Synergism of Malathion Against Resistant House Flies and Mosquitoes

Eighteen tris-substituted derivatives of phosphoric acid were evaluated as synergists for malathion and for several other organophosphate insecticides against resistant strains of the house fly (Musca domestica L.) and the mosquito Culex tarsalis (Coquillett). Many of the materials markedly synergized malathion againt resistant insects, but caused little or no increase in its toxicity to suseeptible insects. In most cases with the insect populations studied the synergists did not overcome resistance present to organophosphates other than malathion. The pattern of synergism differed between flies and mosquitoes, both in range of effective materials and in degree of synergism obtained. It was considered that the synergists worked by blocking the increased ability of resistant insects to degrade either malathion or its oxygen analog, malaoxon. The results provided evidence that with eertain synergists resistance to organophosphate insecticides could be overcome by inhibition of degradation mechanisms.