| Abstract: | The potency of six dietary pyrethroids, as toxicants and inhibitors of weight gain in first- and fourth-instar Tribolium castaneum (Herbst) larvae, decreased in the order of cis-cypermethrin and deltamethrin > trans-cypermethrin and cis-permethrin > fenvalerate and trans-permethrin. Dosages that reduced larval weight also delayed pupation and emergence, probably due to their antifeeding activity. Three oxidase inhibitors (piperonyl butoxide, O, O-diethyl O-phenyl phosphorothioate, and O-isobutyl O-prop-2-ynyl phenylphosphonate), at a dietary concentration of 100 mg kg?1, had little or no effect on the toxicity of trans-permethrin, but strongly synergised the toxicity of cis-cypermethrin by about 3-, 3- and 10-fold, respectively. Piperonyl butoxide also synergised the toxicity of cis-permethrin, trans-cypermethrin and deltamethrin, but not that of fenvalerate. On the other hand, an esterase inhibitor, profenofos, did not enhance the potency of any of the α-cyano-3-phenoxybenzyl pyrethroids. Oxidases appear to be more important than esterases in pyrethroid detoxification by T. castaneum larvae. |
