Development of resistance to pyrethroids in field populations of danish houseflies

Abstract

AbstractHouseflies (Musca domestica) on Danish farms have developed high multiresistance to organophosphorus compounds, after successive use of several OPs, mainly dimethoate, in recent years. Topical application tests 1971–73 with flies from many farms showed that the high OP‐resistance did not involve resistance to pyrethroids (± the synergist piperonyl butoxide (pb)) above a level of 3–7 x, unless field pressure with synergised pyrethrum (py/pb) or other pyrethroids was applied. In 1971–72 moderate to high, often heterogeneous, pyrethroid resistance was found on a few trial farms treated frequently with pyrethroid aerosols (mainly py/pb) and in 1973 on most of 23 trial farms treated intensively with aerosols (or space spray) containing py/pb, bioresmethrin ± pb, tetramethrin/pb or tetramethrin/resmethrin. The effect of field pressure with these different pyrethroids on development of pyrethroid resistance is summarised and discussed. Maximum resistance ratios, R/S at LD50‐LD95, were: py/pb (1:5), 40‐>100; bioresmethrin, 191–770; bioresmethrin/pb (1:5), 55–133; tetramethrin/pb (1:5), 171‐>200; tetramethrin/resmethrin (1:5), 78‐>370. The intensity of selection pressure with pyrethroids is believed to be an important factor. Although py/pb has been widely used as a supplementary fly control on Danish non‐trial farms, pyrethroid resistance has only been found on a few of them.