. We also use this product on a regular basis for tomato hornworms in our covered structures here on the farm. As always, follow label mixing and handling instructions completely and always wear the proper protective clothing. 

Resources

“The Organic Gardener’s Handbook of Natural Pest and Disease Control” Edited by Fern Marshall Bradley, Barbara W. Ellis, and Deborah L. Martin
“The Ultimate Guide to Backyard Bugs Garden Insects of North America” by Whitney Cranshaw
“Vegetable Notes” published by U Mass Extension, written by R Hazzard

Article by Sonya Reynolds, Greenhouse Coordinator, Johnny’s Selected Seeds