| Cotton aphid, Aphis gossypii Glover, is one of the most important pest on cotton. Its special biological properties makes this pest easy to develop resistance to insecticides used frequently. It has been reported that cotton aphid had developed serious resistance to various traditional insecticides. Imidacloprid was introduced as a substitute, and has become the key insecticide for control of this pest since 1990s. However, there is no report on resistance to imidacloprid in field aphid populations so far. For evaluation of the risk for cotton aphids to develop resistance to this insecticide and understanding of the resistance mechanisms, laboratory resistance selection, field resistance survey, resistance mechanism analysis, and molecular cloning of the target genes were carded out. in this research.1. Laboratory selection for imidacloprid resistance in A. gossypiiThe Cotton aphid collected from cotton fields was continuously selected by spraying of imidacloprid every two weeks in the laboratory. After 25 treatments, the resistance was found to increase by 20.03 times and a cotton aphid strain was achieved with moderate resistance level. During the selection, the resistance to imidacloprid increased steadily untill the 19th spraying, and then the resistant level stabilized on about 20 times. It indicates that cotton aphid has the potential to develop moderate resistance to imidacloprid, even though the resistance development is slow.2. Field survey for insecticide resistance in A. gossypiiThe toxicity of imidacloprid, lambda-cyhalothrin and chlorpyrifos to field cotton aphid had been tested with the insects collected from Nanjing, Anyang, Beijing and Tai'an. Results showed that Nanjing aphid population was the most susceptible, and the relative resistance ratio to imidacloprid for the aphids from Anyang, Taian and Beijing were 2.21, 7.63 and 9.53, respectively. Based on the baseline reported in China, the data showed that cotton aphids in all these places had developed high resistance to organophosphorous and pyrethroids, except the moderate resistance of Nanjing population to organophosphorous. The survey also showed that A. gossypii had developed resestance to imidacloprid in some areas, which further confirmed the possibility for A. gossypii to develop imidacloprid resistance. 3. Studies on biochemical mechanisms of imidacloprid resistance in A. gossypiiThe biochemical mechanisms of imidacloprid resistance in cotton aphid were studied by testing the synergism of special enzyme inhibitors and the activity of detoxification enzyme in resistant and wild susceptible strain of A. gossypii. The results showed that the synergism of TPP and PBO was significantly higher in the resistant strain than in the susceptible strain, but DEM did not show any synergism on imidacloprid in both strains. Detoxification enzyme activity test in two strains showed that the activity of esterase (EST) in resistant strain is much higher than the susceptible strain (1.71 folds), however the activity of glutathione-S-transferase (GST) was similar (1.12 folds) in both strains. In addition, treatment with imidacloprid could obviously enhance the esterase activity of aphid populations. Thus, it was concluded that the enhanced esterase and cytochrome P450-monooxygenases detoxification might contribute to the moderate level of imidacloprid resistance in A. gossypii. Thus, some synergists, inhibitors of esterase and P450-monooxygenases, could be used to improve the efficiency of imidacloprid and delay the resistance development in pest control practice.4. Cloning and sequensing of nAChR genes from A. gossypii.With degenerate primers and RACE technique, two new a subunits full length genes and one subunit gene fragment of nAChR has been successfully cloned in A. gossypii, named Agα3, Agα4 and Agα5. The Agαt3 subunit gene is of 2,314 bp with open reading frame (ORF) 1,776 bp encoding 592 amino acids. The Agα4 subunit gene (including Agα4-1 and Agα4-2) is of 1,767 bp with ORF 1,602 bp encoding 534 amino acids, and the Agα5 subunit gene fragment is of 840 bp encoding 280 amino acids. Further analysis demonstrated that the cloned genes from A. gossypii had high identity with other insect nAChR subunits previously reported, especially in Myzus persicae and Sitobion avenae, with the similarity higher than 90%. These work founded the base for further work on A. gossypii nAChR and its possible target resistance mechanisms.In conclusion, this study has confirmed that the cotton aphid can develop resistance to imidacloprid, and in some areas it has actually developed the resistance. Analysis on resistance mechanism showed that the enhanced esterase and cytochrome P450-monooxygenases detoxification contribute to the imidacloprid resistance in A. gossypii, and we suggested some inhibitors of esterase and P450-monooxygenases, could be used to improve the efficiency of imidacloprid and delay the resistance development in pest control practice. Obviously these research results are significant for resistance management. In addition, the newly cloned nAChR subunit genes are also useful for further work on A. gossypii nAChR and its possible target resistance mechanisms.
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