Toxicity and synergism of insecticides against susceptible and pyrethroid resistant neonate larvae and adults of the tobacco budworm lepidoptera noctuidae

Toxicities of insecticides to neonate larvae of susceptible (Stoneville) and pyrethroid-resistant (ICI) strains of the tobacco budworm, Heliothis virescens (F.), were measured using a glass liquid scintillation vial technique. Results showed that ICI neonates were resistant to all pyrethroids tested. We observed no resistance to alternate insecticides including organophosphates, carbamates, avermectin, and rotenone. Several insecticide combinations were synergistic against resistant neonates. Chlordimeform synergized almost all insecticides tested; synergism was usually greater with susceptible than with resistant neonates. Piperonyl butoxide synergized cypermethrin in susceptible and resistant neonates. In combination with chlordimeform, piperonyl butoxide was more synergistic against resistant neonates. The resistance spectrum of the ICI strain suggests that target site resistance to pyrethroids is most important in neonate larvae and that metabolic resistance is of secondary importance. Tests with adult tobacco budworms suggested that both target site and metabolic resistance were present, with target site resistance being more important. The similarity of resistance in adults and neonates confirms the validity of adult monitoring in predicting resistance in neonates, the stage at which insecticide control is usually aimed.