Issue No. 13, Article 2/June 23, 2006
Western Corn Rootworm Adult Emergence Has Begun
Sightings of western corn rootworm adults were reported for the first time in many areas of central and western Illinois during the past week. Matt Montgomery, crop systems educator, Sangamon-Menard Extension unit, observed western corn rootworm adults for the first time near Havana (Mason County) on June 17. Will Mullenix, technical support agronomist, Channel Bio Corp., reported finding a western corn rootworm adult in central Christian County on June 17. He also observed larvae (second and third instars) feeding on brace roots. Kevin Black, an insecticide and fungicide technical specialist with Growmark, reported that Jerry Harbour and Denny Kopp, certified crop specialists with Lincoln Land FS, found corn rootworms in several developmental stages (third instars, prepupae, and pupae) near Auburn (Sangamon County). They also indicated that corn planted as a refuge was severely lodged in one field they scouted.
Corn rootworm larvae, Champaign County, June 19 (courtesy of Joe Spencer, Illinois Natural History Survey).
Kevin Black has suggested that some confusion exists with regard to misidentifying soldier beetles as western corn rootworm adults. There have been many reports of soldier beetles this spring. We are not sure why their densities have been so impressive. Soldier beetles (Family: Cantharidae) are elongate and soft-bodied insects. They are quite similar to lightningbugs (Family: Lampyridae), but they do not produce light. Soldier beetles are beneficial insects, and as adults they can be found most frequently on flowers. Larvae are predators of other insects. The confusion between one soldier beetle species and western corn rootworm adults arises due to the presence of black stripes and yellowish wing covers on both. Beyond this similarity, the beetles are strikingly different. Be sure of your insect (weed or plant pathogen) identification--the first step in a sound IPM program.
Soldier beetle adult (courtesy of Kevin Black, Growmark, Inc.).
Soldier beetle larva (courtesy of Kevin Black, Growmark, Inc.).
How early is this emergence of western corn rootworm adults? The Handbook of Corn Insects published by the Entomological Society of America provides some help in answering this question: "Rate of development of immatures is temperature dependent. Development occurs fastest between 70-75°F (21-24°C). Within the soil temperature range of 64-81°F (18-27°C), the time from egg hatch to adult emergence is 46 and 23 days, respectively. Depending upon seasonal temperatures, adults begin emerging in early July; peak emergence occurs near August 1.Gerald R. Sutter."
Just as this information from the handbook suggests, reports of adult western corn rootworms typically filter into my office around the Fourth of July. It appears that we are about 2-1/2 weeks ahead of schedule this season. Although late May and the first half of June have been very warm, it seems doubtful that temperature alone explains the early emergence of western corn rootworm adults. What else might help explain these early sightings?
Paula Davis and other authors made the following observation in a 1996 journal article (Environmental Entomology 25: 773): "It is evident from our data that rootworm development is not strictly a function of temperature. Adult emergence seemingly is synchronized with time of corn flowering, although the mechanism for this synchronization is not clear." Because of the long-term trend to plant corn earlier and earlier, resulting in earlier flowering, have we begun to shift the developmental schedule of corn rootworm development? Will it become the norm to see western corn rootworm adults emerging initially in mid-June? We definitely need to explore these questions.
Because of the early adult emergence this season, we are likely to see earlier egg laying in cornfields and soybean fields. What is the progression of egg laying by western corn rootworm adults? Chris Pierce, currently in the Department of Entomology at Purdue University, addressed this question while completing his doctorate in the Department of Entomology at the University of Illinois. Chris took weekly soil samples in commercial cornfields and soybean fields during the summers of 1999 through 2001 in Iroquois County, Illinois. During this three-year study, Chris processed tons of soil in his search for western corn rootworm eggs. He determined that 50% of western corn rootworm egg laying had occurred in corn by July 28, July 28, and August 1 for 1999, 2000, and 2001, respectively. Cumulative oviposition (egg laying) in adjacent soybean fields at the 50% level was July 31, July 30, and August 10 for 1999, 2000, and 2001, respectively. Those wanting to monitor variant western corn rootworm adults in soybean fields this summer may wish to consider deploying Pherocon AM traps a few weeks earlier. Instead of late July, mid-July might be a more appropriate starting point. For more detailed information about seasonal oviposition in cornfields and soybean fields, take a look at Chris's recently published paper (Environmental Entomology 35: 676-683).
Please forward your observations concerning corn rootworm damage this season. We will share these findings with our readers. Thanks.--Mike Gray
Davis, P.M., N. Brenes, and L.L. Allee. 1996. Temperature dependent models to predict regional difference in corn rootworm (Coleoptera: Chrysomelidae) phenology. Environmental Entomology 25(4): 767-775.
Pierce, C.M.F., and M.E. Gray. 2006. Seasonal oviposition of a western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), variant in east central Illinois commercial maize and soybean fields. Environmental Entomology 35(3): 676-683 (web link).