In alfalfa, pea aphids (Acyrthosiphon pisum) are attacked by a diverse community of natural enemies, including a specialist parasitoid wasp, Aphidius ervi, and a guild of generalist predators, primarily made up of Nabis and Orius bugs, Coccinellid and Carabid beetles, and web-building spiders. In a field experiment, we manipulated both the parasitoid and the generalist predator guild, and recorded the impact of each class of natural enemy on pea aphid population dynamics. The parasitoid had little impact on aphid population growth initially, but mounted a strong numerical response that eventually caused pea aphid populations to decline. In contrast, the generalist guild had an immediate impact, lowering the aphid’s rate of population growth. However, the generalists did not exert density dependent control, so that pea aphid densities continued to increase throughout the experiment. When both classes of natural enemy were present, pea aphid dynamics reflected the impact of both generalists and specialists – pea aphid population growth was depressed early, and pea aphid densities strongly declined later. Densities of parasitoid pupae were 50% lower in the presence of generalists, indicating that intraguild predation was common. However, the ratio of parasitoids to aphids was not changed, and the impact of the two types of natural enemies was additive when both were present. Laboratory feeding trials indicated that two important predators in the foliage, nabid bugs and coccinellid beetles, preferentially fed on aphids rather than parasitoid pupae, which might explain the lack of an overall change in aphid-parasitoid ratios in the presence of the generalist guild. Overall, the density independent impact of generalists and the density dependent impact of specialists were complimentary, so that biocontrol was most effective when both classes of natural enemy were present. The impacts of generalists and specialists were additive despite a high rate of intraguild predation, because predators did not alter percent parasitism by A. ervi.
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA