| Nilaparvata lugens Stal (Brown planthopper, BPH) is one of the most important pests on rice. Imidacloprid is the key insecticide which has been widely used for control of this pest. At present, there is no any other available insecticides can act as substitute. So, study on imidacloprid resistance is very important for sustainable control of BPH. In this paper, the resistance survey, resistance selection, and insect fitness analysis were carried out to evaluate the risk for BPH to develop high resistance to imidacloprid. With selected susceptible strain and resistant strains, the biochemical and molecular mechanisms for the resistance were also, analyzed. Some valuable breakthroughs had been achieved.1. Field survey for insecticide resistance in BPHThe toxicity of 10 different kinds of insecticides to field BPH had been tested with the insects collected from Guilin, Anqing and Dongtai in 2000 and 2001. Results showed that imidacloprid was the most effective insecticide. BPH was also very sensitive to ethofenprox, which had not been used for control of this pest. But the conventionally used insecticides, such as malathion, fenitrothion, diazinon and fenvalerate, showed little effect on BPH and different level of resistance had been developed to these insecticides. No significant changes in BPH resistance was found among areas and different years. However, obvious decrease in sensitivity was found with imidacloprid. In conclusion, the field population of BPH was still susceptible to imidacloprid. But the resistance development might just start.2. Imidacloprid resistance selection with BPHA root-dipping method featuring systemic uptake of imidacloprid through roots was developed for resistance selection with BPH. This method was simple, convenient, less labor consumption and making rapid increase in the resistance. With this method, a strain of BPH with high resistance to imidacloprid was successfully selected, indicating BPH had the potential to develop high resistance to imidacloprid.During the selection, imidacloprid resistance was found with a phased increase,and there were two rapid increase periods. From the 1st to 9th generation, the resistance developed very slowly. Then, the two rapid increases occurred from 10 to 22nd and 26th to 32nd, and leaving 22nd to 26th (RR 77) and 32nd to 37th (RR 255) as two resistance developing plates. This result indicated that there might be two different mechanisms for imidacloprid resistance in BPH.3. The influence of imidacloprid resistance on the fitness of BPHThe relative fitness of resistant BPH (from Til, T25 and T35) had been tested by constructing life tables and taking susceptible BPH as control. The results showed that all the resistant BPH had obvious disadvantages in their development and reproduction. Comparing with susceptible strain, the 3 strains of resistant BPH had decreased their fitness (0.609, 0.172 and 0.106) dramatically. The fitness of T25 and T35 was even much lower and only 1/5-1/10 that of susceptible one. So, it was concluded that imidacloprid resistance in BPH can significantly depress their fitness, which could result in a quirk recovery of the sensitivity when the population did not contact with imidacloprid. This means that a reasonable resistance management with less imidacloprid use will efficiently delay and even stop the resistance development.4. Biochemical mechanisms for imidacloprid resistance in BPHThe biochemical mechanisms of imidacloprid resistance in BPH were studied by synergism test, cross-resistance analysis, detoxifying enzyme activity test and membrane protein binding experiment. The results showed that significant cross-resistance existed with only the insecticides targeted on nAChR. Esterase and glutathione S-transferase were not related to imidacloprid resistance in this pest. However, PBO had significant synergism on imidacloprid in all BPH strains tested, and the synergism was higher in resistant strains. Cytochrome P450-monooxygenases analysis also proved that the activity increase paralleled with the high resistance (RR 77)...
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