FLIGHT OF HELIOTHIS-VIRESCENS MALES IN THE FIELD IN RESPONSE TO SEX-PHEROMONE

The behaviour of Heliothis virescens males flying upwind in the field in a sex pheromone plume was videorecorded and analysed. Males flew fa ster and straighter, with less counterturning, and heading more direct ly into the wind when they were 9-11 m away from the odour source than when they were 1-3 m away. Regardless of their distance from the sour ce or the windspeed, they maintained an average groundspeed of c. 200 cm s(-1), except when they arrived within 1 m of the source, when thei r groundspeed slowed significantly, Two or more males flying in the pl ume at the same instant often exhibited either extremely straight and directly upwind tracks or else zigzagging tracks with significant coun terturning (as did males flying through the field of view of the camer as at slightly different times). The males' position, either in the ce ntre of the plume's axis or along one side, might explain these differ ences in track straightness, which previous studies with H.virescens h ave shown to be caused by higher frequencies of contact with plume fil aments. When a significant shift in wind direction occurred, males ten ded to make an initial movement in the direction of the shift, perhaps due to latencies of response in both the olfactory and visual systems associated with flying into clean air. The males' behaviour in the fi eld overall was similar to that observed in the wind tunnel, except th at their airspeeds and groundspeeds were significantly higher than tho se observed in the laboratory. The fact that they flew faster in the f ield can be explained both by the significantly higher windspeeds that males need to compensate for in the field to attain a preferred veloc ity of image motion, as well as by a higher height of flight over the ground in the field causing a slower apparent motion of images at a gi ven groundspeed compared with the laboratory.