Health food versus fast food: the effects of prey quality and mobility on prey selection by a generalist predator and indirect interactions among prey species

1. In order to understand the relative importance of prey quality and mobil ity in indirect interactions among alternative prey that are mediated by a shared natural enemy, the nutritional quality of two common prey for a gene ralist insect predator along with the predator's relative preference for th ese prey was determined. 2. Eggs of the corn earworm Helicoverpa zea (Lepidoptera: Noctuidae) were n utritionally superior to pea aphids Acyrthosiphum pisum (Homoptera: Aphidid ae) as prey for big-eyed bugs Geocoris punctipes (Heteroptera: Geocoridae). Big-eyed bugs survived four times as long when fed corn earworm eggs than when fed pea aphids. Furthermore, only big-eyed bugs fed corn earworm eggs completed development and reached adulthood. 3. In two separate choice experiments, however, big-eyed bugs consistently attacked the nutritionally inferior prey, pea aphids, more frequently than the nutritionally superior prey, corn earworm eggs. 4. Prey mobility, not prey nutritional quality, seems to be the most import ant criterion used by big-eyed bugs to select prey. Big-eyed bugs attacked mobile aphids preferentially when given a choice between mobile and immobil ised aphids. 5. Prey behaviour also mediated indirect interactions between these two pre y species. The presence of mobile pea aphids as alternative prey benefited corn earworms indirectly by reducing the consumption of corn earworm eggs b y big-eyed bugs. The presence of immobilised pea aphids, however, did not b enefit corn earworms indirectly because the consumption of corn earworm egg s by big-eyed bugs was not reduced when they were present. 6. These results suggest that the prey preferences of generalist insect pre dators mediate indirect interactions among prey species and ultimately affe ct the population dynamics of the predator and prey species. Understanding the prey preferences of generalist insect predators is essential to predict accurately the efficacy of these insects as biological control agents.