Toxin or medication? Immunotherapeutic effects of nicotine on a specialist caterpillar

1. A core tenant in the field of ecological immunology is that immune responses trade off with other physiological functions due to resource-allocation costs. Caterpillars, for example, tend to exhibit reduced immune responses when reared on more toxic food plants due to a cost from detoxifying or sequestering secondary metabolites, also known as the “vulnerable host hypothesis”. However, support for this hypothesis is mixed, and studies have not yet mechanistically isolated the relative contributions of total plant defenses, specific metabolites, or macro-nutritional quality.     

2. We used the tobacco hornworm (Manduca sexta), a specialist herbivore on plants in the nightshade family (Solanaceae), to investigate tradeoffs in immune response. This system is ideal given the availability of solanaceous plant lines varying in general (i.e., jasmonate-induced) and specific (i.e., nicotine) resistance traits. We also applied a geometric diet stoichiometry approach to examine how phytoch...