The dose makes the poison: plant toxin concentrations and herbivore immunity against pathogens and parasitoids

Hormesis, the phenomenon in which low doses of toxins promote beneficial biological responses and higher doses compromise these responses, offers an underexplored framework for understanding herbivore eco-immunology. Here, we explore how insect herbivores might exploit plant secondary metabolites to enhance immune function. We propose that herbivores experience a 'window of enhanced immunity,' where toxins confer immune benefits at low concentrations, but suppress immune responses at higher concentrations. This concept bridges the interplay between bottom-up (plant defense) and top-down (natural enemy) pressures, providing insights into how herbivores balance challenges posed by exposure to plant toxins and exposure to their natural enemies. We discuss how both generalist and specialist herbivores navigate this balance, highlighting the evolutionary adaptations that influence their strategies. We suggest that the immune systems of specialist and generalist herbivores may both exhibit hormetic responses to plant toxins, although the shape of this relationship likely differs depending on their ability to detoxify and sequester plant toxins.