Corn Rootworm Hatch Under Way

May 26, 2000
The annual corn rootworm hatch is under way! Larry Bledsoe, an entomologist with Purdue University reported the first observation of corn rootworm larvae in root tips on May 22 in southern Tippecanoe County, Indiana. The boundary of southern Tippecanoe County corresponds approximately with Highway 136 that bisects Illinois into northern and southern halves. Larry indicates that he believes the hatch began sometime during the week of May 15, and densities of larvae finally reached the detection level by May 22. First-instar corn rootworm larvae are very difficult to find, due to their very small size and internal root-feeding behavior. Based upon 4-inch soil heat-unit accumulations (base 52°F) from January 1 to May 22, Purdue's observations are right on target (Figure 1).

In last week's Bulletin, I mentioned that I believed larval hatch would begin during the last week of May. I also mentioned that the hatch would not be late this year. Well, it's even earlier than I suspected. In 1999, Purdue reported the first observance of corn rootworm larvae on June 1. So, for now, corn rootworms seem to be about one developmental week in advance of last year's pace. Figure 1 reveals a broad band of counties through central Illinois have accumulated between 300 and 400 heat units (4-inch soil level, base 52°F). As we've mentioned previously, about 380 to 426 heat-unit accumulations are required for 50% of corn rootworm larvae to hatch. For many areas of central and southern Illinois, corn rootworm larvae are hatching within fields that are relatively dry. This translates into excellent larval survival following hatch. By late May, the corn rootworm larval hatch will have begun even in northern counties of Illinois.

We've received a number of calls regarding observances of western corn rootworm adults. These sightings are not western corn rootworm adults. Instead, folks have been observing adult striped cucumber beetles. These insects are often confused with western corn rootworm adults, especially females. The black stripes on striped cucumber beetles extend to the extreme posterior tips of the wing covers. On western corn rootworm females, the stripes fall somewhat short of the posterior tips of wing covers, especially the middle stripe. Male western corn rootworm adults have the black stripes essentially merged across the wing covers. If you still are not convinced of your identification, flip adult beetles on their "backs" and look at the color of the abdomen. Abdomens of striped cucumber beetle are nearly black.

Corn rootworm larva.

As we proceed into the first few weeks of June, we are likely to begin hearing our first reports of corn rootworm larval injury. We'll keep you posted on corn rootworm developments throughout the season.--Mike Gray

Author: Mike Gray