Correlation of Est erase Isozymes to Malathion Resistance in the Small Brown Planthopper (Homoptera: Delphacidae)

Variation of esterase and its importance to malathion resistance was investigated in the small brown planthopper, Laodelphax striatellus (Fallén). It was confirmed that increased activity of malathion carboxylesterase was one of the most important correlative factors for malathion resistance and that the insensitivity of acetylcholinesterase to malaoxon was a rather minor factor in the small brown planthopper. Increased carhoxylesterase activity was detected by aliesterase activity. Several aliesterase isozymes, which were separated by isoelectrofocusing, showed a high activity in the resistant strain. Among them, activities of two isozymes (pI = 5.3 and 5.0) were correlated with malathion resistance, and they were found to be able to hydrolyze both aliphatic esters and 14C-malathion. These two isozymes were found to be components of an E7 band separated by agar gel electrophoresis, a band already known to be controlled by a single, codominant, autosomal factor.