Insecticidal and growth inhibitory effects of some thymol derivatives on the beet armyworm,Spodoptera exigua(Lepidoptera: Noctuidae) and their impact on detoxification enzymes

Thymol is a known natural product with insecticidal activity against several insect species. A recent study on structural modifications of thymol to thymyl esters and their efficacy against Spodoptera litura suggested that such an approach could develop generalized novel insecticides/insect growth inhibitors and requires further studies to establish the efficacy against lepidopterans.Thymol and structurally modified eight esters were evaluated against beet armyworm, Spodoptera exigua using the topical application. Thymyl butanoate was the most toxic compound with a median lethal dose (LD50 ) of 2.33 and 1.62 μg/larva after 24 and 48 h posttreatment, respectively. All thymyl esters were potentially better than the parent compound thymol, except thymyl dibromoacetate, in their efficacy against Spodoptera exigua. Essentially, there were three levels of activity vis-à-vis the compounds used, that is, with the LD50 range of 1.5 to 5.0, 7.0 to 15.0, and > 20 μg/larva, respectively. Ovicidal activity and reduction in larval growth were also determined by treating third instars at sub-lethal doses, that is, LD50 doses of second instars. Thymyl butanoate treated larvae inhibited glutathione S-transferase, carboxylesterase, and acetylcholinesterase activities, whereas the other thymyl esters induced these enzymes.Thymyl butanoate exhibited higher toxicity against Spodoptera exigua and is the first to report about > 15.5× more toxicity than thymol and > 6.5× than thymyl cinnamate, which suggests that the efficacy was species-specific versus the chemical structural variation of the esters. © 2021 Society of Chemical Industry.