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Issue No. 9, Article 2/May 25, 2007

Corn Rootworm Larval Hatch--Accumulated Degree-Days or Calendar Dates?

At this time of year, we receive numerous questions regarding corn rootworm larval hatch. We also hear of many observations of fireflies, which have been associated with rootworm larval hatch in the past. However, one has to wonder about such an association when the first appearance of fireflies occurs, as it did in some areas this year, in early May. It is unlikely that rootworms hatched from overwintered eggs so early. But the question remains: When do rootworm larvae hatch?

The research we rely on to address this question was conducted in the mid- to late 1980s by entomologists at the Illinois Natural History Survey and the University of Guelph and published in 1992 (Levine, E., H. Oloumi-Sadeghi, and C.R. Ellis. 1992. Thermal requirements, hatching patterns, and prolonged diapause in western corn rootworm [Coleoptera: Chrysomelidae] eggs. Journal of Economic Entomology 85: 2425-2432). This is the citation indicated on the relevant page at the degree-calculator Web site. The base temperature used for development is 52°F at a 4-inch depth in the soil. However, we have learned recently that the accumulated degree days indicated at this Web site are for temperatures at a 4-inch depth in soil beneath sod. Obviously, temperatures beneath sod are cooler than temperatures beneath bare soil, so the accumulated degree-days and projected accumulated degree-days for rootworm larval hatch are conservative (i.e., less than beneath accumulated degree-days beneath bare soil). Please keep this in mind when you visit the Web site.

There also has been some confusion about accumulated degree-days and time for 50% hatch of rootworm larvae. On the Web site, we indicate that 50% hatch of rootworms occurs when 684 to 767 degree-days have accumulated. Others, however, have suggested fewer degree-days. You should know that the accumulated degree-days on our Web site are Fahrenheit degree-days, and the research reported in Levine et al. (1992) was published as Centigrade degree-days. We have clarified this in the past, but recent questions suggested we should address the discrepancy again.

Based on the research done by Levine et al. (1992), predicated dates for first hatch in 1986, 1987, and 1988 were May 29, May 18, and May 26, respectively. Predicted dates for 50% hatch for the same years were June 8, May 28, and June 4. Nine to 10 days elapsed between predicted first and 50% hatch.

All of this said, we have learned that in most years, the annual hatch occurs near the end of May or early June. Following are estimated dates of corn rootworm larval hatch for central Illinois over the past 11 years:

  • 2006--May 29
  • 2005--May 31
  • 2004--May 28-29
  • 2003--May 29
  • 2002--May 31
  • 2001--May 16
  • 2000--May 22
  • 1999--June 1
  • 1998--late May (no precise date reported)
  • 1997--June 13
  • 1996--June 12

A quick review of these dates suggests that corn rootworms typically begin to feed on corn roots around the Memorial Day holiday, although there are exceptions. In 1996 and 1997, the larval hatch did not occur until mid-June. In 2001, corn rootworms began to hatch in mid-May. These dates were selected by reviewing articles from the Pest Management and Crop Development Bulletin and are primarily based on observations by Larry Bledsoe, an entomologist at Purdue University.

At least one published scientific article has indicated that calendar date worked as well as degree-days in predicting corn rootworm phenology (Bergman, M.K., and F. T. Turpin. 1986. Phenology of field populations of corn rootworms [Coleoptera: Chrysomelidae] relative to calendar date and heat units. Environmental Entomology 15: 109-112). These entomologists arrived at this conclusion because factors other than soil temperature may influence time of hatch of rootworms in given fields, including soil type, soil moisture, and location of rootworm eggs in the soil. The projections for first and 50% hatch using accumulated degree-days are reasonable guidelines only, but using calendar dates for projecting rootworm hatch has held up pretty well over time.

We welcome any confirmations of the corn rootworm larval hatch when it occurs in your area of the state.--Mike Gray and Kevin Steffey

Mike Gray
Kevin Steffey

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