| Sixty-six plant extracts were screened for their insecticidal activities on the mustard aphid (Lipaphis erysimi Kaltenbach) using the spraying method of Potter. The results showed that three plant extracts, methanol extract of Croton tiglium L. fruit, ethanol extract of Picraasma quassioides bark and methanol extract of Gelsemium elegans Benth rattan, possess significant insecticidal activities and they gave 91100% corrected mortality at concentration of 10mg·ml-1 at 24h. The methanol extract of G. elegans rattan achieved 83.08% mortality against the green peach aphid [Myzus persicae (Sulzer)] and produced the most significant insecticidal activity among the three plant extracts by topical application (1.28 × 10-3mg/insect) method.The methanol extract of G. elegans rattan was isolated using organic solvents of increasing polarity and six parts of different polarities were collected. Each part was estimated of its contact toxicity (1.28 × 10-3mg/insect), repellency (1mg·ml-1) and systemic toxicity (1mg·ml-1) against the green peach aphid. The test results suggest that the chloroform part was most significant because of its higher contact toxicity and repellency effect. The mortality and repellency rate of chloroform part on the aphid 24h after administration were 86.33% and 82.26%, respectively, more significant than the other parts. Each part didn't produce systemic toxicity action within 48h at concentration of 1mg·ml-1. It also indicated that the active components should be in the part of chloroform.On the other hand, the anti-insect activity and toxicity of the other most common vegetable insect pests were tested as well. The results showed that the active part exerted lower activity against the 3rd-instar larvae of diamond back moth (DBM, Plutella xylostella L.) with the immersed method. Only 8.75% mortality was obtained after 72h of administration. With the immersed method, the active part exerted significant contact toxicity against the initial stage of large white butterfly [Pieris rapae (L.)] 5th-instar larvae and the final stage of P. rapae 4th-larvae at 96h and 72h , in which the mortality were 70% and 97.5%, respectively, at the concentration of 10mg·ml-1. The active part at concentration of 2mg·ml-1 produced significant antifeedant effect by leaf disc method on the 4th-larvae of P. xylostella and P. rapae at 24h, in which the antifeedant rate was 97.87% and 99.28%, respectively. At the same time,the growth of the 4th-larvae of P. rapae was tested at the concentration of 0.9mg·ml-1 by the method of leaf disc. The result showed that the active part reduced larvae average weight to 21.32mg/insect in comparison with 182.82mg/insect of the control after 72h of administration. The toxicity against the adult of green peach aphid was tested by the method of topical application at 24h and 48h, and the LD50 were 0.6431 ×10-3mg/insect and 0.3439 × 10-3mg/insect, respectively. The AFC50(by leaf disc method) of the 4th-laevae of P. xylostella and P. rapae at 48h were 174.85 × 10-3mg·ml-1 and 119.56 ×10-3mg·ml-1, respectively.The symptom of insects was recorded in bioassays. The result showed that the chloroform extract produced significant effect on the M. persicae adults and P. rapae larvae. Preliminary research on the action mechanism of chloroform part was performed on the most common vegetable insect pests, which base on the record of the symptom of insects. The results are as follows: (1) With the method of topical application, the activity of acetylcholinesterase (AchE) of treatment was 1.09nmol/min/ml which lower than that of the control. And the result showed that the activity of AchE of green peach aphid was inhibited by the chloroform part of the abstract. (2) The AchE activity of 5th-larvae of P. rapae was inhibited by the chloroform part. The activity of AchE was 1.70nmol/min/mg (Pr) at 4h, three times lower than that of the control. But the activity recovered gradually after 4h. (3) The respiratory intensity of 5th-larva...
|
PDF Full Text Request