Generalist predators disrupt biological control by a specialist parasitoid

Two broad classes of arthropod natural enemies attack insect herbivores: sp ecialists and generalists. The tight dynamical linkage of specialist natura l enemies and their prey may make the specialists able to respond numerical ly to, and perhaps suppress, herbivore outbreaks. Because generalist predat ors may attack not only herbivores, but also the herbivores' specialist nat ural enemies, generalist predators may disrupt control of herbivore populat ions rather than contribute to it. We examined interactions between pea aphids (Acyrthosiphon pisum), a specia list parasitoid wasp (Aphidius ervi) that attacks the aphids, and a common generalist predator, carabid beetles (primarily Pterostichus melanarius). I n two held experiments in alfalfa, we manipulated carabid densities to meas ure their direct (through predation) and indirect (through intraguild preda tion on A. ervi) impact on aphid population dynamics. The first experiment was initiated when plants were short (following cutting), and carabid preda tion of aphids led to an immediate reduction in aphid densities. This reduc tion was short-lived, however, because carabids reduced parasitism, leading to higher aphid population growth rates. Therefore, although the immediate direct effect of carabids was a reduction in aphid densities, this was off set in the longer term by the indirect effect of carabids disrupting the co ntrol of aphids by parasitoids. The second experiment was initiated after p lants had regrown following cutting. When plants were tall, carabids had no direct impact on aphid densities. Nonetheless, the slow indirect effect of carabids on aphid densities through reduction in parasitism still occurred , leading to higher aphid population growth rates in the presence of carabi ds. Additional laboratory experiments demonstrated that carabids are able t o climb into plants and prey upon the immobile parasitoid pupae ("mummies") , whereas antipredator behaviors of the aphids protect them from predation by carabids when plants are tall. Overall, carabid predation caused an immediate but ephemeral decrease in ap hid density. In contrast, the indirect positive effect on aphid density cau sed by reducing parasitism developed later but persisted longer Thus the ge neralist carabid primarily acted as an intraguild predator, interrupting ap hid population control by the specialist parasitoid.