Susceptibility of the Danish housefly, Musca domestica Linné to pyrethroid.

The insecticidal activity of a new pyrethroid, fenvalerate, and the effects of its synergists were evaluated using te 49-s and 381-s strains from Denmark which are both resistant to pyrethroid. It was found that the effect of fenvalerate was poor on the pyrethroid-resistant strains and little was expected from the addition of its synergists. Rapid development of resistance was observed in the 49-s strain during the selection with permethrin.