Effects of Synergists on the Toxicity of Fenvalerate to Pyrethroid-Resistant Diamondback Moth

Synergism of fenvalerate with piperonyl butoxide (PB), NIA-16388 (NIA), S-421, and MGK-264 on both susceptible (S) and fenvalerate-selected (FS) strains of diamondback moth, Plutella xylostella L, were determined in laboratory tests. Further, combinations of fenvalerate and these synergists were tested for their efficacies against resistant strain under field conditions in the Cameron Highlands, Malaysia. Fenvalerate/PB showed higher synergism and efficacy than combinations of fenvalerate with other synergists against FS and pyrethroidsresistant strains in t h e field. A 1:3 ratio for fenvalerate and PB seemed to be optimal for the control of resistant diamondback moth. Combinations of fenvalerate with NIA, S-421 or MGK-264 showed slight synergism on the FS strain in the laboratory tests, whereas combinations with NIA and S-421 showed efficacy similar to that of combination with PB in the field test. S-strain of diamondback moth was selected by fenvalerate alone and in combination with PB. Selection by fenvalerate alone developed resistance, whereas selection by fenvalerate/PB resulted in a somewhat lower level of resistance. This suggests that fenvalerate/PB may be effective in preventing or retarding the diamondback moth from developing resistance.