Dose-related upwind anemotaxis and movement up odour gradients in still air in the presence of methyl eugenol by the wild tobacco fly, Bactrocera cacuminata

The behaviour of Bactrocera cacuminata (Hering) in wind varied according to the concentration of methyl eugenol (0, 95, 327 and 500 mu g m(-3), respec tively). General locomotor activity (as measured by mean distance moved in 5 min, regardless of direction) was not significantly different in the firs t two treatments but was significantly lower in the others. Most flies in t he fourth treatment did not move more than one body length. In the first tw o treatments, the rate and pattern of movement of most flies was basically similar, with walking in tortuous paths interspersed with short flights and usually no obvious bias in direction. However, 32% of flies in the second treatment did move in a biased direction, achieving upwind anemotaxis of at least 400 mm, but only 2-8% did so in the other conditions. Flies moved up a concentration gradient to a source of methyl eugenol in still air when r eleased at a distance of 100, 150 or 200 mm. With one exception, no more th an 40% did this within 3 min of release (whether or not the olfactory stimu lus was augmented by a visual one). However, 77% responded when released 10 0 mm from a combined olfactory and visual stimulus. Visual augmentation of an olfactory stimulus may also be responsible for far fewer flies flying ou t of the vicinity at distances up to 150 mm, but not 200 mm.