RESPONSE OF STRAINS OF HOUSEFLY, MUSCA-DOMESTICA (DIPTERA, MUSCIDAE) TO COMMERCIAL BAIT FORMULATIONS IN THE LABORATORY

Field strains of housefly collected from animal units were exposed to the insecticide baits, Golden Malrin and Alfacron in laboratory cage t ests. Flies were exposed to each bait alone in no-choice tests and to the bait plus sugar in choice tests. Golden Malrin was generally effec tive against the strains tested in no-choice tests; the lowest recorde d knockdown after exposure for 48 h was 82%. Eight strains, however, g ave a reduced knockdown when exposed in choice tests (between 60 and 8 7%). Golden Malrin was ineffective against one strain in choice tests with 7% knockdown at 48 h. With Alfacron 12 strains exhibited knockdow n 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. C orrelation coefficients were calculated for results from cage tests an d levels of resistance to the active ingredients assessed using topica l application and feeding test methods. There was a positive correlati on between cage and feeding test results. Correlation was, however, le ss significant with choice test results than with no-choice results. T here was no correlation between cage test and topical results. It is s uggested that behavioural resistance was responsible for the reduced r esponses shown by some strains in choice tests compared to no-choice t ests. The results demonstrate the importance of using a laboratory met hod which allows for behavioural differences when monitoring for resis tance 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 int ensive animal units.