Medicinal effects of plant chemistry on the immune system of Manduca sexta (Lepidoptera: Sphingidae)

Related wild plant species of crops are known to serve as reservoirs for shared insect pests, but less understood is how weeds in the environment might affect the survivorship of parasitoids providing biological control services in cultivated fields. Plant toxins can be fatal to parasitoids or could be medicinal and enhance the insect’s own immune system lowering rates of parasitism. We investigated this using Manduca sexta and its specialist parasitoid, Cotesia congregata, to examine host performance and immune activity across a gradient of wild and cultivated plant species in the Solanaceae. Caterpillar performance was assayed by measuring wet-weight at seven days and immunocompetence was determined by measuring phenoloxidase activity. We found that food plant influenced larval growth, with individuals performing better on weeds. Immunity was not different across food plants, except tobacco were phenoloxidase activity was enhanced. As a follow up experiment we reared M. sexta on an isogenic tobacco line varying in nicotine measuring the same parameters. Nicotine lowered larval development, but increased phenoloxidase activity, suggesting enhanced immunity. Together, these findings suggest that related solanaceous weeds in the landscape do not affect parasitoid biological control efficacy in agricultural systems, except for cultivated tobacco due to the presence of nicotine.