Antifeedant Activity and Biochemical Responses in Spodoptera exigua Hübner (Lepidoptera: Noctuidae) Infesting Broccoli, Brassica oleracea var. alboglabra exposed to Piper ribesioides Wall Extracts and Allelochemicals

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 (FI 50 ) of 26.25 µg/cm 2 and feeding deterrence index (FDI) of 91.8%, which was slightly lower than the positive control (cypermethrin, FDI = 100%; FI 50 0.027 µg/cm 2 ). The most effective feeding deterrent compounds against S. exigua were pinostrobin and pinocembrin with FDI range of 77 to 90% and FI 50 values of 14.39 and 19.38 µg/cm 2 . In the greenhouse, the larvae treated on potted B. oleracea at FI 50 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