Effects of foliar applied jasmonic acid and seed applied imidacloprid on phytohormone expression and twospotted spider mite populations

Cotton responds to insect injury by eliciting signal pathways that result in the production of metabolites that have been demonstrated to deter insect feeding, reduce insect fitness, and attract predators. Furthermore, neonicotinoid seed treatments have been shown to suppress these natural secondary metabolites increasing the prevalence of phytophagous arthropods on seedling cotton.  Therefore, we investigated the effects of selected elicitors on the production of plant defenses using phytohormone analysis and measuring natural populations of two-spotted spider mite, Tetranychus urticae, on mid-bloom cotton after foliar applications of Jasmonic Acid (JA), etaxazole, and seed applied imidacloprid. Our research indicates that neonicotinoid insecticide seed treatments suppress naturally occurring secondary metabolites resulting in significantly larger populations of two-spotted spider mites infesting cotton throughout the season. One secondary metabolite, JA-ILE, was determined to be present in significantly higher levels in the non-treated control plots as compared to all other treatments. JA-ILE is a conjugate of jasmonic acid and isoleucine and is an important signaling pathway in plants to herbivore induced responses. Foliar applications of JA produced no greater responses of secondary metabolites in insecticide treated seed or non-treated seed. However, spider mite infestations elicited a metabolic response that may have masked any impact JA had on phytohormone concentrations. Finally, etaxazole, a chitin biosynthesis inhibitor, significantly reduced mite populations in all plots tested. These findings provide further support that neonicotinoid insecticides suppress secondary natural metabolites in plants resulting in decreased resistance to phyophagous arthropods.