Each spring as we approach mid-to-late May, questions begin to surface regarding the timing of the annual corn rootworm larval hatch. In general, this yearly event and the Memorial Day Holiday weekend coincide reasonably well for the latitude that cuts across much of central Illinois. As many of us recall, last year shattered this pattern and western corn rootworm adults and severe root damage were observed during the first week of June in western Illinois. Because hatching of larvae is a heat-driven biological event, there is approximately a 1-month window in which I have observed this phenomenon through the years – from mid-May to mid-June. By using a biofix of January 1 and a base 52⁰ F soil threshold, an estimated 379 heat units have accumulated for Champaign by May 19. Approximately 50% of western corn rootworm larvae are expected to have hatched when 684 to 767 heat units (base 52⁰ F, soil) have accumulated from January 1. Heat-unit totals for DeKalb, Monmouth, and Springfield from January 1 to May 19 of this year are 209, 381, and 373, respectively. Based upon these accumulations and the recent warm weather, I anticipate that larvae will begin to hatch as we get late into the Memorial Day Holiday weekend and throughout the last week of the month.
I encourage the readers of this Bulletin to visit the pest degree-day website maintained by the Illinois State Water Survey, Prairie Research Institute.