Published online by Cambridge University Press: 10 July 2009
A strain of Musca domestica Linnaeus with low pyrethroid resistance was divided into four sub-strains, each was subjected to three selections in the laboratory with either alphacypermethrin, cypermethrin, deltamethrin or permethrin. Each selected strain was then tested for cross-resistance to the other pyrethroids and to synergized pyrethrins. Pyrethroid resistance was increased by all selecting agents by a factor of between 1.5 and 3.6 as measured at the KD50, thus demonstrating that the newer persistent pyrethroids had a similar potential to select for pyrethroid resistance as did permethrin. A field trial using a commercial alphacypermethrin formulation was carried out in an intensive animal unit against a population of M. domestica with a high pyrethroid resistance. After a high initial kill, the population rapidly recovered and a second application failed to control the population. Pre- and post-trial M. domestica samples were taken and resistance to alphacypermethrin and synergized pyrethrins was shown to have increased from 54.5 to 263.6 and 15.2 to 28.4 at the KD50, respectively.