ARTHROPOD MUSEUM NOTES
Number 102 • May 8, 2013 • Jeffrey K. Barnes
Genus and species: Mythimna unipuncta (Howarth)
In the Entomological Society of America’s list of common names of insects and related organisms, 10 distinct species are referred to as some type of “armyworm”. Examples include the beet armyworm, fall armyworm and southern armyworm, but only one species is known simply as the armyworm. Some people distinguish this species from others by calling the true armyworm. Armyworms are moth larvae - caterpillars - that travel in multitudes from field to field. After consuming all available food in one area, the entire “army” of caterpillars marches on to another area with a fresh food source. True armyworms are primarily pests of plants in the grass family, including turf grass, small grains, and corn, but they will also feed on alfalfa, vegetables, fruit trees, ornaments and various wild plants when preferred food sources are not readily available. They are mostly a problem in the spring. After that, parasites and predators tend to keep populations in check. In North America, armyworms are most abundant east of the Rocky Mountains.
Armyworms pass the winter underground in the larval or pupal stage. They winter only in the South, and adults migrate northward each year. The caterpillar body is normally grayish green or grayish brown with a broad dark stripe dorsally and along each side. The head capsule is yellowish brown with a dark net-like pattern. After emerging from pupae and mating in the spring, adult females deposit eggs in clusters sheltered among vegetation. Often many eggs are deposited in a single locality, resulting in high populations of caterpillars. Adults live for about one to three weeks. Larvae complete development in 20–30 days in the warmer months. Young larvae skeletonize foliage, but older instars eat holes in leaves or consume entire leaves, feeding only at night and hiding during the day beneath debris or soil at ground level. Young larvae consume so little that they often go unnoticed. Once they reach the final instar they are highly mobile and voracious, often seeming to appear in large numbers out of nowhere to devastate crops. A complete generation requires 30–50 days, and the number of generations per year varies with location. In southern Canada there are 2 generations per year, in the northern United States there are 2–3, and in the southern states there are 4–6.