Effects of background contrast on visual attraction and orientation of Tabanus nigrovittatus (Diptera: Tabanidae)

The effect of relative intensity contrast of an object against a background on the visual attraction of host-seeking adults of Tabanus nigrovittatus was examined on a salt marsh in Massachusetts in 1982 using small test panels against large background panels. Attraction of T. nigrovittatus to grey panels was significantly greater when contrast against grey background panels was highest. Maximum numbers were attracted to high-intensity blue test panels against low-intensity grey backgounds and low-intensity saturated blue panels against all-grey backgounds regardless of intensity. The latter indicated the attractive effect of hue independent of intensity. Flies collected on test and backgound panels were clumped on the test panel (32.6%), along the boundary of the test panel and the backgound panel (59.9%) and from 15.2 to 38.0 cm above gound level (85%). Direct observation revealed that 60% of the flies approaching the panel landed and all did so below the height of initial approach. Only 20% landed immediately; the remainder flew past the panel first, then returned. The majority of flies (85.5%) approached panels at 30.4-53.2 cm above ground level.