The status and development of insecticide resistance in Danish populationsof the housefly Musca domestica L

Samples of housefly (Musca domestica) field populations were collected from Danish livestock farms in 1997. The tolerance of the first-generation offs pring was determined for a number of insecticides. Dose-response values wer e obtained by topical application for the pyrethroids bio-resmethrin and py rethrum, both synergised with piperonyl butoxide, and the organophosphate d imethoate. The organophosphates azamethiphos and propetamphos and the carba mate methomyl were tested in discriminating dose feeding bioassays. Resista nce was low to moderate in mast of the populations for most of the compound s tested, but this study also revealed the existence of high resistance to pyrethroid, organophosphate and carbamate insecticides in some populations. The resistance factors at LD50 for bioresmethrin/piperonyl butoxide ranged between 2 and 98, and for pyrethrurulpiperonyl butoxide between 2 and 29. Our results indicate that pyrethroid resistance in Denmark is increasing, s ince four of the 21 farms showed more than 100-fold resistance at LD95 a le vel of resistance only observed once before. Resistance factors at LD50 for dimethoate ranged from 9 to 100, and showed two distinct trends: populatio ns with either decreasing or increasing resistance. Resistance to azamethip hos was found to be widespread and high. Although two strains with high met homyl and propetamphos resistance were observed, methomyl and propetamphos resistance is moderate and appears not to be increasing. (C) 2001 Society o f Chemical Industry.