Herbivore Immunity Exhibits a Biphasic Relationship with Plant Defense Toxin Concentrations

Plant chemical defenses are known to influence herbivore performance, yet their effects on herbivore immunity are poorly understood. In this study, we manipulated both the mean and variance of the concentration of a plant toxin in an artificial diet to assess the effects on herbivore immune function and survival. Our results revealed a biphasic immune response across increasing mean toxin concentrations, with exposure to low toxin concentrations enhancing immune function, creating a 'window of enhanced immunity', and exposure to higher concentrations suppressing it relative to toxin-free diets. Encountering variable toxin levels may alter this relationship by mixing toxin levels from within and outside this window. Importantly, we found that when the mean toxin concentration is just outside of the window of enhanced immunity, exposure to low variability in toxin concentrations enhanced herbivore immunocompetence compared to when they were exposed to high or no variation. Elevated immunity correlated with increased survival under pathogen challenge.