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Grammia geneura caterpillar eating Crotalaria pumila (Fabaceae) (left); Estigmene acrea caterpillar eating Senecio longilobus (Asteraceae) (right). Both plant species contain pyrrolizidine alkaloids (PAs) that are highly phagostimulatory for both caterpillar species. The PAs are sequestered and associated with resistance to parasitoids. In E. acrea, the PAs are metabolized into components of male pheromones. In both species, PAs stored in adult males are transferred to females during mating. Photos by Michael S. Singer.
Recent experiments show that G. geneura and E. acrea caterpillars increase their taste for specific toxins, such as pyrrolizidine alkaloids, in 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. Testing this hypothesis and other aspects of foraging plasticity, such as host-plant switching, is a current focus of research in my lab.