CHARACTERIZATION OF GENERAL ESTERASES FROM SUSCEPTIBLE AND PARATHION-RESISTANT STRAINS OF THE GREENBUG (HOMOPTERA, APHIDIDAE)

A susceptible and two parathion-resistant strains of the greenbug, Sch izaphis graminum (Rondani), exhibit three different patterns of genera l esterase isozymes in native polyacrylamide electrophoresis gels. Cha racterization of general esterase activity using alpha-naphtholic este rs as model substrates indicated that the three strains differed in is ozyme composition. The type-II strain, which had the highest level of resistance, exhibited the highest levels of general esterase activity under all assay conditions, and the type-I strain had consistently hig her levels than the susceptible strain, In all three strains, these es terases were more active toward alpha-naphtholic esters with side chai ns of six or fewer carbon atoms. alpha-Naphthyl propionate was the opt imal substrate for both susceptible and type-I strains, and alpha-naph thyl butyrate for type II. Over 90% of esterase activity was localized in the cytosolic fraction of type-II greenbugs. In susceptible and ty pe-I greenbugs, the activity was distributed equally between the cytos olic and microsomal fractions. Differences in kinetic properties of th e general esterases from the three strains also were evident, further indicating differences in isozyme composition. Although the three stra ins differed in properties of the general esterase activities, the dif ferences do not provide sufficient discrimination to distinguish relia bly among the three strains using single aphid activity measurements.