To date, we have not received any confirmation that corn rootworm larvae have hatched in Illinois. Soil heat unit accumulations (base 52°) at the 4-inch level, from January 1 through May 19, 2003 (Figure 1), suggest that late May is still a good general target date for larvae to begin their hatch. As we've reported previously, after 380 to 426 soil heat units have accumulated, from January 1, approximately 50% of corn rootworm larvae should have hatched. Across the southern third of the state, we have accumulated the necessary number of heat units for this annual event to begin. However, the southern one-third of Illinois is not typically an area of the state where corn rootworms are considered a primary insect threat. For sure, corn rootworms may cause damage in any area of the state; however, their economic threat in southern Illinois is more sporadic in nature.
By late May, I anticipate receiving a few reports that larval hatch has occurred across central Illinois counties. As Figure 1 depicts, heat units in the northern one-third of Illinois are expectedly well behind even those in central areas of the state. I do not anticipate reports of larval hatch in northern Illinois counties until sometime during the first week of June.
We look forward to your observations from the field regarding the corn rootworm hatch.--Mike Gray