Following the 1984–85 housefly (Musca domestica L.) resistance survey, a similar survey was carried out between January 1990 and April 1992 in order to monitor changes in UK housefly resistance. Samples of fly populations were collected from 35 farms throughout England and Wales. Dose‐response data were obtained by topical application and feeding test methods. For both methods the knockdown (KD) after 48 h was used for all the dose‐response comparisons. The ranges of resistance factors for the topical application method were for methomyl, 1·6–20·0 at KD50 and 4·5–34·4 at KD95; for azamethiphos, 2·5–58·5 at KD50 and 5·0–2604 at KD95; for pyrethrins+piperonyl butoxide, 1·2–9·6 at KD50 and 1·6–14·7 at KD95; and for permethrin, 2·2–118·8 at KD50 and 4·3–200·0 at KD95. The ranges of the resistance factors for the feeding tests were for methomyl, 1·2–56·1 at KD50 and 3·1–80·0 at KD95, and for azamethiphos, 3·9–125·0 at KD50 and 4·4–380·0 at KD95. The means of resistance factors for all of the insecticides showed increases over those obtained in a previous survey carried out in 1984–85. The significance of the results for housefly control in intensive animal units in the UK is discussed.

Field strains of housefly collected from animal units were exposed to the insecticide baits, Golden Malrin and Alfacron in laboratory cage tests. Flies were exposed to each bait alone in no-choice tests and to the bait plus sugar in choice tests. Golden Malrin was generally effective against the strains tested in no-choice tests; the lowest recorded knockdown after exposure for 48 h was 82%. Eight strains, however, gave a reduced knockdown when exposed in choice tests (between 60 and 87%). Golden Malrin was ineffective against one strain in choice tests with 7% knockdown at 48 h. With Alfacron 12 strains exhibited knockdown responses between 57 and 87% at 48 h in no-choice tests. In choice tests, 17 strains gave knockdown responses of less than 50% at 48 h. Correlation coefficients were calculated for results from cage tests and levels of resistance to the active ingredients assessed using topical application and feeding test methods. There was a positive correlation between cage and feeding test results. Correlation was, however, less significant with choice test results than with no-choice results. There was no correlation between cage test and topical results. It is suggested that behavioural resistance was responsible for the reduced responses shown by some strains in choice tests compared to no-choice tests. The results demonstrate the importance of using a laboratory method which allows for behavioural differences when monitoring for resistance to insecticides formulated as baits. Behavioural differences in housefly strains should be considered when testing the efficacy of new bait formulations. It is also suggested that insecticide baits should not be used continuously and exclusively to control houseflies in intensive animal units.