Complementary suppression of aphids by predators and parasitoids

Abstract

Biocontrol practitioners have long debated the relative merits of specialist parasitoids versus generalist predators as biological control agents. Predators and parasitoids exhibit a broad array of ecological differences that might lead them to play different, and perhaps complementary, roles in pest suppression. However, predators often feed on parasitoids with the potential to disrupt biological control. We compared suppression of the woolly apple aphid (Eriosoma lanigerum) when attacked solely by the specialist parasitoid Aphelinus mali, to aphid suppression when the parasitoid was joined by a diverse community of generalist predators (primarily earwigs, syrphids, predatory bugs and spiders). In a series of exclusion-cage experiments conducted on apple trees (Malus domestica) placed in the field, we found that the parasitoid alone significantly slowed aphid population growth. However, consistently flat or declining aphid densities were seen only when the parasitoid was paired with the generalist predator guild. This suggested that predators and parasitoid exerted complementary impacts, with the most effective aphid suppression achieved when the two natural enemy classes were present together. Consistent with this observation, percentage aphid parasitism was not consistently reduced in the presence of predators; this implies relatively weak predator–parasitoid interference. Altogether, our results suggest that the conservation of a diverse community of natural enemies, including both predators and parasitoids, is an effective way to strengthen suppression of this, and perhaps other, aphid species.