Ishaaya, I., Division of Entomology, Agricultural Research Organisation, The Volcani Center, Bet Dagan, 50-250, Israel
Elsner, A., Division of Entomology, Agricultural Research Organisation, The Volcani Center, Bet Dagan, 50-250, Israel
Ascher, K.R.S., Division of Toxicology, Agricultural Research Organisation, Volcani Center, Bet Dagan, 50-250, Israel
Casida, J.E., Pesticide Chemistry and Toxicology Laboratory, Department of Entomological Sciences, University of California, Berkeley, California, 94720, United States

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. Copyright © 1983 John Wiley & Sons, Ltd