author(s)
R Surya Raj, N Muthukrishnan, N Sathiah and K Prabakar

abstract
Baseline susceptibility of neonicotinoid insecticide viz., imidacloprid 17.8% SL was carried out in rice whitebacked planthopper during 2019-20 at Tamil Nadu Agricultural university, Coimbatore. F3 generation of TNAU susceptible WBPH population was taken as base population and preliminary range tests were conducted to determine 50% mortality (LC₅₀). Initially doses were fixed from wider to narrow range. Different concentrations of insecticides giving the mortality range from 20-80% were taken for the study. Each concentration of insecticides was replicated thrice by releasing 15 adult female insects per mylar cages. After fixing the X dose for susceptible population, dose for the other population were fixed at different concentration. Laboratory cultured TNAU susceptible population and populations from Coimbatore, Bhavani and Nagapattinam regions were treated. The results revealed that Nagapattinam populations showed higher LC₅₀ and higher resistance level followed by Bhavani and Coimbatore population against the imidacloprid 17.8 SL. The resistance ratio varied from 2.8 - 6.2 fold among the populations against imidacloprid 17.8% SL.