Function and mechanism of self-medication in an arctiid caterpillar

The current focus on tri-trophic interactions has emphasized adaptive plasticity of plant defense and behavioral responses of carnivores such as parasitoids, with relatively little consideration for counter-adaptations by herbivores. This study investigates herbivore self-medication behavior: changes in ingestion of plant toxins as resistance against parasitoids. Grammia incorrupta (Lepidoptera: Arctiidae) caterpillars are dietary generalists with behavioral and physiological specializations to pyrrolizidine alkaloids (PA), which they pharmacophagously ingest and sequester from certain host-plant species, providing resistance against parasitoids. Our previous work has demonstrated that this caterpillar can self-medicate with PA by increasing PA ingestion following parasitoid infection, thus enhancing anti-parasitoid resistance. Here we show that the feeding response to PA by G. incorrupta caterpillars changes over the duration of parasitoid infection. Experimentally parasitized caterpillars did not increase ingestion of PA until 48 h after larval parasitoids hatched from eggs. Because parasitoid infection initially triggers the caterpillar’s immune response, we hypothesized that low consumption of PA early in the infection reflects negative effects of dietary PA on immune function. We observed that artificially immune-challenged caterpillars avoided PA ingestion. However dietary PA did not change immune response in standardized bead melanization assays. Field-parasitized caterpillars with early-stage parasitoid infections tended to decrease ingestion of PA and PA-plants, but increased their ingestion of a highly nutritious plant containing flavonoids. Experiments are underway to determine if G. incorrupta caterpillars use macronutrients or anti-oxidant compounds, such as flavonoids, at the onset of parasitoid infection to enhance immunological resistance against parasitoids.