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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. My empirical focus has been on polyphagous woolly bear caterpillars because they are informative, fascinating, and charismatic. Experiments with two polyphagous caterpillar species (Grammia geneura and Estigmene acrea, both Arctiidae) show that their normal preference for toxic plants amounts to a sacrifice of food quality (i.e. growth efficiency) for defense against parasitoids (and possibly predators) (Singer et al. 2004a,b). The dietary generalism of these caterpillars can be seen as an adaptive response to the trade-off in the value of particular plants in providing the caterpillars with enemy-reduced space vs. high food quality (Singer et al. 2004a, Singer and Stireman 2003). The dietary inclusion of host-plants that are primarily useful for defense and those that are primarily useful for their food quality allows these insects to balance their needs for growth and defense. Ongoing work on this project is currently focused on chemical ecology and tri-trophic interactions involving nematodes. |
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