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Early Corn Rootworm Larval Hatch Anticipated: An Update on Heat-Unit Accumulations

May 18, 2001
During the past several days, we've received a few reports concerning the first sightings of fireflies or lightningbugs. As many veteran corn rootworm observers know, the first observation of these luminescent insects often coincides with the hatch of corn rootworm larvae. However, there is no biological or ecological linkage between these two events. Western and northern corn rootworm eggs have a developmental threshold of 52°F. About 380 to 426 degree-day accumulations are required for 50% of the larvae to hatch. Robert Scott, Illinois State Water Survey, has provided us with Figure 1, which depicts soil heat-unit (base 52°F) accumulations at the 4-inch level from January 1 through May 13, 2001. According to Robert's calculations, the soil heat-unit accumulations (January 1 to mid-May) for 2001 are very similar to last year's for this period. Last year, first-instar corn rootworm larvae were detected initially in root tips on May 22 by Purdue University entomologists in Tippecanoe County. The boundary of southern Tippecanoe County corresponds approximately with Highway 136, which bisects Illinois into northern and southern halves. I believe that by early next week (May 21), the corn rootworm larval hatch will be under way in much of central Illinois. This represents an early hatch.

If the dry and warm conditions persist across much of east-central and central Illinois counties, good-to-excellent survival of larvae is anticipated following the hatch. In addition, soil insecticide performance will not be enhanced by these dry soil conditions. Please let us know when you detect corn rootworm larvae in your fields, and we'll pass along the observation to all our readers.--Mike Gray

Corn rootworm larva.

Author: Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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