LARVAL FEEDING AND MOVEMENT IN 2 PLUTELLA-XYLOSTELLA (LEPIDOPTERA, PLUTELLIDAE) POPULATIONS EXPOSED TO DISCRETE DEPOSITS OF PERMETHRIN

Feeding and movement by two Ohio populations of diamondback moth (Plutella xylostella L.) on leaf discs treated with individually applied, uniformly-sized droplets of permethrin was video-recorded. Incorporation of fluorescent dye in the permethrin solution allowed comparison of the proportion of leaf area eaten with proportion of deposits consumed and measurement of distance travelled across the leaf surface between contacts with droplets. There was a direct relationship between proportion of leaf area eaten and proportion of deposits consumed (slope approximately 1.0). Feeding by the more physiologically susceptible population was unaffected by drop size (100-220-mu-m, 50 drops disc-1 in each case) and the proportion of the leaf disc consumed was much less than that recorded for the more resistant population at the two smaller drop sizes (100-mu-m and 150-mu-m). These data indicate that the P. xylostella larvae showed no behavioural avoidance of deposits when feeding, but ingestion of permethrin inhibited feeding in the more susceptible population. Two equivalent measures of avoidance when moving, the average distance travelled between contacts with droplets and the average number of contacts per unit distance travelled, were not different for the two populations, indicating no difference in the average ability of insects from these populations to avoid deposits of permethrin. The average distance travelled and the average number of contacts with droplets during the 90-min period was significantly greater for the more susceptible population, indicating that the behaviour of the two populations differed. These differences are consistent with a post-contact irritation in the more physiologically susceptible larvae.