Evaluation of horn flies (Diptera : Muscidae) from a pyrethroid susceptible colony for general and permethrin esterase activities

In this study we describe a nonradioactive single-fly microassay for permet hrin hydrolysis. We used this assay with a microplate assay for general est erase activity to evaluate the permethrin hydrolyzing and general esterase activities of aging pyrethroid-susceptible male and female ham flies, Haema tobia irritans (L.). We found substantial gender- and age-related differenc es regarding general esterase activity, permethrin sensitivity, and permeth rin hydrolyzing activity within the colony. Extracts of female flies collec ted 48 h after receiving their first blood meal yielded significantly great er esterase activity than male extracts. Aging female flies were more toler ant of permethrin than were male flies. In addition, a positive correlation was found to exist between the general esterase activity of aging females and their ability to hydrolyze permethrin.