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Grammia incorrupta caterpillars eating Plagiobothrys arizonicus (Boraginaceae) (left) and Plantago patagonica (Plantaginaceae) (right) at a field site near Oracle, AZ, USA. Plagiobothrys and several other host plants contain pyrrolizidine alkaloids (PAs) that are highly phagostimulatory for G. incorrupta. The PAs are sequestered and associated with resistance to parasitoids. Plantago and several other host plants contain iridoid glycosides (IGs) that are also feeding stimulants, sequestered and associated with resistance to parasitoids. Photos by Tom Chase (MA ’07).
Recent experiments show that G. incorrupta caterpillars increase their taste for specific toxins, such as PAs and IGs, in putatively medicinal plants when the caterpillars are parasitized (Bernays and Singer 2005). We hypothesize that parasitized caterpillars self-medicate in nature by increasing their consumption of medicinal plants relative to more nutritious plants of limited medicinal value. Controlled experiments in my lab conducted by Kevi Mace (MA ’08) show that G. incorrupta caterpillars gain resistance against parasitoids by ingesting PAs and that most caterpillars increase their ingestion of PAs when parasitized (Singer, Mace, and Bernays, unpublished).
Understanding the function and mechanism of self-medication by G. incorrupta is a current focus of NSF-funded research in my lab. Post-doctoral fellow, Dr. Angela Smilanich, is leading our efforts to understand how self-medication involves the caterpillar’s immune system. Visit her web page for more information.