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Isozyme studies revealed the existence of an autosomal, male-limited esterase locus (Est-m) in species of the virgifera group of Diabrotica found in the United States. The locus is characterized by the presence of active and null alleles, and particular taxa are monomorphic or polymorphic for the presumptive Est-m variants. Investigations of the properties of an Est-m form present in the western corn rootworm, Diabrotica virgifera virgifera LeConte, produced data to indicate that the enzyme is a carboxylesterase and a glycoprotein with a relative molecular weight of 78,000 ± 8,680 SD. Additional work has shown that Est-m activity is localized in the accessory glands of virgin, adult males 12-14 d old. Moreover, the enzyme is passed to females during mating. The molecular properties of Est-m, its sex-limited expression in the accessory glands of males, and its subsequent occurrence in mated females suggest strongly that it functions in some way in reproduction.