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Update on Heat Unit Accumulations for Corn Rootworm Larval Hatch

May 15, 2003

The annual "watch" for the corn rootworm larval hatch continues. Soil heat unit accumulations (base 52°F) at the 4-inch level, from January 1 through May 12, 2003 (Figure 1), indicate that the corn rootworm larval hatch may be very similar or slightly ahead of last year's hatch (late May). After 380 to 426 soil heat units have accumulated, from January 1, approximately 50% of corn rootworm larvae should have hatched. Although it's too early to predict accurately the severity of corn rootworm problems this season, the early planting and average-to-above-average densities of western corn rootworm adults in 2002 could contribute to some management challenges this year. If precipitation patterns change during the next few weeks and a dry spell occurs, this development also would enhance larval survival during hatch. Saturated soils would interfere with larval establishment. Corn rootworm larvae orient toward roots based on the concentration of carbon dioxide in the soil profile resulting from the respiration of roots. Roots that are in waterlogged soils don't respire as efficiently, and the production of carbon dioxide is impaired. The dispersal of first-instar larvae through the soil also likely is impaired in saturated soils.

Corn rootworm eggs below a dime.

Corn rootworm larva.

We'll let you know when the larval hatch has been confirmed.--Mike Gray

Author: Mike Gray

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

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