Effects of selected synergists on insecticide toxicity in tobacco budworm (Lepidoptera: Noctuidae) in laboratory and field studies

Synergism of insecticide toxicity by piperonyl butoxide (PBO), triphenyl phosphate (TPP), and diethyl maleate (DEM) was examined in field-collected strains of the tobacco budworm, Heliothis virescens (F.), from Louisiana, Oklahoma, and Texas in 1992 and 1993. Pretreatment with PBO significantly (P less than or equal to 0.05) increased the toxicities of cypermethrin (9 of 18 strains), methomyl (3 of 9 strains), profenofos (2 of 12 strains), and thiodicarb (2 of 5 strains). However, PBO pretreatment significantly (P less than or equal to 0.05) decreased (i.e., antagonized) the toxicity of thiodicarb in 2 of the 5 strains tested. Similarly significant (P less than or equal to 0.05) antagonism of toxicity following TPP pretreatment was measured in bioassays with cypermethrin (2 of 5 strains) and profenofos (1 of 3 strains). Neither synergism nor antagonism was apparent in tests with DEM. In a replicated small-plot field test. 2 applications of cypermethrin (0.09 kg [AI]/ha) plus PBO (1.12 kg [AI]/ha) significantly (P less than or equal to 0.05) reduced numbers of tobacco budworm larvae and damaged squares compared to treatments with cypermethrin alone. Demonstration of synergism of cypermethrin toxicity by PBO in both laboratory and field studies suggests that metabolic resistance to pyrethroids is an increasingly important consideration in the management of midsouth populations of the tobacco budworm. However, penetration of radiolabeled cypermethrin into 5th instars was decreased significantly (P less than or equal to 0.05) following pretreatment with PBO, suggesting that some of the effects measured in tests with this com pound were not related to inhibition of metabolism.