Interaction between synergists and permethrin in adults of the red flour beetle, Tribolium castaneum (herbst)

Abstract The potential synergistic activities of chlordimeform, S,S,S‐tributyl phosphorotrithioate (DEF), and piperonyl butoxide were studied by exposing adults of the red flour beetle Tribolium castaneum (Herbst) to treated filter paper discs. Each of the chemicals was individually of low toxicity to T. castaneum. However, when tested in various combinations, chlordimeform: DEF (1:1 and 1:4) exhibited a strong synergistic interaction, but other pairs (chlordimeform: piperonyl butoxide, 1:4, or DEF: piperonyl butoxide, 1:4) exhibited no synergism. When tested at experimentally determined optimum ratios with the synthetic pyrethroid permethrin, synergism was observed for permethrin:chlordimeform (1:2) and for permethrin: DEF (1:2) but not for permethrin: piperonyl butoxide (1:2). The triple mixture permethrin:chlordimeform:DEF (1:2:2) was twice as toxic as permethrin:DEF (1:2) but only slightly more toxic than permethrin: chlordimeform (1:2). Although chlordimeform and DEF interact synergistically, and each synergises permethrin by unrelated mechanisms, these interactions were not expressed independently in a mixture of all three substances tested against T. castaneum.