University of Illinois

No. 8/May 16, 1997

Insects in Wheat Fields

People scouting wheat fields are beginning to find insects. Noel Troxclair, Extension educator in IPM at the Marion Extension Center, observed first and second instars of cereal leaf beetles in wheat fields in Jefferson and Hamilton counties. However, the larvae were not numerous. Not surprisingly, he also found as many as two to six aphids per plant in some of these fields. Most of the aphids (98 percent) were bird cherry-oat aphids; only 2 percent of them were greenbugs. Although entomologists have never been able to associate economic yield losses in wheat with infestations of bird cherry-oat aphids, the presence of many aphids early in the season deserves our attention. Noel believes that the cool temperatures are holding back the parasitoids that usually suppress early season populations of aphids in wheat. If aphids begin building their colonies in the absence of natural enemies, their numbers could escalate rapidly.

Although we have received no reports of armyworms in wheat, entomologists at Purdue University have been capturing armyworm moths in light traps over the past few weeks. Consequently, armyworm larvae could be feeding and growing on leaves closer to the ground. However, they should not become economically threatening until they become larger and begin feeding on leaves closer to the heads, especially the flag leaves.

If you are scouting wheat fields for cereal leaf beetles, aphids, and armyworms, you should be able to identify the insects you are looking for. In issue no. 5 (April 25) of this Bulletin, we described cereal leaf beetles, so here we offer descriptions of armyworm larvae and the aphids you might encounter in wheat. Young armyworm larvae, the ones you might find right now, are pale green in color and have a looping habit when they crawl. When the larvae are full grown, they are approximately 1-1/2 inches long and have distinct longitudinal white, brown, and orange stripes, most notably the orange stripes just beneath the spiracles on each side of the body. Black stripes on the prolegs are also noticeable.

Several aphid species can be found in wheat. English grain aphids are green and have long, narrow cornicles ("tailpipes" that protrude from the rear of the abdomen) that are entirely black. The bird cherry-oat aphid is olive green, with a red-orange band across the rear of the abdomen; the tips of its cornicles are black. The greenbug, the most threatening aphid species, is bright green, with a darker stripe along the middle of its back. The tips of the cornicles are black. Sketches of these three aphids' rear ends are shown in Figure 4.

It's probably too early for economic levels of any of these insects in wheat to have developed. However, their presence in any field should be noted. As wheat matures, feeding by any of these insects could become threatening if their densities are large enough and the flag leaves are affected. If you encounter large numbers of cereal leaf beetles, aphids, or armyworms in wheat, let us know so we can share the information.
Kevin Steffey, Extension Entomology, (217) 333-6652