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 phytochemical toxicity and nutritional quality interactively impact insect immunity and performance. We predicted that as plant toxicity increased immune activity and herbivore performance would decrease.

3. Increasing food plant toxicity reduced insect growth and development, as predicted, but contrary to our hypothesis, plant toxicity did not trade off with immune parameters. Surprisingly, specific plant chemicals, in this case nicotine, appeared immunotherapeutic, stimulating the phenoloxidase (PO) immune response of M. sexta. Available nutrients in artificial diets, mainly protein, also strongly impacted insect growth, but did not affect PO activity, while diets supplemented with nicotine enhanced the PO and melanization response.

4. This work highlights how specific secondary metabolites, and not overall plant toxicity, impact the immune response. Importantly, our data also suggest an alternative mechanism (i.e., immune enhancement) for reduced parasitoid performance when reared from hosts on toxic plants via tri-trophic interactions.

Data for all associated experiments to conduct the statistical anyalases in the paper are presented in the excel file with each tab coresponding to a different set of experiments.

Please contact the first auther if you have further questions on using this data set.