MOVEMENT OF CORN-ROOTWORM BEETLES, DIABROTICA-SPP (COLEOPTERA, CHRYSOMELIDAE), AT CORNFIELD BOUNDARIES IN RELATION TO SEX, REPRODUCTIVE STATUS, AND CROP PHENOLOGY

Canopy and bidirectional Malaise traps were used to estimate the flight activity of northern corn rootworms, Diabrotica barberi Smith & Lawrence, and western corn rootworms, D. virgifera virgifera LeConte, above the plant canopy and at the lateral boundaries of cornfields. Capture of beetles in both types of traps was greatest during pre- and post-flowering intervals, particularly for northern corn rootworms, indicating increased flight activity in response to changes in habitat quality. Based on comparison of ratios of northern/western corn rootworms in the field with those ratios in traps, northern corn rootworm beetles were generally more active at field boundaries than western corn rootworm. There were marked differences in flight activity between males and females and between reproductively immature and mature females of both species. In general, Malaise traps captured a greater number of males and canopy traps a greater number of females in relation to their abundances in source fields. In comparison with source field populations, a greater number of western, but not northern, corn rootworm females captured in canopy traps had immature ovaries, suggesting that vertical movement may represent the initiation of migratory flight. Bidirectional trap results suggest that northern corn rootworm females move from cornfields when silks and pollen become unavailable and later return to fields for oviposition. Comparison of field data with computer simulations for both species suggested that there was substantial emigration from the early-planted field and that most of the beetles in the late-planted field were immigrants. Seasonal trends in flight activity tend to reflect these patterns; however, data from bidirectional Malaise traps.failed to provide strong evidence of greater emigration over immigration in the early-planted field or an opposite trend in the late-planted field. Overall, flight activity measured in passive traps at field boundaries may be a poor index of site-specific patterns of beetle movement, especially for western corn rootworms.