INTRAGUILD PREDATION AMONG APHID PREDATORS - CHARACTERIZATION AND INFLUENCE OF EXTRAGUILD PREY DENSITY

Intraguild predation (IGP), a common interaction in invertebrates and vertebrates, affects the abundance and distribution of many species. S everal parameters influence the magnitude and direction of IGP: feedin g specificity, size, mobility, and aggressiveness of the protagonists, as well as extraguild prey density. Under laboratory conditions, we s tudied IGP among three species of aphid predators, Aphidoletes aphidim yza (Diptera: Cecidomyiidae), Chrysoperla rufilabris (Neuroptera: Chry sopidae), and Coleomegilla maculata lengi (Coleoptera: Coccinellidae), which commonly attack the potato aphid (extraguild prey) Macrosiphum euphorbiae (Homoptera: Aphididae). We characterized the levels and sym metry of IGP among the various stages of the predators in the absence of extraguild prey. The aphid specialist A. aphidimyza was more vulner able to IGP than C. rufilabris and C. maculata, two generalist predato rs. The C. maculata/A. rufilabris interaction was symmetric (mutual IG P), whereas the C. maculata/A. aphidimyza and C. rufilabris/A. aphidim yza interactions were asymmetric, in favor of the coccinellid and the lacewing, respectively. Sessile and low mobility stages of all species were extremely vulnerable to IGP. Generally, the larger sized individ ual won confrontations. For similar sizes, lacewing larvae were superi or to coccinellid larvae. We also tested whether IGP decreases when ex traguild prey are introduced into the system. Data from five predator combinations revealed three types of responses: (1) an exponential dec rease in IGP (lacewing instar I vs. coccinellid instar I); (2) a const ant IGP (lacewing instar III vs. coccinellid instar I); (3) a constant IGP at low densities decreasing at high densities (lacewing instar II I vs. gall midge old larva). Four theoretical scenarios are derived fr om these responses. Each is discussed according to the ecological attr ibutes of the protagonists. This study shows that IGP is influenced by factors inherent to the predators and external factors such as extrag uild prey density. The results are discussed further in the context of aphid biological control.