The joint action of malathion and IBP against malathion-resistant and -susceptible strains of Anopheles stephensi

Malathion resistance in an Anopheles stephensi strain from Pakistan is dependent on a single gene, which results in increased degradation of malathion to the monocarboxylic acid. Mixtures of malathion and the fungicide IBP (O,O-bis(1-methylethyl) S-phenylmethyl phosphorothioate) were tested against this resistant strain and a laboratory susceptible strain. The mixtures were more toxic to both the resistant and susceptible insects than either IBP or malathion alone. The high degree of synergism with the mixtures against the susceptible strain would not be expected if IBP were simply competing with malathion for a common carboxylesterase detoxification enzyme. The reason for the high degree of synergism in the susceptible strain is unknown.