Issue No. 9, Article 2/May 23, 2008
Late Corn Rootworm Larval Hatch Anticipated This Spring
Not surprisingly, this year's cool, wet spring is expected to delay the hatch of corn rootworm larvae across Illinois. In most years, hatch begins in late May. In 2006, confirmation of the larval hatch occurred on May 29 for central Illinois. Last year, hatch was relatively early, estimated at May 18. A list of previously confirmed hatches, (since 1996) is provided at the end of this article, thanks to Larry Bledsoe at Purdue University. Late hatches have occurred in other years. For instance, in 1996 and 1997, corn rootworm larvae did not begin to hatch until about mid-June across central Illinois. We may observe a similar late hatch this season. Based on the pest degree-day calculator, roughly 200 degree-days (base 52°F, 4-inch soil depth) have accumulated from January 1 to May 19 in central Illinois. In northern Illinois, approximately 100 degree-days (base 52°F, 4-inch soil depth) have accumulated during this time frame. These accumulations are taken from temperatures recorded beneath sod, so they should be viewed conservatively. Bare soil will accumulate heat units more rapidly.
According to research by Eli Levine and others published in 1992 (Journal of Economic Entomology 85: 2425-2432), 50% of corn rootworm larvae can be expected to have hatched when 684 to 767 degree-days (base 52°F, 4-inch soil depth) have accumulated. For much of Illinois, including southern Illinois, we are far behind the number of degree-days necessary to trigger larval hatch.
What are the implications of a late corn rootworm larval hatch? Early planting of corn followed by a late hatch of larvae can place increased pressure on soil insecticide performance. Likewise, early planting of Bt corn followed by a very late hatch could increase the chances for more root injury. However, early planting of corn has not taken place for much of Illinois this spring. The late hatch of larvae and the late planting of corn may simply delay emergence of corn rootworm adults and the silking and tasseling of corn. Unfortunately, this may increase the odds that these biological events occur during the hottest period of the summer. We welcome any confirmations of the corn rootworm larval hatch when it occurs in your area of the state.
Larval Hatch--Central Illinois
1998--late May (no precise date reported)