Description: The armyworm gets its name because it travels in small insect armies and consumes just about everything in its path. Armyworm caterpillars (1-3/4 inch long) are most active at night and hide under garden debris during the day. They skeletonize leaves of lettuce, cole crops, beans and corn. In tomatoes they make shallow gouges in fruit.
Markings on newly hatched caterpillars are usually hard to distinguish, older larvae have distinctive stripes that run the entire length of the body. Fall armyworms (Spodoptera frugiperda) are brown with yellow stripes, beet armyworms (Spodoptera exigua) are green with light stripes. Adults are gray, mottled moths (1-1/2 inch wingspan) with a small white dot in the center of each forewing and dark margins on the hind wings.
Life Cycle: Armyworm eggs are laid in fluffy masses on crowns of seedlings and on leaves of older plants. In 5-10 days tiny caterpillars hatch and begin feeding for several weeks. They then pupate and emerge as adults 10 days later. One to six generations per year. Some armyworm species overwinter as caterpillars in the soil, other species migrate south.
Armyworm Control: Outbreaks of this pest are not common because they have many natural predators. Use pheromone traps to determine the main flight period for moths, then release trichogramma wasps to parasitize the newly laid eggs. Beneficial insects, such as lacewing, ladybugs and minute pirate bugs feed on armyworm eggs, as well as the young larval stage.
Monitor plants closely in early spring or summer months for signs of insect damage. Caterpillars will often be found feeding on the undersides of leaves and on new growth. Handpick pests that are discovered and drop them in a bucket of soapy water. Applications of Dipel Dust (Bt-kurstaki) or Monterey Garden Insect Spray (spinosad) will also kill caterpillars. Use botanical insecticides if pest levels become intolerable.
Photo Credit: Purdue Entomology/John Obermeyer