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), triphen yl phosphate (TPP), and diethyl maleate (DEM) was examined in field-co llected strains of the tobacco budworm, Heliothis virescens (F.), from Louisiana, Oklahoma, and Texas in 1992 and 1993. Pretreatment with PB O significantly (P less than or equal to 0.05) increased the toxicitie s of cypermethrin (9 of 18 strains), methomyl (3 of 9 strains), profen ofos (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 tes ted. Similarly significant (P less than or equal to 0.05) antagonism o f toxicity following TPP pretreatment was measured in bioassays with c ypermethrin (2 of 5 strains) and profenofos (1 of 3 strains). Neither synergism nor antagonism was apparent in tests with DEM. In a replicat ed 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 c ompared to treatments with cypermethrin alone. Demonstration of synerg ism of cypermethrin toxicity by PBO in both laboratory and field studi es suggests that metabolic resistance to pyrethroids is an increasingl y important consideration in the management of midsouth populations of the tobacco budworm. However, penetration of radiolabeled cypermethri n into 5th instars was decreased significantly (P less than or equal t o 0.05) following pretreatment with PBO, suggesting that some of the e ffects measured in tests with this com pound were not related to inhib ition of metabolism.