The fate of some organophosphorus compounds applied topically to peach‐potato aphids (Myzus persicae (Sulz.)) resistant and susceptible to insecticides

The fate of [14C]‐dimethoate and [14C]‐parathion after topical application to strains of Myzus persicae resistant and susceptible to organophosphorus compounds was investigated. In 4 h approximately 65% of the dimethoate or para‐thion applied was lost from the aphids by evaporation, and this limited the amount available for penetration into the insect. Only approximately 25% of the dose applied penetrated, of which a very small proportion was excreted as metabolites. There was no difference in the amounts penetrating into the resistant and susceptible strains, indicating that penetration was not a factor contributing to the observed resistance. These findings emphasise that evaporation from the cuticle can greatly influence results when insecticides are applied topically.