Toxicity and Synergism of Insecticides Against Susceptible and Pyrethroid-Resistant Third Instars of the Tobacco Budworm (Lepidoptera: Noctuidae)

Abstract

Third-instar tobacco budworms, Heliothis virescens (F.), susceptible or resistant to pyrethroids were tested by exposure to residues of insecticides in glass liquid scintillation vials. These bioassays were done to find alternative insecticides or insecticide combinations for control of larvae resistant to pyrethroids, to evaluate the effect of synergists on insecticide toxicity and resistance, and to establish the main mechanisms of resistance present. Insecticides effective for control of resistant third-instar tobacco budworms included profenofos, sulprofos, acephate, endosulfan, methomyl, and thiodicarb. Cypermethrin combined with chlordimeform, piperonyl butoxide, or both was highly effective against resistant larvae. With other insecticides, neither piperonyl butoxide nor chlordimeform produced more than five-fold synergism. The only synergistic insecticide combination against resistant larvae was cypermethrin plus methyl parathion. Thus, use of other combinations had no advantage compared with use of alternative insecticides. High-level cypermethrin resistance, resistance to methyl parathion, and synergism of these insecticides by piperonyl butoxide provide evidence for metabolic resistance in this life stage. Therefore, we conclude that metabolic resistance seems to be the most important resistance mechanism in large tobacco budworms.