Essential oil antifeedants against armyworms: promises and challenges

Plant secondary metabolites are fascinating weapons in the fight against herbivores. Of note, products of the plant secondary metabolism can be highly useful in developing insecticides for insect pest management. In this framework, the present review focuses on a group of plant secondary metabolites, i.e., essential oils (EOs), and a major group of insect pest species, armyworms, Spodoptera spp. (Lepidoptera: Noctuidae), with a major focus on antifeeding responses. Among all tested EOs, only the ones extracted from Angelica archangelica, Artemisia nakaii, Piper hispidinervum, P. sanctifelicis, Pulegium vulgare and Tanacetum parthenium showed good antifeedant efficacy (i.e., ED50<10 mu g/cm2) against Spodoptera littoralis or S. litura. EO major constituents showing promising antifeedant activity include pulegone, 11 alpha-epoxyeremophil9-en-8-one (ligudicin A), piperitone epoxide and thujone, all showing ED50<1 mu g/cm2. Other promising compounds are dehydrofukinone, germacrone, piperitenone and piperitenone oxide, showing ED50<5 mu g/cm2. Overall, considering the sparse literature on the topic and the lack of standardized methods for testing EOs and their major constituents as antifeedants on armyworms, a call for standardization of armyworm antifeedant tests is presented.