Discussions of intraguild predation (IGP) have assumed that the nutritional quality of intraguild (IG) prey is similar to that of any other prey available to the IG predator. It has been suggested therefore that generalist predators do not distinguish between healthy and parasitized aphids and thus function as facultative predators of parasitoids. More recent studies have shown that predators may selectively ingest prey of various nutritional qualities and that predators may have higher body nitrogen composition than herbivores. If so, predators may preferentially feed on other predators (i.e., IG prey) to increase their nitrogen intake. We used a system composed of larvae of the coccinellid Coccinella undecimpunctata (IG predator), the parasitoid Aphidius cohmani (IG prey), and their shared food source, the green peach aphid Myzus persicae, to test whether (1) the predator preferentially feeds on parasitized over healthy prey, (2) predatory behavior on parasitized aphids changes with time elapsed after parasitism, and (3) parasitized and healthy aphids differ in their suitability for predator development. Results indicate that, given a choice, C. undecimpunctata larvae show no preference for either parasitized or healthy prey items, regardless of parasitoid age. Feeding time, however, increased with parasitoid age, leading to a reduction in the number of prey consumed per unit time. Mummified prey were protected from predation by C. undecimpunctata larvae but the larvae were able to feed and complete development on mummies with a broken cuticle. These predators, however, had an extended developmental time and lower pupal and adult weights in comparison to larvae fed healthy aphids. That parasitized aphids are inferior prey for C. undecimpunctata larvae should act to reduce intensity of IGP in the field and the negative impact IGP has on herbivore suppression.
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