Biochemical and Genetic Aspects of Malathion-Specific Resistance in the Indianmeal Moth (Lepidoptera: Pyralidae)1

Malathion resistance in a strain of Plodia interpunctella (Hübner) was highly specific for malathion and was suppressed by nontoxic carboxylesterase inhibitors. Fifth instars of the resistant strain had 33 times as much malathion carboxylesterase activity but only 0.30 times as much α-naphthyl acetate esterase activity as larvae of a susceptible strain. Resistance was controlled by a single autosomal gene or closely linked set of genes. Resistance and malathion carboxylesterase were inherited as dominant and codominant factors, respectively, and were genetically linked. Thus, malathion resistance in this insect is due to an altered esterase, just as is true for several dipterans. Comparison of 10 α-naphthyl acetate esterase isozymes resolved by electrophoresis revealed seven interstrain differences.