Development of resistance in the bean aphid, Aphis craccivora Koch., to various synthetic pyrethroids with special reference to change in susceptibility of some important aphid species during the last one and a quarter decade.

The relative toxicity of emulsions of seven pyrethroids was evaluated against the adults of Aphis craccivora Koch. in the laboratory. On the basis of LC50 values, lambdacyhalothrin, alphametrhin, decamethrin, cypermethrin, fenvalerate, fenproprathrin were respectively 57.08, 16.00, 9.997, 4.35, 3.81, 1.72 times more toxic than pyrethrin. Amongst the three aphid species, A. craccivora was more susceptible than M. persicae to all the pyrethroids tested, whereas L. erysimi was more resistant than A. craccivora except to pyrethrin. The most toxic insecticide among the pyrethroids against the three aphid species was decamethrin. A comparison of LC50 values for pyrethroids determined during the last one and a quarter decade revealed that there has been a considerable shift in the level of susceptibility to these pyrethroids. There was about 33.9, 14.8 and 14.5-fold increase in the LC50 values of emulsifiable concentrates of cypermethrin, decamethrin and fenvalerate, respectively. Pyrethrin also showed 14.7-fold increase but after a lapse of twenty-three years i.e., nearly twice the period during which resistance developed to synthetic pyrethroids.