An untapped plant defense: Eggplant’s steroidal glycoalkaloid solasonine confers deterrence against the Oriental leafworm Spodoptera litura

Abstract

The eco-friendly natural insecticides’ demand is increasing with the increasing awareness about the synthetic insecticides’ hazards. We observed that the larvae of polyphagous and multi insecticide resistant pest Spodoptera litura Fabricius show differential abundance on five eggplant (Solanum melongena L.) varieties. To test the hypothesis that eggplant varieties’ varying steroidal alkaloid contents caused such differential abundance, we quantified solasodine, solamargine, and solasonine in these varieties and analyzed their effects on larval host choice, growth, and nutritional indices. We also examined their suitability as botanical insecticides by their foliar application. Larvae showed lower abundance on the high solasonine containing varieties than the low solasonine containing ones. In the choice assays conducted using pure compound-complemented and control artificial diets, 76% of larvae showed an aversion to the solasonine complemented diet. UPLC-ESI-QTOF-based metabolomics of the larval hemolymph and frass revealed that the larvae do not metabolize these alkaloids. The excretion efficiency determination assays revealed that the larvae excrete all of the ingested quantities of these compounds. The larval nutritional indices analysis revealed that the solasonine ingestion reduced the larval efficiency to convert food into body substance by 56% and the growth rate by 51%. Foliar application of solasonine on five eggplant varieties deterred >90% of larvae. Thus, eggplant’s solasonine acts as an antifeedant, which S. litura cannot detoxify. Since solasonine is already a part of eggplant’s agroecosystem, it is likely to be degraded by the naturally occurring microbes. Therefore, integrating this botanical insecticide in eggplant’s pest management can be advantageous over the synthetic insecticides.