Possible mechanism of diazinon negative cross-resistance in pyrethroid-resistant horn flies (Diptera: Muscidae)

The underlying mechanism responsible for enhanced toxicity of the organophosphate diazinon in eight pyrethroid-resistant horn fly, Haematobia irritans (L.), field populations was investigated. In vitro enzyme assays and in vivo pyrethroid synergist studies showed increased activity of the mixed function oxidase system (measured as O-demethylase activity) in five of the eight populations with, in most instances, a subsequent increase in diazinon metabolism. We suspect that enhanced toxicity of diazinon (or negative cross-resistance), as observed in pyrethroid-resistant horn fly field populations, may result from in vivo accumulation of the toxic metabolite diazoxon