Field studies were designed to more clearly determine how adult western corn rootworm, Diabrotica virgifera virgifera LeConte, population distribution patterns are altered over time by changes and contrasts in corn (Zea mays L.) plant phenology using whole-plant beetle counts as the sampling tool. In 1994, studies were conducted in a model system consisting of a late-planted corn strip placed in the middle of an early-planted cornfield. The system was replicated over three fields. Large-scale variation was modeled using trend-surface regression analysis to describe the relationship between beetle counts and distance from the center of the late-planted strip. In each field, the beetle distribution became greatly skewed toward the late-planted strip when the strip was either in the tassel or silk stage and the surrounding field was ≥ blister stage. In 1995, studies were conducted in and at the interface of two adjacent cornfields that were planted 9 d apart. Count data were analyzed to quantify population density changes at different sampling locations within and among fields over time as crop phenology changed. Rapid positive or negative changes in beetle densities occurred within and among fields as contrasts in corn phenology changed. In both years, the stability of the beetle distribution was strongly influenced by the length of time that a contrast in plant phenology was maintained between adjacent patches of corn. Directional movements of beetles toward pollinating corn and associated semiochemicals could be inferred from both 1994 and 1995 data analyses. Data suggest that contrasts in crop phenology at the interface and among cornfields should be considered when developing beetle sampling programs and interpreting scouting data to improve the accuracy of rootworm management decisions.
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