Comparison of predation in annual versus perennial agroecosystems: Aphid predation in soybean versus alfalfa in Manitoba

A field study was conducted in four pairs of neighboring alfalfa and soybean fields to compare natural enemy suppression of soybean and pea aphids. Natural enemy manipulations consisted in complete exclusion, ground predator exclusion and open controls using field cages. I compared the impacts of these manipulations on populations of sentinel soybean aphids on both crops using potted soybean plants, and on pea aphids on alfalfa stems. I found strong predation on both aphid species in both crops at all locations. There was no difference in final aphid number between ground predator exclusion and open control, suggesting that predation by foliar natural enemies was not increased by ground predators. Predation between soybean aphid and pea aphid was also similar, suggesting effective assemblages of foliar predators in both crops. These results suggest that the assemblage of generalist predators present in alfalfa fields can be very effective suppressing soybean aphid in nearby fields.