An Update on Soil Heat-Unit Accumulations and the Corn Rootworm Larval Hatch

May 19, 2000
As we conclude the first half of May, questions concerning the projected date for the annual corn rootworm larval hatch begin to surface. Western and northern corn rootworm eggs have a developmental threshold of 52°F. About 380 to 426 degree-day accumulations are required for 50 % of the larvae to hatch. Robert Scott, Illinois State Water Survey, has provided us with Figure 1, which depicts soil heat-unit (base 52°F) accumulations at the 4-inch level from January 1 through May 15. According to Robert's calculations, heat-unit accumulations for 2000 are very similar to those of 1998 and 1999. On June 1 of last year, entomologists at Purdue University confirmed that larval hatch had occurred in central Indiana. Because of the similarity of this year's heat-unit accumulations with those of the past two seasons, I anticipate that corn rootworm larvae will begin to hatch across central Illinois during the last week of May. The hatch will not be late this season. Recall that in 1996 and 1997, larvae did not begin to hatch across central Illinois and Indiana until mid-June. Late hatches can result in more performance pressure on soil insecticides, particularly if products have been applied early in the season (early to mid-April). If below-normal soil moisture conditions persist through late May, expect excellent larval survival following the hatch. In addition, soil insecticide performance is typically not enhanced by very dry soil conditions. We have a long way to go this season regarding the continuing corn rootworm saga, so stay tuned.--Mike Gray

Author: Mike Gray