Aerial movement of western corn rootworm (Coleoptera : Chrysomelidae): Diel periodicity of flight activity in soybean fields

The western corn rootworm, Diabrotica virgifera virgifera LeConte, has alte red its movement in east-central Illinois to lay eggs in soybean fields, th ereby reducing the effectiveness of crop rotation for western corn rootworm control. Because western corn rootworm larvae cannot survive on soybean ro ots, adult western corn rootworm population dynamics in these fields is gov erned by aerial movement. Malaise trap samples From soybean fields were use d to describe temporal variation in western torn rootworm immigration and e migration during 1997, 1998, and 1999. Sixty-eight percent of western corn rootworm collected in the soybean fields were female. A diel periodicity in western corn rootworm immigration and emigration was observed on days cond ucive to beetle night in soybean fields. Flight activity was low during the early morning (500-700 hours solar time) when air temperatures were cool. Aerial movement peaked between 7000 and 1100 hours. During the first half o f this interval (700-900 hours), immigration of western corn rootworm to th e soybean field exceeded emigration, thereafter emigration exceeded immigra tion. Before sunset (1700-1900 hours), night activity increased. Western co rn rootworm were not caught in the malaise traps between sunset and sunrise . Micrometeorological measurements from days when western corn rootworm cap tures were large indicate that western corn rootworm movement may be gated to late morning, when wind speed and air temperature gradients above the so ybean crop Frequently create unstable atmospheric conditions that facilitat e night. Because of the diel periodicity and high day-to-day variability of western corn rootworm movement to soybean fields, scouting tools, such as sticky traps and vial traps that integrate captures over an entire day or m ultiple days, are likely to provide more reliable western corn rootworm pop ulation estimates in soybean fields than sampling techniques that measure w estern corn rootworm abundance at single points in time (e.g., plant counts and sweep net samples).