Effect of prey density on intraguild interactions among foliar- and ground-foraging predators of aphids associated with alfalfa crops in Chile: a laboratory assessment

Abstract

Among the aphidophaga guild present in alfalfa [Medicago sativa L. (Fabaceae)], coccinellids forage on aphids most of the time on the foliage and carabids on the ground. The result of their combined effect can be synergistic, additive, or antagonistic, but this may depend on the prey density and interacting predatory species. In this study, we first determined, under laboratory conditions, the relative tendencies of Therioaphis trifolii (Monell), Acyrthosiphon pisum (Harris), and Aphis craccivora Koch (all Hemiptera: Aphididae) to drop in the presence and absence of two predators: Eriopis connexa (Germ) and Hippodamia variegata (Goeze) (both Coleoptera: Coccinellidae). These experiments showed that T. trifolii and A. pisum dropped more frequently than A. craccivora, and dropping was more frequent in the presence of H. variegata. We also determined the functional responses to aphid densities of five beetle species (Coleoptera: Carabidae) commonly found in alfalfa fields in Chile. All carabid species consumed aphids, with Trirammatus aerea (Dejean) being one of the most voracious. Then, we tested the hypothesis that the interactions between both coccinellid species and T. aerea would be antagonistic at low prey densities, because of competition or intraguild predation, and synergistic as prey density increases. For this, we recorded aphid consumption when predators were alone, or in combinations of a foliar and a ground predator, for five prey densities. For all predators and combinations, aphid consumption increased continuously with aphid density, and more prey were eaten when a foliar- and a ground-foraging predator were combined than when either predator was present. But, contrary to our expectations, we found that the interaction of these foliar- and ground-foraging predators was additive for all prey densities. Our results suggest that coexistence of these species would not interfere with aphid biological control in alfalfa.