Reports of True Armyworm in Hay in Central & Western Kentucky

This past week, there were a few reports of large numbers of armyworms in hay fields collecting on equipment as the fields were being cut. This is a different species from the fall armyworm that had an outbreak in soybeans, alfalfa, and pastures two years ago. In general, true armyworm, also know just as armyworm, tends to be more of a spring pest of grasses and corn. Fall armyworm, as its name implies is more of a late season pest beginning mid-July until frost.

Figure 1. True armyworm characteristically feeds on leaf margins (Photo: Ric Bessin, UK).

Habits & Description

Infestations usually first develop in fields of small grains or other grasses. 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 corn crops, 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 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. Armyworm overwinters as partially grown larvae in grasses or small grain fields in Kentucky. 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 April and May and feed on nectar for 7 to 10 days before beginning to lay eggs. There are three to four generations per year in Kentucky. 

As with fall armyworm, true armyworm usually feeds at night preferring to feed on the succulent leaves in the whorl first. During the day, armyworms are found in the soil or underneath ground cover. Ragged leaf feeding on leaf margins in the spring and early summer is consistent with armyworm feeding.

Monitoring & Management

In hay fields and pastures, treatments for armyworm is based on monitoring. As with fall armyworm, true armyworm often hides under debris on sunny days, so monitoring is best done in the late afternoon or early evening. The same threshold is used for both true armyworm and fall armyworm: 2 to 3 per square foot. Materials listed for fall armyworm control in ENT-17 are also effective against true armyworm.

By Ric Bessin, Entomology Extension Specialist

Posted in Grains
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