BEHAVIORAL-RESPONSES OF THE HORN FLY (DIPTERA, MUSCIDAE) TO SELECTED INSECTICIDES IN CONTACT AND NONCONTACT ENVIRONMENTS

Pyrethroid resistant and susceptible populations of horn flies were ex posed to several insecticides including 3 pyrethroids, 2 organophospha tes, 1 carbamate, DDT, and 3 synergist combinations. Horn flies were e xposed to log-dose concentrations ranging from 5 x 10(-6) to 5 X 10 de grees mg/cm(2) in both contact (irritancy) and noncontact (repellency) environments. This is the Ist clarification of the mechanisms of stim ulus-dependent behavioral resistance in horn flies; the data demonstra te the existence of both irritancy and repellency. Significant differe nces in flight responses were observed between populations, indicating hypersensitivity of resistant flies in both contact and noncontact en vironments for several compounds. Resistant horn flies had lower sensi tivity thresholds for permethrin, fenvalerate, pirimiphos-methyl, DDT, and fenvalerate with piperonyl butoxide (PBO) when compared with susc eptible flies in the contact environment. Lower sensitivity thresholds for resistant horn flies were also observed for permethrin, diazinon, and diazinon-PBO in the noncontact environment. In addition, the flig ht response curve for the pyrethroid resistant population increased fo r all compounds in the contact environment and most compounds in the n oncontact environment, whereas the response curve remained constant or decreased in tile pyrethroid susceptible population. The increased ir ritability and repellency seen with tile addition of the synergist in a resistant population exhibiting knockdown (physiological) resistance suggests that nongeneralized intoxication may be the sensory mechanis m by which behavioral resistance occurs.