TOP-DOWN EFFECTS IN SOYBEAN AGROECOSYSTEMS - SPIDER DENSITY AFFECTS HERBIVORE DAMAGE

Spiders are abundant predators in many agricultural systems. The goal of this study was to determine if the influence of spiders as predators could cascade through the food web and affect primary producers. We examined the effects of spider augmentation and removal in soybean monocultures to assess their impact on the damage experienced by the plants. In two of three years (1990 and 1992), leaf damage was reduced in areas where spiders were added, and in one of two years (1992), there was increased damage in areas where spiders were removed. The biomass of insects killed by the spiders was positively correlated with spider mass and the leaf damage near augmentation sites was negatively correlated with the mass of insects killed by the spiders. In this study, small changes in spider density had a significant localized effect on the plants, especially in years when damage due to pest insects was high. These data also verified that trophic cascades and top-down control are in operation at measurable levels in soybean food webs.