The armyworm, or true armyworm, is a common early season pest that can cause occasional losses in corn and should be monitored for in the spring. Infestations usually first develop in fields of small grains or in other grass cover crops. In conventional tillage systems, partially-grown larvae can migrate into corn fields from grassy waterways or wheat fields, damage is usually first noticeable around the field margins adjacent to these areas.
The name armyworm derives from its behavior of migrating in large numbers into fields similar to invading armies. In no-till or reduced tillage systems, infestation may cover the entire field. In these systems, eggs may be laid on grasses within the field prior to planting and herbicides may force armyworms to feed on corn as the weeds or cover crop dies. Cool, wet, spring weather usually favors armyworm development. The full-grown 1-1/2 inch armyworm has a greenish brown body with a thin stripe down the center and two orange stripes along each side. The head is brown with dark honeycombed markings. Eggs are small, greenish white, globular, and laid in clusters of 25 or more on the leaves of grasses. The adult moth is tan with a tiny white spot in the center of the forewings. The moth has a wingspan of about 1-3/4 inches. Armyworms overwinter as partially grown larvae in grasses or small grain fields. When warm spring temperatures return, armyworm feeding resumes. Armyworms may move onto corn during this period. When feeding is completed, larvae pupate just below the surface of the soil. Adults of the first generation emerge in May and June and feed on nectar for 7 to 10 days before beginning to lay eggs. There are three to four generations per year in Kentucky.
Biological: The armyworm is preyed upon by a number of insects, notably a tachinid fly, Winthemia quadripustulata, which lays its eggs on the backs of the armyworms, mostly on the fore part of the body. The fly larvae hatching from these eggs bore into the armyworms and kill them. Armyworms are also preyed on by several ground beetles and certain parasitic wasps. Perhaps the most efficient insect enemy of armyworm is an extremely small, black, scelionid parasite, Telenomus minimus, that deposits its eggs inside the eggs of the armyworm. Braconid parasites, Apanteles spp., attack armyworms then they are partly to nearly fully grown and thus prevent an excessive increase in the next generation, but they do not kill armyworms until aftermost of their feeding has been done. The egg parasite, on the other hand, by preventing the eggs from hatching, stops all
damage by these insects.
Monitoring : Monitoring for armyworms early in the year is very important. Since armyworm outbreaks usually originate in fields of small grain or grasses, especially where there is a very rank growth of vegetation, or where the grain has fallen down and lodged, such situations should be watched, especially during May, and if high numbers of young armyworm larvae are found, the use of an insecticide boarder spray may be warranted.
Chemical Control: On average, less than 2 percent of the sweet corn acreage in Kentucky is annually sprayed with insecticides to control armyworms.
Ambush (permethrin)- Apply as a foliar treatment at 6.4 to 12.8 fl oz per acre to a limit of 76.8 lbs per acre per season
Asana XL (esfenvalerate)- Apply as a foliar treatment at 5.8 to 9.6 fl oz per acre to a limit of 96 fl oz per acre per season.
Baythroid 2 (cyfluthrin)- Apply as a foliar treatment at 1.6 to 2.8 fl oz per acre to a limit of 10 applications per season.
Pounce 3.2 EC (permethrin)- Apply as a foliar treatment at 4 to 8 fl oz per acre to a limit of 48 oz per acre per season.
Sevin 80 WSP (carbaryl)- Apply as a foliar treatment at 1-1/4 to 2-½ lb per acre to a limit of 8 applications per acre per season. Allow at least 7 days between sprays.
Warrior 1EC (lambda-cyhalothrin)- Apply as a foliar treatment at 2.56 to 3.84 fl oz per acre to a limit of 3.84 pints per acre.
Corn Leaf Aphids