Corn rootworm egg hatch in Iowa can occur from late May to mid June depending on soil conditions, but the average hatching date is around June 6. Research suggests about 50 percent of larval hatch occurs between 684-767 degree days (base 52°F, soil). The cool spring weather in 2011 has definitely slowed down predicted egg hatch this year. Reports from surrounding states have also indicated a delayed egg hatch. This map show the accumulated soil temperatures in Iowa this year. The southwestern region is experiencing 50 percent larval hatch now, and the rest will approach it within 7-10 days depending on the temperature.
Saturated soils will diminish overall corn rootworm pressure, and the high adoption of Bt corn should decrease populations in most fields in 2011. However, every field should be scouted for corn rootworm larval feeding regardless of the seed selection (i.e., scout even if Bt proteins are used).
Corn rootworm rescue treatments should be made in June, but are not very practical or effective, as the corn is too tall for the product to sufficiently penetrate the soil and reach the larvae. But sampling and evaluating root systems this year will help assess corn rootworm management and seed selection for 2012.