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Abstract Background Plant secondary metabolites or mixtures in extracts or essential oils are well known to enhance the activity in binary mixtures. The present study is the first to report that thymol synergistically or additively enhances the activity of P. ribesioides extracts and isolated compounds against S. exigua larvae at sublethal doses. Results Thymol was synergistic when are mixed with hexane extract; however, if the hexane extract level was higher (LD30) than the thymol level (LD10), the reaction was antagonistic. CH2Cl2 extract and thymol were more toxic than the extract or thymol alone, and EtOAc extract was synergized by thymol if the components were combined at similar levels (1:1 thymol:EtOAc extract at the LD10 or LD30). MeOH extract individually had moderate insecticidal activity, but all combinations with thymol were synergistic as binary mixtures. Isolated compounds, piperine, phenethyl cinnamamide and cinnamic acid represented synergistic, additive, and antagonistic action after combining with thymol (1:1 at the LD10 or LD30). Detoxification enzymes after exposure of insects to treatments showed isolated compounds + thymol could inhibit CE, GST and AChE reaction of S. exigua exceptional being piperine + thymol, which induced detoxification enzyme activity. Conclusion The synergistic activity was extract- and dose-specific. The impact on detoxification enzymes was variable and dependent on the composition of the extract and the doses of extract and thymol used in a binary mixture. In this metabolic model, the major insect compound in an extract may become detoxified, whereas a minor compound will act unimpeded, showing a lower LD50 than acting alone. This model suggests that thymol synergizes with extract components differently, which could depend on the specific metabolites in the extract and the dose applied. Such studies will help design effective insecticides based on natural plant mixtures and a synergistic compound. Graphical abstract

Abstract Background Spodoptera litura (Lepidoptera: Noctuidae), the tobacco cutworm, is an prominent agricultural pest. To reduce the use of chemical pesticides that cause health problems and that leave residue in the environment, some botanical pesticides have been developed. Our research aimed to evaluate the insecticidal efficacy of Acorus calamus L. extract for the control of S. litura under laboratory conditions. Results After 24 h of topical application, the A. calamus methanolic crude extract showed toxicity against secondary instar S. litura larvae (LD50 ~ 7.438 µg/larva). Chrysin (5,7-dihydroxy flavone), one of the isolated phenolic compounds, showed optimal control efficiency (LD50 ~ 2.752 µg/larva) and showed a reduction in carboxylesterase activities, which have detoxification reduction roles in larvae. Conclusion Chrysin in the crude extract of A. calamus may be an active compound to control this pest, and it may be applied as an alternative to minimize the usage of chemical insecticides. Graphical Abstract

Abstract Background Spodoptera exigua Hübner (Lepidoptera: Noctuidae) is a widely occurring insect pest of several crops conventionally controlled by pyrethroids and organophosphates hazardous for the environment and human health. Thus, the alternatives are biocide-based phytochemicals. Accordingly, the Piper ribesioides Wall. (Piperales: Piperaceae) plant, well distributed in the northern regions of Thailand (Nan Province), was used due to its known bioactivity against insects. The objective was to determine the feeding deterrent activity of P. ribesioides extracts and isolated allelochemicals under laboratory conditions and correlate the efficacy under greenhouse conditions after the extracts were applied to S. exigua larvae infesting potted Brassica oleracea var. alboglabra (Bailey) Musil plants. Another objective was to look at the impact of spray applications on detoxification enzymes to check the possibility of resistance development against such natural extracts. Results Ethyl acetate extract deterred feeding of larvae better than other extracts with the concentrations causing 50% feeding inhibition (FI50) of 26.25 µg/cm2 and feeding deterrence index (FDI) of 91.8%, which was slightly lower than the positive control (cypermethrin, FDI = 100%; FI50 0.027 µg/cm2). The most effective feeding deterrent compounds against S. exigua were pinostrobin and pinocembrin with FDI range of 77 to 90% and FI50 values of 14.39 and 19.38 µg/cm2. In the greenhouse, the larvae treated on potted B. oleracea at FI50 concentrations (determined in laboratory experiments), ethyl acetate extract gave the highest mortality of 63.33% within 24 h of first spray and total of 73.33% after 24 h of the second spray. Impact on detoxification enzymes (24 h post-treatment) was determined from survived 3rd instars of S. exigua using spray applications. Inhibition of carboxylesterase (CE) was 1.94-fold after hexane extract treatment. However, ethyl acetate extract inhibited glutathione-s-transferase (GST) 1.30-fold. Conclusions Ethyl acetate extract of P. ribesioides twigs and isolated pinostrobin and pinocembrin compounds were potential antifeedants against S. exigua larvae. The data obtained also showed that such antifeedant levels of treatment could be used in greenhouse or field trials directly as an extract after establishing the efficacy of extracts and the active compounds therein under laboratory conditions. Graphical Abstract

Abstract Background Dengue is a major problem for humanity. Most people use insecticides to eliminate larvae of Aedes aegypti, which requires heavy chemicals use that affects the environment and human health. Therefore, in this research, the focus was on the larvicidal efficacy of pure compounds from the leaves and twigs of Maerua siamensis against the larvae of A. aegypti. Results Larval mortality was observed after a 24-h exposure. The 1H-indole-3 acetonitrile glycosides cappariloside A and cappariloside B and the triterpene lupeol showed strong larvicidal effects (24-h LC50 = 71.14, 99.79 and 133.03 ppm). After 48 h, cappariloside B caused the most potential mortality with an LC50 of 1.56 ppm and lupeol had the highest lethal concentration at LC50 = 158.71 ppm. Additionally, consistency was observed between the toxicity tests and detoxification enzyme activity. Most compounds, except for lupeol and vanillin, reduce the activity of glutathione-s-transferase, whereas no significant differences were between control and treated groups for carboxylesterase. Conclusions Cappariloside A and cappariloside B are good potential larvicide agents. They showed larvicidal activity against Ae. aegypti larvae with LC50 = 71.14 and 99.79 ppm at 24 and 48 h, respectively.

Various extracts prepared from twigs of Piper ribesioides were evaluated against third instars of Aedes aegypti using the WHO guidelines. Highest toxicity was obtained with hexane extract with an LC50 of 363.32 ppm 48 h after treatment. Piperine was isolated as the active compound from hexane extract and was about 4.5 times more active as compared to the hexane extract (LC50 = 87.59 ppm 48-h post-treatment). Dichloromethane and ethyl acetate extracts were significantly similar in toxicity against the larvae, which was apparently due to the presence of significantly similar quantity of piperine in these extracts as well. Hexane and dichloromethane fractions showed significant induction of acetylcholinesterase (AChE), which was about 48- and 17-fold higher than controls. However, 2.5-fold increase in glutathione-s-transferase was also recorded after larvae were exposed to piperine. Such an induction indicates the potential for resistance development; however, this need to be established with detailed experiments designed specifically for establishing biochemical resistance. However, it was obvious that piperine did not influence AChE,; therefore, the effect on AChE due to hexane and dichloromethane extracts must be a consequence of some other constituents in those extracts. [ABSTRACT FROM AUTHOR]