Comparative Study of Malathion Toxicity and General Esterases in Larvae and Adults from a Field Population of Oxya chinensis (Thunberg) (Orthoptera: Acridoidea)

The susceptibility of Oxya chinensis to malathion was compared in larvae and adults from a field population, collected from Jinyuan outskirt, Shanxi Province. The results showed that Oxya chinensis was more susceptible to malathion in the adult stage than in the larval stage. The LD50 values for malathion susceptibility of Oxya chinensis were 4.94 and 2.44 mg g-1 body weight in the larvae and adults respectively. The results indicated that the larvae were 2.02-fold less susceptible to malathion than the adults. The general esterases and the kinetics were characterized and compared between the two life stages and between females and males. Larval preparations of Oxya chinensis were more active than adult preparations in females and males. The larvae showed 1.18-, 1.49-, and 1.17- fold higher specific activities than the adults in females with α-NA, α-NB and β-NA respectively. In males, the ratios were 1.34-, 1.70-, and 1.06-fold. Female preparations were more active than those of males in the adults. The reverse results were observed in the larvae where male preparations were more active than female preparations. Kinetic studies showed that Km values of general esterases hydrolyzing α-NA, α-NB, and β-NA in the adult stage were 1.36-, 1.32- and 1.39-fold respectively, higher than those in the larval stage in females. In males, the ratios were 1.24-, 2.14-, and 1.20-fold. The esterase from male insects had a higher affinity (lower Km value) to the substrate than those from females. The results also showed that the Vmax values of general esterase hydrolyzing α-NA, α-NB, and β-NA in the two stages were similar. From the results of bioassays and biochemical analyses, it has been inferred that a higher level of resistance to malathion in larvae than in adults would appear to result from differences in the expression of resistance mechanisms in these two life stages. Enhanced esterase activities appeared to play a major role in resistance to malathion in both larvae and adults. From the analysis of inhibition in vitro, the esterases in the two life stages were B-type, and carboxylesterases were predominant enzymes in the composition of the esterases in the two stages.

A Photosensitivity Insecticide, 5-Aminolevulinic Acid, Exerts EffectiveToxicity to Oxya chinensis (Orthoptera: Acridoidea)

5-Aminolevulinic acid （ALA）, a major photosensitivity insecticide, has attracted increasing attention as a new type of highly efficient, environmental friendly pesticide to be used to control the pest. To examine whether or not ALA acts effectively to grasshopper, Oxya chinensis and elucidate the detoxification mechanism of ALA, the susceptibility to ALA was assessed in O. chinensis and two major metabolic detoxification enzymes including glutathione S-transferases （GSTs） and general esterases （ESTs）-specific activities were compared in different development stages and different body sections of O. chinensis treated by ALA and the control. The results showed that the ALA exhibited obvious toxicity to the grasshopper in different development stages. In the low-dose treatment （0.0597 mmol L-1）, the mortalities of O. chinensis reached a significant level （55.5% in the 1st instar nymphs, 61.4% in the 2nd instar nymphs, 71.4% in the 3rd instar nymphs, and 64.4% in the 4th instar nymphs. But, there was no dose-dependent toxic effect. Thereby, we proposed that ALA has the potential for acting as photosensitivity insecticide for controlling O. chinensis. GSTs activity assays using CDNB and DCNB as substrates indicated that the thorax and abdomen of the different instar nymphs treated by ALA showed 1.52-5.56 fold significantly increased GSTs activities compared with the control. However, for the ESTs-specific activity assay, there was no significant difference between O. chinensis treated by ALA and the control within different instar nymphs, when a-NA, a-NB and b-NA were used as substrates. Therefore, GSTs-mediated metabolic detoxification as evidenced by significantly increased GSTs activities might contribute to protect against oxidative damage and oxidative stress by ALA in O. chinensis.

Toxicological and biochemical characterizations of malathion sensitivity in two field populations of Oxya chinensis （Orthoptera： Acridoidea）

We evaluate comparative toxicity of malathion in the two populations of the grasshopper Oxya chinensis, collected from Daixian and Fanshi of Shanxi province, China. General esterases and acetylcholinesterase （ACHE） from the two populations were characterized and compared. LD50 of the Daixian population （7.58 μg/g body weight） was 2.02-fold higher than that of the Fanshi population （3.75μg/g body weight）. General esterase-specific activities in the Daixian population were 1.91,130 and 1.85-fold higher than those in the Fanshi population, when α-NA, α-NB and β-NA were used as a substrate, respectively. Kinetic studies of general esterase showed that Vmax values of general esterases hydrolyzing α-NA,α-NB and β-NA in the Daixian population were 2.15-, 1.12-, and 1.47-fold, respectively, higher than those in the Fanshi population. The AChE activity of the Fanshi population was 1.54-fold higher than that of the Daixian population. Kinetic analysis of AChE showed that significant differences were presented between the two populations in the Km values; and the Vmax value in the Fanshi population was higher than that in the Daixian population. Inhibition studies of AChE indicated that AChE from the Daixian population was 2.56-, 2.80-, and 2.29-fold less sensitive to inhibition by paraoxon, chlorpyrifos-oxon, and demeton-S-methyl, respectively, than that from the Fanshi population. These biochemical characterizations of general esterases and AChE were consistent with malathion bioassay in the two populations. It is inferred that the reduced sensitivity of altered AChE and increased general esterase activities play an important role in the differences of insusceptibility of Oxya chinensis to malathion between the two populations.