MULTIPLE RESISTANCE IN THE LARGER HOUSE-FLY MUSCA-DOMESTICA IN GERMANY

Populations of the housefly Musca domestica isolated from farms in dif ferent German districts with strong resistance problems were compared to laboratory strains with varying resistance spectra Resistance again st pyrethroids, organophosphates and carbamates was tested using impre gnated filter papers, and by topical application using a susceptible h ousefly strain (origin WHO) for comparison. The multi-resistant fly st rains tested had a strong resistance against these insecticide groups, ranging from 37- to >10 000-fold for organophosphates and 150- to >66 00-fold for pyrethroids. The constituent enantiomer pairs of the alpha -cyano-pyrethroid cyfluthrin were tested, as was beta-cyfluthrin. With respect to multi-resistant fly strains, the isomers II and IV had the best activity, with LD,, values of 0.012 and 0.014 mu g per fly, resp ectively. In addition, different groups of insect growth regulators (j uvenile hormone analogues, chitin synthesis inhibitors and one triazin e derivative) were tested in a special larvicidal test. The chitin syn thesis inhibitors were quite effective against multi-resistant M. dome stica strains except for one strain with strong resistance against chi tin synthesis inhibitors, developed after extensive treatments with be nzoylphenylureas for several years. The fly strains tested were not re sistant against cyromazine. Additionally, the insecticides were combin ed with the synergists piperonyl butoxide, tributylphosphorotrithioate (DEF) and Cibacron blue and tested against the fly strain with the st rongest resistance spectrum ('Grimm') in comparison to the susceptible strain ('WHO-N'). Piperonyl butoxide had the greatest effect on the e fficacy of cyfluthrin followed by Cibacron blue and DEF. In a parallel investigation with susceptible and resistant house fly strains, diffe rent enzyme activities related with resistance mechanisms were tested, e.g. glutathione S-transferase (3.5-fold) and mixed-function oxidase (2.3-fold). Implications of these results for management of insecticid e resistance in M. domestica are discussed.