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Grammia incorrupta caterpillar eating Crotalaria pumila (Fabaceae) (left); Estigmene acrea caterpillar eating Senecio longilobus (Asteraceae) (right) at field sites in Santa Cruz Co., AZ, USA. 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. 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 incorrupta [formerly G. 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.
I place this work and the ideas arising from it in a broader theoretical context in a chapter of the book “Specialization, Speciation, and Radiation,” edited by Kelley Tilmon (Singer 2007). Liz Bernays and I also discuss this and related work in the context of chemical ecology of Arctiidae in a chapter of the book “Tiger Moths and Woolly Bears,” edited by Bill Conner (Singer and Bernays 2008). |
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