Manipulation of ground cover vegetation in apple orchards and vineyards had a substantial impact on the abundance of soil dwelling and foliage inhabiting arthropods. Systems with cover crops were generally characterized by lower densities of phytophagous insects, less fruit damage caused by insects on the trees, larger populations and more species of natural enemies and increased predation of artificial prey. Cover crops that remained in full bloom throughout the season, that produced more biomass and supported higher numbers of alternate prey, seemed to harbor the largest complex of predators and parasites. Apparently, cover crop manipulation can directly affect colonization of insect pests which discriminate among trees with and without cover beneath, and can also help retain populations of soil and foliage inhabiting natural enemies through the provision of alternate food and habitat. The design of proper cover crop-orchard mixtures can result in enhanced biological control of specific pests in existing orchards and vineyards.