Intraguild predation among the hoverfly Episyrphus balteatus de Geer (Diptera : Syrphidae) and other aphidophagous predators

Aphidophagous predators compete for the same prey species. During their for aging activity they frequently encounter heterospecific aphid predators. Th ese situations can lead to intraguild predation and may disrupt biological control efforts against aphids where more than one predator species is pres ent. We investigated the behavior of larvae of the hoverfly Episyrphus balt eatus de Geer and its interaction with three other aphid predators: the lad ybird Coccinella septempunctata L., the lacewing Chrysoperla carnea Stephen s, and the gall midge Aphidoletes aphidimyza (Rondani). Interspecific inter actions: between predators were examined in arenas of different sizes and i n the presence of extra-guild-prey. The outcome of interactions between E. balteatus larvae and the other predators depended predominantly on the rela tive body size of the competitors. Relatively large individuals acted as in traguild predators, while relatively smaller individuals became intraguild prey. Eggs and first- as well as second-instar larvae of E. balteatus were highly susceptible to predation by all other predators, whereas pupae of E. balteatus were preyed upon only by the larvae of C. carnea. Interactions b etween A. aphidimyza and E. balteatus were asymmetric and always favored th e latter. Eggs and first- as well as second-instar larvae of E. balteatus s ustained intraguild predation irrespective of the size of the arena or the presence of extraguild prey. However, the frequency of predation on third-i nstar larvae of E. balteatus was significantly reduced. This study indicate d that the same species can be both intraguild predator and intraguild prey . It is suggested that combinations of predators must be carefully chosen f or success in biological control of aphids. (C) 2001 Academic Press.