On June 13, John Shaw, Illinois Natural History Survey, observed second instars feeding on roots in experimental corn plots located in Champaign County. Producers in central Illinois shouldn't be too surprised to see the first adult western corn rootworms of the season during the last week of June. Evaluating soil insecticide performance for corn rootworm larval control requires a little effort. The following questions and answers are designed to provide some additional input on this topic. Locating corn rootworm grubs in mid-June requires some effort.
First instars are quite small, and they tend to burrow into root tissue and feed internally.
Figure 1 provides some information that can be used to determine larval instars for corn rootworms. Don't be alarmed. Remember that soil insecticides are placed in-furrow or in a narrow band during planting. Untreated areas between rows effectively serve as a rootworm refuge. Because the entire rootworm larval population within a treated field is not exposed to a soil insecticide application, resistance to commonly used products has not occurred for decades.