Toxicological and Oviposition Suppression Responses of Field Populations of the Australian Sheep Blowfly, Lucilia cuprina (Wiedemann) (Diptera: Calliphoridae) to the Pyrethroid Cypermethrin

Pyrethroid oviposition suppressants are an alternative to cyromazine for the prevention of flystrike on sheep by organophosphate resistant sheep blowflies. Laboratory bioassays were used to determine the relationships between toxicological responses to, and oviposition suppression by, cypermethrin in Australian field populations of L. cuprina, which might indicate pre‐selection for resistance. There was no correlation between the toxicological responses of adult and larval blowflies to cypermethrin in over 100 populations sampled. Toxicological responses of field larvae were substantially different from that of the susceptible strain LS but the oviposition suppression response and adult toxicological response of LS was within the range of field responses. As well, larval responses to cypermethrin were significantly correlated with those to diazinon. This suggests a low‐level cross‐resistance between the two compounds. Although organophosphate resistance is widespread in field populations, and despite 13 years of pyrethroid use on sheep, the low‐level cross‐resistance to cypermethrin has not interfered with susceptibility to oviposition suppression by cypermethrin.