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Mechanisms of resistance to organophosphorus insecticides in Culex tarsalis.

A 55.6-fold level of resistance to parathion in larvae of a multiresistant strain of Culex tarsalis Coquillett was shown to be due to slower penetration and increased metabolism of the toxicant. After 4 h exposure, 1.9× more water-soluble metabolites and 0.4× less intact parathion were recovered from the resistant strain relative to the susceptible. Comparable amounts of paraoxon were isolated from both strains. During the same period, larvae of the susceptible strain absorbed 1.3× as much parathion as resistant larvae.

In vivo tests revealed strong synergism of organophosphorus compounds in the resistant strain by DEF (S, S, S-tributyl phosphorotrithioate) but antagonism or no effect by piperonyl butoxide. In vitro assays indicated that resistance was not due to acetylcholinesterase insensitivity to inhibition by organophosphates.