discovered microorganisms in the mid 1800s. The phenomenon of antagonism (a microorganism's ability to sustain its life by parasitizing another organism) was recognized in the late 1880s, but its occurrence in agricultural soils and its role in suppression of plant pathogens and other pests was not fully recognized until early in this century. It is now firmly established that many practices implemented by early agriculturalists enhance the diversity and activities of soil microorganisms, including biological-control organisms. Ironically, the importance of natural processes performed by indigenous biological-control organisms are demonstrated most clearly when they are destroyed by chemical or physical means. If indigenous biological-control organisms are destroyed by sterilization or fumigation of soil, for example, an unaffected indigenous or an introduced pathogen can cause far more damage than would be possible in untreated soil. Many of the cultural practices developed throughout the evolution of agriculture are still used today.

Crop rotation (successive planting of different crops in the same field) is a proven cultural strategy to suppress weed, arthropod, and pathogen pests (National Research Council, 1989b). Optimizing the benefits of crop rotations requires knowledge of the effects of diversified plantings on a cropping system. Rotating a primary crop with another cash crop can limit growth of a pest that feeds on the primary crop (National Research Council, 1989b). Root food sources of soil-borne arthropods as well as inoculum densities of plant pathogens are severely reduced by crop rotation. For instance, populations of plant-parasitic nematodes—a persistent problem in continuous soybean cropping system—are nearly eliminated by switching to corn in alternate growing seasons. Crop rotation alters a soil environment, which can reduce the devastating effects of many soil-borne pests.

Other cropping practices used to encourage growth of beneficial organisms and suppress pest populations include use of green manures (a legume crop plowed under to increase soil fertility), cover crops (crops grown for ground cover), and intercropping (interspersing crops). An example of intercropping is the use of trap crops grown in rows beside a primary crop to both provide habitat for beneficial arthropods and divert pest arthropods from preying on the primary crop. Knowing the effects of cropping on ecosystem components can lead growers to a diversity of plant resources useful in crop-production systems.

A combination of cultural practices is synergistic and can be more effective than a single tactic alone against an agricultural pest. For example, against weeds, a grower may cultivate a field to prevent germination of weed seeds, then plant the crop to ensure that the crop has a competitive advantage in access to limited water, light, and nutrient sources. Manipulation of planting and harvesting schedules also can have a negative impact on pest populations (National Research Council, 1989b; Ferris, 1992; Schroth et al., 1992); however, optimizing these schedules requires prior knowledge of crop-arthropod life cycles and host-free stages of the pest (Edwards and Ford, 1992). With knowledge of



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