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Résumé :
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Determining the phenological and diel concurrence of potentially overlapping predator guilds in a given system provides a gauge of the potential prevalence and importance of synergistic prey suppression. The phenology of pea, aphids (Acyrthosiphon pisum) and ground- and foliar-foraging predators was determined through a combination of pitfall and sweep-net sampling in alfalfa. These results revealed diverse guilds of ground-and foliar-foraging predators with a high degree of phenological overlap among individual species. One of the most common ground-foraging predators, Harpalus pennsylvanicus, was found to be significantly more active at night, but also to maintain moderate activity levels during the day. When tested individually in simple arenas with full access to aphids, the aphid-consumption rates of five common ground-foraging predators (H. aeneus, H. pennsylvanicus, Amara familiaris, A. octopunctatum, and Philonthus spp.) were not significantly different from that of Coccinella septempunctata, a foliar-foraging predator that is held to be one of the most important aphid consumers in alfalfa. Thus, both the seasonal occurrence and daily activity levels of these two aphid predator guilds overlap, and the ground-foraging predators, which are often overlooked as sources of aphid predation, were shown to be effective aphid consumers. Three key elements emerge from the alfalfa-aphid-predator system as general predictors of the probability for the synergistic predation on pests by members of a predator complex: (1) synchrony of predator species in the complex, (2) predator-induced escape behavior of prey resulting in habitat switching and encounter with a new predator guild, and (3) minimal negative interaction (intraguild predation or interference) between the predator species. These elements can aid in identifying systems where the conservation or augmentation of ground-foraging predators or other interactive predator guilds will be most effective.
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