Synergistic interaction of thymol with Piper ribesioides (Piperales: Piperaceae) extracts and isolated active compounds for enhanced insecticidal activity against Spodoptera exigua (Lepidoptera: Noctuidae)

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