Reversal Of Resistance To Insecticides And Ecological Fitness To Rice Varieties Of Nilaparvata Lugens (Stal) Populations Collected From Rice Fields

Nilaparvata lugens (Stal), brown planthopper (BPH) is one of the serious insect pests of rice in China and other Southeast Asian countries. In recent years, the outbreak of BPH gradually increased and caused devastating damage to rice production. So far, chemicals are still main measures to control BPH. However, because of the long-term, successive and irrational use of chemical insecticides, the BPH developed high resistance to many insecticides. It’s crucial to clarify the resistance of BPH field populations in order to promote the reasonable application of insecticides and management of BPH. In addition, the adaptation of insect-resistant rice varieties is considered as the most economical, effective and environmental friendly strategy in the control of insect pests. In this study, BPH populations collected from rice fileds were massively reared in the lab free of insecticides for more than14generations, and the susceptibility change of BPH population against insecticides and ecological fitness on different rice varieties were monitored by generations. The results are as follows:1. Susceptibility variation of BPH field populations reared successively in the lab free of insecticidesThe resistances of BPH populations against imidacloprid, chlorpyrifos, fipronil and BPMC were declined with the increase of generations. The resistance ratio of Hangzhou and Jinhua BPH population against imidacloprid was declined significantly with generations, while the resistances ratio against chlorpyrifos and BPMC did not decrease on a large scale. The resistance ratio agianst fipronil was declined to a certain degree. The resistance ratio of Hangzhou BPH population against imidacloprid was declined with generations from359.94-fold at F1to6.50-fold at F14compared with susceptible strain Pila. The susceptibility of Hangzhou population after F8against imidacloprid remained stable. The resistance ratio of Jinhua BPH population against imidacloprid was declined with generations from1027.93-fold at F1to50.08-fold at F15. The susceptibility of Jinhua population against imidacloprid after F9remained stable. The resistance ratio of Hangzhou BPH population against chlorpyrifos was decreased from9.90-fold at F1to5.94-fold at F14. The format of reversion of BPH population to chlorpyrifos had differences with that to imidacloprid. The susceptibility of Jinhua BPH population to chlorpyrifos from F1to F7remained stable, while from F7to F9the resistance ratio declined from11.06-fold to4.63-fold. The resistance ratio of Hangzhou population agianst fipronil was declined from68.09-fold at F1to10.22-fold at F14, the susceptibility from F10to F14did not show significant change. The resistance ratio of Jinhua BPH population at F1and at F4was respectively166.23-fold and122.03-fold, and declined significantly at F7(31.32-fold). The resistance level remained stable. The resistance ratio of Hangzhou BPH population to BPMC was declined with generations from8.16-fold at F1to4.08-fold at F14, the resistance ratio of Jinhua BPH population to BPMC was declined with generations from10.03-fold at F1to2.04-fold at F15.2. Fitness on different rice varieties of BPH field populationsOn different resistant rice varieties (susceptible TN1, resistant IR26and IR36), nymphal duration and weight of newly hatched female adult from both Hangzhou and Jinhua were significantly affected by rice varieties, generations in lab and their interaction, meanwhile the nymphal survival rate was significantly affected by generations. The male brachypterous percentage of Hangzhou BPH population was affected by generations and interaction of rice varieties and generations, while no difference was found in female brachypterous percentage in Jinhua BPH population. The female proportion was not affected by generations and their interaction. On other rice varieties (TIC-19with cry1C gene, and T2A-1with cry2A gene, and their parental MH63), the nymphal duration of BPH from two population were both significantly affected by rice varieties, generations and their interaction except for female nymphal duration of Jinhua population. The nymphal survival of two populations was significantly affected by generations; meanwhile nymphal survival rate of Hangzhou population was significantly affected by interaction of rice varieties and generations. Weight of newly hatched female adult from Hangzhou was significantly affected by rice varieties and generations, while that from Jinhua was significantly affected by rice varieties, generations and their interaction. The proportion of female adults was not affected by rice varieties, generations and their interaction (except Hangzhou BPH population). Male brachypterous percentage of Hangzhou BPH population was affected by generations and interaction of rice varieties and generations, while no difference was found in female brachypterous percentage.3. Changes of detoxification enzymes in BPH populations free of insecticides in the lab for generationsEST and GST activities of BPH populations free of insecticides in the lab for generations were determined. Results showed that the EST activity of Hangzhou and Jinhua population and GST activity in Hangzhou population declined with generations, while GST activities of Jinhua populations remained stable as generation increased. ANOVA indicated that EST and GST activity of the two populations were significantly affected by generation, rice variety and their interaction.4. Detection of mutant nAChR gene associated with imidacloprid resistance in BPH populations free of insecticides in the lab for generationsChanges on the susceptibility variations in the BPH populations free of insecticides in the lab were observed. Therefore, the susceptibility of BPH against imidacloprid increased fast compared with susceptible population. In order to clarify the reversion mechanism, the gene frequencies of mutant nAChR associated with imidacloprid resistance in BPH populations were detected by PCR with specific primers. Results showed that no mutant nAChR gene was founded in Hangzhou population and Jinhua population for generations. These results suggested that the resistant reversion was not resulted from the mutant nAChR, but other reasons.