Formamidine Synergism of Pyrethroid Toxicity to Twospotted Spider Mites (Acari: Tetranychidae)

The toxicity of binary mixtures of formamidines and pyrethroids to Tetranychus urticae Koch was investigated and compared with the toxicity of their individual components. A synergistic interaction was observed with 85 of the 104 formamidines examined in binary mixtures with cypermethrin; 29 of these improved toxicity 5-fold or greater. With regard to the different pyrethroids, there was evidence for suggesting that formamidine synergism generally was greatest with cypermethrin and deltamethrin, intermediate with fluvalinate, flucythrinate, and fenpropathrin, and least with fenvalerate and permethrin. The four best overall pyrethroid synergists were N′-(2,4-dimethylphenyl)-N-methy 1-N-(pheny Ithio)formamidine (Upjohn-42564), N′-(2,4,6,-trimeth yIpheny1)-N,N-dimeth yIformamidine, N′-(3,4-methylenedioxypheny 1)-N ,N-dimethylformamidine, and chlordimeform. The greatest synergism of cypermethrin was a 24-fold enhancement by the methylenedioxyphenyl formamidine; synergism by this formamidine of other pyrethroids ranged from 8.9-fold to only 2.5-fold. Structure-activity relationship studies clearly indicated that the ability of formamidines to effect synergism of pyrethroids was not inextricably associated with their lethality in spider mites. Thus, highly active synergists included some formamidines that were directly lethal to spider mites and some that demonstrated little, if any, direct toxicity to this organism.