Contact toxicity and antifeedant activity of binary mixtures of piperine and β-asarone against the crop pests, Spodoptera litura and Mythimna separata (Lepidoptera: Noctuidae)

Plants produce arrays of secondary metabolites that act synergistically to deter and/or intoxicate herbivores. In this study, we evaluated the toxicity of piperine and beta-asarone, individually and in binary mixtures, to examine their synergistic effects against larvae of two generalist herbivores, Spodoptera litura and Mythimna separata. Acute toxicity of piperine after topical application was lower than that of beta-asarone for second instar larvae of both S. litura and M. separata (LD50 53.65 and 7.52 mu g/larva, respectively, after 24 h). However, all mixtures produced synergistic, antifeedant effects. Mixtures of either 56.65 or 7.52 mu g/larva of piperine + 18 mu g/larva of beta-asarone were highly synergistic in term of their acute toxicity (synergistic factor 11 and 44 for S. litura and M. separata, respectively). The more concentrated mixture also had the strongest antifeedant effects when presented on artificial diet to both species, S. litura and M. separata (56.64% and 62.30% at 24 h, respectively). Our results suggest that the combination of piperine and beta-asarone is a greater deterrent than either pure compound alone. Use of this mixture may provide better crop protection rather than that offered by single compounds for controlling lepidopteran pests.