Imidacloprid susceptibility survey and selection risk assessment in field populations of Nilaparvata lugens (Homoptera: Delphacidae)

Imidacloprid has been used for many years to control planthopper Nilaparvata lugens (St?l) (Homoptera: Delphacidae) in China. To provide resistance assessment for the national insecticide resistance management program, we collected a total of 42 samples of the planthoppers from 27 locations covering eight provinces to monitor their dose responses and susceptibility changes to imidacloprid over an 11-yr period (1996-2006). Results showed that most field populations maintained susceptibility from 1996 to 2003 except for a population from Guilin, Guangxi, in 1997, which showed a low level of resistance to imidacloprid. However, surveys conducted in 2005 indicated that 16 populations from six provinces quickly developed resistance with resistance ratios ranging from 79 to 811. The data collected in 2006 revealed that the resistance levels in 12 populations collected from seven different provinces decreased slightly (RR = 107-316), except the Tongzhou population (Jiangsu Province), which developed 625-fold resistance. Dominant and intensive use of imidacloprid in a wide range of rice, Oryza savita L., growing areas might be a driving force for the resistance development. Migration of the insect also significantly boosted the resistance levels due to extensive and intensive use of imidacloprid in emigrating areas and continuous postmigration sprays of the chemical. In addition, laboratory resistance selection using imidacloprid showed that resistance ratio increased to 14-fold after 27 generations, suggesting that quick resistance development might be associated with more frequent applications of the insecticide in recent years.