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Issue No. 7, Article 1/May 21, 2010

Corn Rootworm Larval Hatch: Current Projections

Each year as we move past the mid-point of May, questions begin to surface about the likely timing of the annual corn rootworm larval hatch. For many years, some have suggested that this biological event coincides with the first sightings of lightningbugs (fireflies). Although these unrelated events may from time to time have occurred at about the same point, it has been my experience that lightningbugs are typically noticed before the corn rootworm hatch, especially over the past 5 to 10 years.

In numerous articles published in the Bulletin through the years, I have indicated that corn rootworm larval hatch most often occurs within a few days on either side of the Memorial Day holiday. In an attempt to be more precise, entomologists have estimated that approximately 50% of the corn rootworm hatch occurs when 684 to 767 degree days (52°F) have accumulated from January 1 (biofix date). A quick glance at degree-days across the north-central region of the United States (base 50°F) since January 1 indicates that about 600 have accumulated across central Illinois through May 17. For much of northern Illinois, the number is roughly 400 to 450. These data suggest that for central Illinois, the hatch may be just around the corner. Entomologists at Purdue University annually dissect corn roots looking for the first instars; so far they have not reported any hatch. As soon as this annual event is confirmed, I will let readers know. If the hatch occurs later than predicted over the next several years, we may begin to build a body of evidence suggesting that the extensive use of Bt corn hybrids may mean we are selecting for corn rootworm larvae that hatch later.--Mike Gray

Mike Gray

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