Effect of Pyrethroids on Knockdown and Lack of Coordination Responses of Susceptible and Resistant Diamondback Moth (Lepidoptera: Plutellidae)

Abstract

Among DDT and the 10 pyrethroids tested, permethrin, cypermethrin, deltamethrin, and fenvalerate were most toxic against a field strain of Plutella xylostella (L.) that had high levels of resistance to all insecticides tested. Knockdown effect of these compounds against larvae and adults of susceptible and resistant strains was evaluated by a residual-film method. Symptoms of poisoning are described. When larvae or adults could no longer stand and had to lie on their sides, they were considered knocked down. Pyrethroids with primary alcohol esters produced more acute symptoms than those with secondary alcohol esters. Responses of resistant larvae and adults were less pronounced than those of susceptible ones. In the resistant strain, addition of synergist piperonyl butoxide (PB) to the pyrethroids produced symptoms displayed by the susceptible strain. Tetramethrin, which was not highly toxic, was the most potent knockdown agent against susceptible larvae. Knockdown action of all pyrethroids appeared much more slowly in resistant larvae; fluvalin ate and flucythrinate were least effective as knockdown as well as killing agents. Addition of synergists PB, S,S,S-tributyl phosphorotrithioate, and chlordimeform generally enhanced knockdown action of most pyrethroids in resistant larvae, but had only limited effect in susceptible larvae. Maximal knockdown of adults by fenvalerate was reached 60–90 min after exposure; PB synergized this action of fenvalerate and reduced recovery of the moths within 24 h.