Caterpillars Ingest Plant Toxins to Fight Off Parasites

For humans, and for the woolly bear caterpillar, eating right is beneficial to our health. Michael Singer of Wesleyan University is researching how the woolly bear caterpillar uses chemicals from the plants it eats to defeat parasites. Specifically, Singer and his team are focusing on the caterpillar's ingestion of plant toxins that kill the parasites, as well as dietary nutrients that enhance its immune system.

Grammia incorrupta, the woolly bear caterpillar, feeds on a variety of plant species that differ in their nutritional content and toxicity, which in turn affects the caterpillar's immune defenses. Many caterpillars become victims of parasitoids, a larval stage of a parasitic wasp or fly. Parasitoids feed and develop in the caterpillar host, eventually killing it. Caterpillars and other insects can sometimes overcome parasitoid infections with an immune response called encapsulation. If encapsulation is successful, host cells completely surround the parasite and kill it.

Singer hypothesized that the caterpillar's ingestion of plant toxins, called pyrrolizidine alkaloids (PA), may serve as a secondary defense against parasitoids. Plants containing PA were previously observed to provide G. incorrupta with resistance against parasitoids. Singer and his team observed that PA ingestion enhances the survival of caterpillars infected by Exorista mella, a parasitic fly, and that PA ingestion reduces the survival of unparasitized caterpillars. As predicted, a parasitized caterpillar will increase its intake of PA above amounts eaten by unparasitized caterpillars, a rare example of self-medication by an invertebrate animal.

For a caterpillar like G. incorrupta, making adaptive foraging choices requires enough plant diversity in its habitat to meet its nutritive and defensive needs. Other herbivores, wild and domestic, might have similar requirements to maintain healthy populations. Moreover, this clear case of self-medication by a caterpillar suggests that other invertebrates as well as vertebrate animals might have evolved to forage for medicine in addition to food. This nascent field, sometimes called Darwinian medicine, offers great promise for advancing understanding of the links between behavior, health, and the environment.