Role of Visual Contrast in the Alighting Behavior ofHarmonia axyridis(Coleoptera: Coccinellidae) at Overwintering Sites

Abstract

The multicolored Asian lady beetle Harmonia axyridis (Pallas) is often a pest during autumn, when large numbers take flight and subsequently land on and enter buildings. Open field experiments were conducted during the autumn flight to examine the role of linear contrast in visually attracting beetles. White targets printed with 61 by 15-cm stripes that varied in orientation and degree of contrast were covered in insect adhesive, attached to a white background panel, and erected in sites known to be attractive to flying beetles. Flights occurred when temperatures rose above 21°C. Diurnally, peak flight occurred between 1400 and 1600 hours but shifted progressively earlier within that framework as the flight season advanced. Beetles significantly chose targets with high contrast, black stripes; the number of beetles landing on all other contrast levels did not differ significantly from controls. Vertically positioned stripes attracted more beetles than horizontal ones, but not significantly. The effects of high contrast were absolute rather than relative. The number of beetles landing on targets with 50% contrast stripes did not increase when these were the highest contrast targets available. Results suggest that visual intensity contrast is key to understanding the behavior of H. axyridis in autumn and call into question some commonly accepted ideas regarding choice of overwintering sites. First, during migratory flight, H. axyridis is not visually attracted to the color white per se, and second, it is unlikely that, during flight, pheromones are involved in beetle orientation.