The western corn rootworm, Diabrotica virgifera virgifera LeConte, has altered its movement in east-central Illinois to lay eggs in soybean fields, thereby reducing the effectiveness of crop rotation for western corn rootworm control. Because western corn rootworm larvae cannot survive on soybean roots, adult western corn rootworm population dynamics in these fields is governed by aerial movement. Malaise trap samples from soybean fields were used to describe temporal variation in western corn rootworm immigration and emigration 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 conducive to beetle flight 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 of this interval (700–900 hours), immigration of western corn rootworm to the soybean field exceeded emigration, thereafter emigration exceeded immigration. Before sunset (1700–1900 hours), flight activity increased. Western corn rootworm were not caught in the malaise traps between sunset and sunrise. Micrometeorological measurements from days when western corn rootworm captures were large indicate that western corn rootworm movement may be gated to late morning, when wind speed and air temperature gradients above the soybean crop frequently create unstable atmospheric conditions that facilitate flight. 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 multiple days, are likely to provide more reliable western corn rootworm population estimates in soybean fields than sampling techniques that measure western corn rootworm abundance at single points in time (e.g., plant counts and sweep net samples).
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