University of Illinois

No. 12/June 13, 1997

"Caterpillars" in Wheat

On June 5, Dennis Bowman, crop systems educator at the Decatur Extension Center, found some "caterpillars" in wheat in Shelby County. Two of the larvae were armyworms, but the rest were sawflies (technically, sawflies are not caterpillars, which are larvae of moths and butterflies). Both armyworms and sawflies feed on leaves, but sawflies do not cause economic damage. Therefore, it's important that you identify caterpillars in wheat accurately.

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 about 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 also are noticeable. Sawfly "caterpillars" are the larval stage of insects in the same order (Hymenoptera) as bees and wasps. The sawflies Dennis Bowman found were about 1/2 inch long and light yellow in color. Stripes are not evident on sawflies. However, the best way to distinguish between armyworm and sawfly larvae is to count the numbers of prolegs (false legs on the abdomen): Armyworms have five pairs of prolegs (including the anal pair), sawflies have six to eight pairs of prolegs.

As you scout in wheat fields, you are liable to encounter either armyworms, sawflies, or both. Remember, armyworms can cause economic damage (the threshold is six or more nonparasitized larvae per foot of row); sawflies do not. Don't overreact to leaf-feeding injury, especially on the lower part of the plants.

Kevin Steffey, Extension Entomology, (217)333-6652