Biochemical characterization of chlorphoxim resistance in adults and larvae of the Simulium damnosum complex (Diptera: Simuliidae)

Chlorphoxim resistance is found in forest strains of Simulium sanctipauli Vajime & Dunbar, in some areas of the Onchocerciasis Control Programme in West Africa. There is an increase in esterase activity in both adults and larvae in the resistant compared to the susceptible strain with the general esterase substrate 1-naphthyl acetate. This appears to be associated with the temephos and chlorphoxim resistance which are found in the same resistant strain. It is possible that one or more esterases are qualitatively and/or quantitatively changed in the resistant strain compared to the susceptible. Preliminary studies indicated that esteratic hydrolysis and oxidative cleavage were both involved in chlorphoxim metabolism in S. damnosum Theobald sensu latu. There was evidence of an increase in detoxication by both these routes in the resistant compared to the susceptible strain of the cytospecies S. sanctipauli. The increase in oxidative detoxication seen in the resistant strain was associated with a slight, but non-significant increase in cytochrome P450 as measured by the sodium dithionite reduced carbon monoxide difference spectra.