No. 6 Article 7/May 10, 2012

Corn Rootworm Larval Hatch: Earliest in at Least 35 Years

Long-time Purdue University entomologist Larry Bledsoe has reported observing corn rootworm larvae (1st instars) on May 8 in Tippecanoe County, Indiana (north and west of Indianapolis). Larry says this was the earliest he has found corn rootworm larvae in 35 years! He estimates that hatch occurred between May 4 to May 6.

Before this observation, May 16 (2001) had been the earliest date in the past 17 years that larval hatch had occurred. Generally, the initial hatch occurs near the end of May and into early June. In fact, in the past four years, Larry's first reports of larvae were in early June.

A review of Bulletin articles since 1996 reveals the following estimates of hatch (for central Illinois and Indiana), based on Purdue University observations provided by Larry Bledsoe.

What are the potential implications for corn producers this season? By May 7, 89% percent of corn acres were planted across Illinois, as reported by the USDA NASS Illinois Field Office in Springfield. Unlike in previous recent years, corn rootworm larvae this spring will be feeding on small-rooted corn plants in drier soil conditions. We have had wet springs in recent years, and corn rootworm larvae struggled to survive in saturated soils at the time of hatch.

This will not be the case in many fields this spring. As of May 7, moisture conditions in top soil across the state were 6% short, 68% adequate, and 26% surplus (USDA, NASS, Springfield). I anticipate good larval establishment this season, and where densities are high, significant pressure will be exerted on the root systems of corn plants throughout May and into mid-June. With a larval hatch some 3 to 4 weeks early, anticipate the early arrival of adults this year as well, possibly by mid-June.

If you encounter significant rootworm injury on Bt plants this spring, please let me know. I would like to share these observations with readers. Thanks.--Mike Gray

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