Pirimiphos-methyl resistance in two stored product mites, Acarus siro and Acarus farris, as detected by impregnated paper bioassay and esterase activity assays

The response to pirimiphos-methyl, in one strain of Acarus farris and two s trains of Acarus siro, was assessed using an impregnated filter paper bioas say and by the selection of adults following exposure to pirimiphos-methyl. It was concluded that one of the strains of A. siro was resistant to pirim iphos-methyl and that a major resistance mechanism was involved. The second strain of A. siro gave a response similar to that of a laboratory strain u nexposed to organophosphates and was considered to be susceptible. The A. f arris strain responded to selection at the ED50 but not at the ED99, and it was concluded that a minor resistance mechanism is present in this strain. Assays of esterase activity were used to attempt to identify the biochemic al mechanisms involved in the resistance detected by the bioassays. The A. farris and susceptible A. siro strains showed similar levels of esterase ac tivity but the esterase activity of the resistant A. siro strain was signif icantly greater. An increase in esterase activity followed selection of bot h the A. farris strain and the resistant A. siro strain. An acetylcholinest erase assay showed no significant difference between the susceptible and pi rimiphos-methyl selected strains of A. siro. The results suggest that ester ases are involved in the resistance to pirimiphos-methyl found in A. siro a nd A. farris but that in A. siro, at least, other mechanisms may also be pr esent.