Initial behavioural responses of Aphis gossypii to defined deposits of bifenthrin on chrysanthemum

Abstract

Uniformly sized droplets (60–160 μm in-flight diameter) of bifenthrin were applied to the abaxial surface of portions of chrysanthemum leaves. Adult Aphis gossypii were exposed to the freshly dried deposits and their activity during the next 20 min was video-taped using a camera placed beneath the treated area. On unsprayed or water-sprayed control leaves very little movement was recorded (mean distance walked ∼0·5 cm aphid −1) but aphids walked >2·0 cm during the exposure period in response to particular combinations of droplet size, droplet density and concentration of active ingredient (a.i.). Records of distance walked described the data satisfactorily, in most cases, when regression analysis was performed to show the effect of changing droplet density at each drop size and concentration combination. However, the description was improved when the number of aphids that moved during the experimental period, and the number that walked or dropped off the treated area, were also used to derive an irritancy index. Using this index, it was established that the irritancy was not dose dependent as a 50-fold range in quantity of a.i. cm −2 elicited the same response when presented to the aphids in deposits of different quality. However, there was some evidence for a minimum effective dose at the lowest concentration. The implications of these data are discussed in relation to the dose transfer process and the longer-term effects of sub-lethal behavioural responses.