BACKGROUND: 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. RESULTS: 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 LD(50) of 2.33 and 1.62 microg/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 S. exigua. Essentially, there were three levels of activity vis-a-vis the compounds used, i.e., with the LD(50) range of 1.5 to 5.0, 7.0 to 15.0, and >20 microg/larva, respectively. Ovicidal activity and reduction in larval growth were also determined by treating third instars at sublethal doses, i.e., LD(50) doses of second instars. Thymyl butanoate treated larvae inhibited glutathione S-transferase, carboxylesterase, and acetylcholinesterase activities, whereas the other thymyl esters induced these enzymes. CONCLUSION: Thymyl butanoate exhibited higher toxicity against S. exigua and is the first report to show about >15.5x more toxic than thymol and >6.5x than thymyl cinnamate, which suggests that the efficacy was species-specific versus the chemical structural variation of the esters. This article is protected by copyright. All rights reserved.
        
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Pengsook A, Tharamak S, Keosaeng K, Koul O, Bullangpoti V, Kumrungsee N, Pluempanupat W (2021) Insecticidal and growth inhibitory effects of some thymol derivatives on the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae) and their impact on detoxification enzymes Pest Manag Sci
Pengsook A, Tharamak S, Keosaeng K, Koul O, Bullangpoti V, Kumrungsee N, Pluempanupat W (2021) Pest Manag Sci