Behavioral Responses of the Horn Fly (Diptera: Muscidae) to Selected Insecticides in Contact and Noncontact Environments

Abstract

Pyrethroid resistant and susceptible populations of horn flies were exposed to several insecticides including 3 pyrethroids, 2 organophosphates, 1 carbamate, DDT, and 3 synergist combinations. Horn flies were exposed to log-dose concentrations ranging from 5 X 10 -6 to 5 X 10° mg/cm 2 in both contact (irritancy) and noncontact (repellency) environments. This is the 1st clarification of the mechanisms of stimulus-dependent behavioral resistance in horn flies ; the data demonstrate the existence of both irritancy and repellency. Significant differences in flight responses were observed between populations, indicating hypersensitivity of resistant flies in both contact and noncontact environments for several compounds. Resistant horn flies had lower sensitivity thresholds for permethrin, fenvalerate, pirimiphos-methyl, DDT, and fenvalerate with piperonyl butoxide (PBO) when compared with susceptible 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 flight response curve for the pyrethroid resistant population increased for all compounds in the contact environment and most compounds in the noncontact environment, whereas the response curve remained constant or decreased in the pyrethroid susceptible population. The increased irritability and repellency seen with the addition of the synergist in a resistant population exhibiting knockdown (physiological) resistance suggests that nongeneralized intoxication may be the sensory mechanism by which behavioral resistance occurs.