Properties of Esterases from Lygus hesperus Knight (Hemiptera: Miridae) and the Roles of the Esterases in Insecticide Resistance

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Abstract
Esterases from Lygus hesperus Knight and their roles in insecticide resistance were studied with field populations. The esterases are B-type esterases; carboxyl esterases are predominant in susceptible (S)and resistant (R)populations. The optimal pH and temperature were 7.8-8.0 and 40-45°C, respectively. Km(Michaelisconstant) determined with I-naphthyl acetate at 43°C was similar in the Sand R populations; it was 8.8 × 10-6 M for males and 1.3 × 10-5 M for females. Vmax (maximal velocity) was 27.0 and I36.1 μmol/min per mg protein for the Sand R populations, respectively. The esterases in the R population did not differ from the esterases in the S population, either in response to inhibition by eserine, paraoxon, or trichlorfon or in affinity to substrate (I-naphthyl acetate). However, esterase activity was 4.2 to 6.1 times greater in the R population than in the S population. These results suggest that there are quantitative rather than qualitative differences between the two populations. The greatly increased activity in the R population is apparently due to the production of more carboxyl esterases, which may play an important role in insecticide resistance.