Susceptibility and Mechanisms Underlying the Relative Tolerance to Five Organophosphorus Insecticides in Tobacco Budworms and Corn Earworms

Laboratory investigations were conducted to explain the higher insecticide tolerance in tobacco budworm (TBW) relative to corn earworm (CEW). The acute toxicities of five organophosphorus (OP) insecticides to TBW and CEW laboratory colonies were first measured using a standard topical application procedure. In vivo inhibition of acetylcholinesterase (AChE) and aliesterases (AliE) by the five compounds was determined for both species. In vitro inhibition of acetylcholinesterase and aliesterases by the corresponding oxons was also determined. With the exception of profenofos, all OPs were more toxic to the CEW colony than to the TBW colony. The tolerance ratios ranged from 1.3 for profenofos to 18.3 for chlorpyrifos. No significant differences in the amount of AChE activity in the two species were found. In vitro AChE inhibition studies revealed that AChE of the TBW colony had a lower sensitivity to inhibition by most of the oxons than that of the CEW colony. AChE sensitivity varied among different OPs but did not correlate with toxicity. Aliesterase activity toward three substrates (4-nitrophenyl butyrate, 4-nitrophenyl valerate, and 1-naphthyl butyrate) at three larval ages (small, medium, and large) was found to be significantly greater in the TBW colony than in the CEW colony. There was no significant difference in protein content in the two species, except in medium-size larvae. Substrate specificity of aliesterase from both species was 4-nitrophenyl butyrate > 1-naphthyl butyrate > 4-nitrophenyl valerate. All oxons except methyl paraoxon were potent in vitro inhibitors of AliE. However, TBW AliEs were less sensitive to inhibition than those of CEW. Both TBW and CEW larval homogenates increased the I50 of both paraoxon and chlorpyrifos oxon to bovine brain AChE, indicating that the oxons were detoxified by larval homogenates. Percentage detoxication was higher for TBW than for CEW and was greater for chlorpyrifos oxon than for paraoxon. In TBW, the AliE inhibitor DEF (S,S,S-tributyl phosphorothioate) synergized both methyl parathion and chlorpyrifos, and the level of synergism correlated with anti-AliE activity of the insecticide alone. Thus, a combination of lower sensitivity of AChE and higher specific activity and detoxication capacity of AliE appear to largely explain the higher degree of insecticide tolerance in tobacco budworm than in corn earworm.