BACKGROUND Thymol, a natural phenolic monoterpene originating from Thymus vulgaris, is recognized as a safe and potent botanical insecticide to many insects. The structural modification of thymol into thymyl esters is a potential approach for the development of novel insecticides, which showed more toxicity than thymol. However, there are no reports on the insecticidal activity of thymyl esters to control Spodoptera litura. RESULTS Thymol was structurally modified into ten thymyl esters by esterification using a new reagent, PPh3/Br3CSO2Ph. The insecticidal activity of these compounds was examined against the second instars of Spodoptera litura using a topical application. Among the ten thymyl esters evaluated, thymyl cinnamate was the most toxic with LD50 = 0.41 and 0.34 μg/larva after 24 and 48 h posttreatment, respectively. In addition, thymyl cinnamate‐treated larvae showed increasing carboxylesterase and acetylcholinesterase activities in vivo experiment, whereas glutathione S‐transferase activity showed no significant difference. CONCLUSION Thymyl cinnamate were first reported to exhibit toxicity against S. litura 2.41–2.46 fold more efficient than thymol. However, the detailed biochemical interactions are necessary for further development of novel insecticides. © 2019 Society of Chemical Industry

Background: Spodoptera litura larvae are polyphagous insects that have become a significant pest in recent years. The spread of this pest has led to the continuous usage of insecticides on crops. Some plant extracts have been used as a mixture to control insect pests and improve productivity. Methods: A plant-based mixture was mixed at a ratio of 1:1 v/v to demonstrate the effect on contact toxicity, feeding (no-choice test), and enzyme activities on S. litura. The active compounds of P. retrofractum and A. calamus were isolated by preparative thin-layer chromatography (PTLC). Results: Our results showed that binary mixtures from P. retrofractum and A. calamus exhibit the highest contact toxicity and antifeedant activity at a 1:1 ratio of LD 30 :LD 10 dose (3.213 µg/larva P. retrofractum + 3.294 µg/larva A. calamus). The main active ingredient from each crude extract was (2E,4E,14Z)-N-isobutylicosa-2,4,14-trienamide from P. retrofractum, and beta-asarone and alpha-asarone from A. calamus. Additionally, A. calamus seems to be the synergistic compound. Some compound mixtures increased the glutathione-S-transferase activities in vivo; whereas, almost no significant differences in esterase activities were noted. Conclusion: The results indicated that the ethanolic crude extracts of P. retrofractum and A. calamus mixtures could be used as the pesticidal compound and to develop a binary mixture formulation for controlling lepidopteran pests. However, the toxicity of this mixture to mammals needed to be explored before commercial development.

Culex quinquefasciatus is a common domestic mosquito that is widespread in many areas of Thailand and serves as a southeastern vector of Japanese encephalitis. The present study investigated the acute toxicity of crude extracts and alkaloid compounds of Piper retrofractum (Piperales: Piperaceae) in Cx. quinquefasciatus third instar larvae. P. retrofractum was sequentially extracted using hexane, dichloromethane, ethyl acetate, and methanol, and the crude extracts were tested on mosquito larvae. Detoxification and neuroenzymes were analyzed to establish the mode of action. Acute toxicity was assessed on Poecilia reticulata (Cyprinodontiformes: Poeciliidae) to determine the possibility of toxicity in a nontarget species. Our results showed crude hexane extract had the highest toxicity in Cx. quinquefasciatus (0.9 ppm). Piperine and piperanine, which are alkaloid compounds from the crude hexane extract, showed LC50 values of 0.27 and 2.97 ppm, respectively, after 24 h of exposure. All the crude extracts showed low toxicity in P. reticulata compared with that in the mosquito larvae. The carboxylesterase, glutathione-S-transferase, and acetylcholinesterase activities in Cx. quinquefasciatus were reduced after treatment with all the extracts and the two alkaloid compounds. Thus, P. retrofractum shows larvicidal effects against Cx. quinquefasciatus and low toxicity for nontarget species. Thus, P. retrofractum could be a choice for controlling Cx. quinquefasciatus.