Chlordimeform synergies synthetic pyrethroids against both Heliothis zea and H. virescens. Synergism is greater against the latter species. The precise mechanism of the synergism is not known but probably involves target site effects rather than inhibition of metabolism. In preliminary test: with pyrethroid resistant insects from western Texas evidence was obtained that chlordimeform was more synergistic with a resistant strain than with laboratory insects.