POSSIBLE MECHANISM OF DIAZINON NEGATIVE CROSS-RESISTANCE IN PYRETHROID-RESISTANT HORN FLIES (DIPTERA, MUSCIDAE)

The underlying mechanism responsible for enhanced toxicity of the orga nophosphate diazinon in eight pyrethroid-resistant horn fly Haematobia iritans (L.), field populations was investigated. In vitro enzyme ass ays and in vivo pyrethroid synergist studies showed increased activity of tile mixed function oxidase system (measured as O-demethylase acti vity) in five of the eight populations with, in most instances. a subs equent increase in diazinon metabolism. We suspect that enhanced toxic ity of diazinon (or negative cross-resistance), as observed in pyethro id-resistant horn fly field populations, may result from in vivo accum ulation of the toxic metabolite diazoxon.