Stage-Specific Insecticidal Activity of Thymol Against Colorado Potato Beetle and Time-Dependent Detoxification Gene Expression.

The contact toxicity of thymol, alpha-terpinene, and beta-pinene against Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) was evaluated by topical application. Thymol exhibited markedly higher insecticidal activity (51.67% mortality at 5%) than the other monoterpenes. Dose-mortality tests showed 100% third-instar larval mortality at 7.5% thymol. Probit analyze revealed third-instar larvae (LC50 = 3.82%) as the most sensitive stage, followed by fourth-instar larvae (LC50 = 6.69%) and adults (LC50 = 17.82%); adults were 4.66-fold more resistant. Gene expression analyses demonstrated significant time-dependent induction of glutathione S-transferase (GST; 2.83-fold at 24 h) and delayed induction of cytochrome P450 monooxygenase (CYP450; 1.91-fold at 24 h), while carboxylesterase (CarE) showed no significant change. These results support thymol as a promising biopesticide candidate targeting early larval stages, with GST showing the strongest transcriptional induction among the detoxification genes examined.