Factors affecting resistance to insecticides in house-flies,Musca domesticaL. (Diptera: Muscidae). IV. The population biology of flies on animal farms in south-eastern England and its implications for the management of resistance

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Abstract
Ways in which the bionomics and dynamics of populations ofMusca domesticaL. can influence the development of insecticide resistance, and how resistance genes spread within and between farms was investigated in a three-year study of the biology and movement of flies on 63 pig-rearing farms in south-eastern England. House-flies survived winter only on 12 ‘overwintering’ farms where they bred in heated pig-rearing houses (‘closed buildings’) throughout the year. In late spring they appeared out doors, and their descendents founded populations on neighbouring ‘summer’ farms where pigs breed only in unheated (‘open’) buildings. There, flies reached peak numbers in August–September and died out by mid-November. Gene flow within and between farms was studied indirectly by mark-release-recapture of colour-marked adults, and directly by monitoring the diffusion of the visible marker genebwb(brown body) introduced into indigenous house-fly populations. Although movement between open buildings within a farm was unrestricted, dispersal between farms was limited, and gene flow between even adjacent closed buildings was indirect, and required more than one generation. Likewise, indirect and gradual gene flow during summer probably accounted for the similarity in type and frequency of other independent genetic markers of local overwintering populations. Thus closed buildings played a key role in house-fly ecology and population genetics. Unfortunately, control with persistent insecticides in these buildings ensures efficient resistance selection, ultimately resulting in its spread to all pig farms. Less selective control practices are needed at these sites.

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