Boundaries between cropland and natural plant communities provide food, water, and cover for wildlife. Similarly, field borders com­ posed of natural plant communities, especially those that include flowering plants, may have a marked effect on natural enemies of crop pests within crop fields. Monoculture crops typical of much of the Midwest do not provide adequate sources of food (nectar, pollen) or shelter and breeding sites for these natural enemies. Nectar and pollen are produced by a variety of native Midwestern plants; as these plants flower in succession throughout the growing season, they provide a continuous food source for the natural enemies (parasites) of pests attacking adjacent crops.