Differential susceptibilities to pyrethroids in field populations of Chilo suppressalis (Lepidoptera: Pyralidae)

To assess the feasibility of pyrethroids for rice insect control, we examined susceptibilities of six field populations of rice stem borer Chilo suppressalis (Walker) to 10 pyrethroids using the topical application method in laboratory in 2004 and 2005. Our results showed that the seven pyrethroids with high fish-toxicity (i.e., β-cyfluthrin, λ-cyhalothrin, β-cypermethrin, deltamethrin, S-fenvalerate, α-cypermethrin, and fenpropathrin) were more effective against C. suppressalis than the three compounds with low fish-toxicity (i.e., cycloprothrin, etofenprox, and silafluofen). The results also showed that all 10 of the pyrethroids were much more effective than methamidophos and monosultap for C. suppressalis control. In addition, we found that susceptibilities of some field populations of C. suppressalis to some high fish-toxicity pyrethroids were significantly reduced, and our results indicated that a Ruian (RA) field population showed a year-to-year variation in susceptibility to most tested pyrethroids between 2004 and 2005. Our data indicated that the tolerance levels increased dramatically in RA population, especially to β-cyfluthrin and deltamethrin. This study provided the first assessment of resistance to pyrethroids in field populations of C. suppressalis. In addition, a close correlation between resistance ratios to the 10 compounds and differences of the structures of these compounds was established in the RA05 population, which was resistant to most of the pyrethroids tested while it was still very susceptible to fenvalerate with no cross resistance. Finally, the feasibility and precaution were discussed in selecting pyrethroids as alternatives to replace high toxicity organophosphates for C. suppressalis control and insecticide resistance management.