No. 8 Article 3/May 27, 2011

Predicting Corn Rootworm Hatch: Degree Days, Calendar Date, or Firefly Sightings?

By late May of each growing season, I have been asked multiple times to estimate the timing of the annual corn rootworm larval hatch. Through the years, I've learned that the calendar is a much more reliable indicator of hatch than firefly (lightningbug) sightings. A review of previous articles in the Bulletin reveals the following estimates of hatch for central Illinois and Indiana: 1996, June 12; 1997, June 13; 1998, late May (no precise date reported); 1999, June 1; 2000, May 22; 2001, May 16; 2002, May 31; 2003, May 29; 2004, May 28; 2005, May 31; 2006, May 29; 2007, May 18; 2008, June 4; 2009, June 1; 2010, June 3 (second instars observed).

For many seasons, these estimates were graciously provided by entomologists with the Department of Entomology, Purdue University. Years when hatch occurred very early (mid-May) included 2000, 2001, and 2007. Hatch was very late (mid-June) in 1996 and 1997. These extremes represent about a 1-month range in larval hatch. During this 15-year period, hatch occurred in late May and early June 10 times. So odds are pretty good that in most years, the annual corn rootworm hatch will begin around the Memorial Day holiday.

Research published in the early 1990s by entomologists with the Illinois Natural History Survey indicated that 50% of larval hatch should have occurred when 684 to 767 degree-days (base 52°F, soil) had accumulated since January 1 (biofix). Through May 24 of this year, 382 degree-days (base 52°F) have accumulated in Champaign. These totals and projections indicate that hatch will most likely begin during early June across central Illinois, similar to the pattern observed in recent years.

It's difficult to predict western corn rootworm densities this growing season. If saturated soils are common across the state in early June, rootworm pressure will be diminished. The last few years, populations of this pest have been down in most of Illinois. I do not anticipate this changing significantly this year. Reasons discussed in previous articles in the Bulletin include saturated soils during larval hatch over the past few seasons, widespread use of pyrethroid and fungicide tank mixes applied to corn and soybean fields, and the increasing popularity of Bt hybrids.

Please share with me your observations on western corn rootworms this season, especially if you observe unusual and unexpected levels of root damage.--Mike Gray

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