Usually, the first corn rootworm beetles are found in corn by July 4 or shortly thereafter. One of my graduate students, Yong-Lak Park, spotted several western corn rootworm males in a Clinton County field on July 2, which is a couple of days earlier than average, but not unusual.
Iowa State University
Tollefson, Jon J., "Corn rootworms are back!" (2002). Integrated Crop Management News. 1733.
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