Since 1993, western corn rootworm (WCR) beetle, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), the most serious insect pest of corn, have altered their movement behavior in east-central Illinois to lay eggs in soybean fields, reducing the effectiveness of crop rotation for WCR control. Now it appears that adult WCR have changed their behavior to feed on soybeans as well. On most days from late July to early September, WCR fly between corn and soybean fields to feed and lay eggs. Many of the questions relevant to the development of improved IPM for WCR require an understanding of the timing of WCR presence in soybean fields and the extent of the "problem area".
WCR movement into a soybean field is related to solar radiation, wind speed and direction, air temperature, and precipitation with consideration of biological factors that also influence flight. WCR flight activity and meteorological variables were measured above the canopy of an 1.64 ha soybean field in east-central Illinois between late July and early September, 1997. On 14 days, insect traps were sampled at 30-min intervals. The day-to-day variation in WCR movement was large. Overall, 61% of the WCR collected were female, although this proportion rose throughout the growing season. A diel periodicity in WCR immigration to and emigration from the soybean field was observed on days conducive to beetle flight. From 601 to 1000 solar time, immigration of WCR to the soybean field exceeded emigration. Flight activity peaked between 1001 and 1200, with emigration exceeding immigration. The abundance of WCR in the field reached a minimum at this time. By 1600, more WCR were again flying into the soybean field than leaving, and WCR abundance in the soybean field reached a maximum just before sunset. WCR were not caught in the malaise traps between sunset and sunrise.
Interfield movement of WCR occur over a wide range of solar radiation loads, air temperatures, and wind speeds. Darkness, air temperatures below 15 oC or above 31 oC, and wind speeds in excess of 2.0 m s-1 prohibit aerial movement of WCR. Within these limits, atmospheric factors had only little influence on the biologically-driven seasonal and daily temporal patterns of WCR flight activity. Atmospheric conditions were conducive to WCR flight during 62% of the 10-min measurement intervals during the growing season when biological factors enabled WCR to move among fields. Interfield WCR movement was substantially reduced by weather conditions throughout entire diurnal periods during about one-third of the days when female WCR beetles were abundant at the study site in 1997.
Region-wide sampling and modeling reveal a strong influence of convective storms on long-distance WCR dispersal. The new strain of WCR has moved from east-central Illinois eastward through Indiana to Ohio and Michigan, but is spreading only slowly to the west.