Abstract
By taking regular observations on the number of house flies (Musca domestiea L.), stable flies (Stomoxys calcitrans (L.)) and Fannia species in barns of eastern Norway, it has been shown that stable flies make up a large part of the fly population during the summer months. Of 5954 flies observed in 11 barns during the summers of 1956 and 1957, 74.5% were stable flies, 15.3% house flies and 10.2% Fannia species. Four field-collected strains of stable flies showed differences of up to 5 to 6 times in their tolerance to DDT and lindane, when exposed to blotting paper treated with each of the insecticides. A higher level of resistance to DDT was built up in the laboratory by selecting one of the strains for this insecticide. After two laboratory generations the exposure time required to kill or knock down 50% of the flies (LT50) increased from 312 to 1440 minutes in the females, and from 78 to 918 minutes in the males. At the same time the LT50 for females from an unselected strain decreased from 312 to 78 minutes. Because of difficulties in the rearing technic some of the data were based on a low number of flies, and the cultures had to he discontinued after two laboratory generations.
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