SUSCEPTIBILITY OF STABLE FLIES (DIPTERA, MUSCIDAE) FROM SOUTHEASTERN NEBRASKA BEEF-CATTLE FEEDLOTS TO SELECTED INSECTICIDES AND COMPARISON OF 3 BIOASSAY TECHNIQUES

Insecticide susceptibility of field populations of stable flies, Stomo xys calcitrans (L.), was assayed using 3 exposure techniques: treated filter papers, treated glass petri dishes, and topical applications. B oth topical applications and residual exposure to treated glass surfac es were suitable for testing susceptibility of stable flies to permeth rin, stirofos, or methoxychlor. Residues on filter papers yielded inco nsistent results with stirofos and methoxychlor. Significant concentra tion-mortality regression lines were generated with permethrin residue s on filter papers, but approximate to 1,000 times more insecticide wa s required to produce a toxic response when compared with permethrin r esidues on glass. Because of higher variability in response and the gr eater amount of insecticide required, residues on filter papers do not appear appropriate to test insecticide susceptibility in stable flies . Paired comparisons of field (F) and susceptible (S) stable flies res ulted in field to susceptible ratios significantly >1.0 only when the flies were treated topically, which suggests that topical application is more sensitive than residues on glass for the insecticides tested. Topical treatment with permethrin resulted in one FS(LD90) of 1.8-fold . Topical treatment with methoxychlor resulted in one FS(LD90) of 3.4- fold. However, the magnitude of these ratios is not larger than the si gnificant differences observed within the susceptible laboratory colon y from one generation to another. Intense exposure to insecticides is not known to have occurred in these field populations, indicating that the observed differences are the result of natural variation among st able fly populations and unrelated to prior selection with insecticide s.