Adaptive plasticity is theoretically a mechanism for organisms to maintain high fitness in the face of a varying environment. Empirical evidence for adaptive plasticity suggests that it is a widespread and important ecological phenomenon. One branch of adaptive plasticity that has not been thoroughly investigated is self-medication. Self-medication is the ingestion of medicinal substances in response to illness. If ingestion of medicinal substances increased fitness of ill individuals, but decreased fitness of healthy individuals, plasticity in feeding behavior would be adaptive. We used a natural tri-trophic system – plant secondary compounds, caterpillars, and parasitoids, -- to study adaptive plasticity in the form of self-medication.  

Results/Conclusions

Grammia incorrupta (= G. geneura) (Lepidoptera: Arctiidae) caterpillars were shown to self-medicate by increasing their ingestion of pyrrolizidine alkaloids (PAs) when parasitized by Exorista mella (Diptera: Tachinidae). As predicted by adaptive plasticity theory, PA ingestion was beneficial for sick individuals and costly for healthy individuals. Dietary pyrrolizidine alkaloids increased the survival of parasitized caterpillars by conferring resistance against parasitoids and reduced the survival of unparasitized caterpillars. In a feeding choice assay, most parasitized caterpillars specifically increased their intake of nutritionally inert PAs at the expense of nutritional food, and, in doing so, increased their likelihood of survival. Previous work showed that PAs elicit stronger taste responses in parasitized caterpillars. This provides a mechanism for self-medication that has not been considered in other systems and bypasses the need for the learning generally associated with self-medication. Due to the ubiquity of parasitism, it is possible that, as a form of adaptive plasticity, self-medication is a prominent factor shaping multitrophic interactions and, thereby, shaping communities.