Malathion Resistance in Indianmeal Moths (Lepidoptera: Pyralidae) Infesting Stored Corn and Wheat in the North-Central United States1

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Abstract
Forty-three strains of Pladia interpunctella (Hübner) were collected from grain bins on farms in nine midwestern states. Thirty-nine of the 43 strains were more than 17-fold resistant to malathion. None of 10 resistant strains tested were cross-resistant to chlorpyrifos methyl. Of six resistant strains assayed for malathion carboxylesterase activity, all had greatly elevated levels over those of a susceptible strain. The a-monoacid of malathion was the major hydrolysis product in all six resistant strains. When esterase electrophoretograms were prepared of 16 strains, ranging from purely susceptible to purely resistant, a good correlation was observed between resistance level and the staining intensity of certain esterase bands. We conclude that carboxylesterase-type malathion-specific resistance is epidemic in Indianmeal moth populations throughout the U.S. grain belt.