enous biological controls), soil-borne arthropods, nematodes, and pathogens, plant viruses, rangeland weeds, row-crop weeds, and aquatic weeds.

The U.S. Office of Technology Assessment (1993) estimated that more than 4,000 plants, arthropods, and plant pathogens of foreign origin have established free-living populations in the United States. Some once-exotic species clearly are beneficial; soybean and wheat are now primary crops throughout much of the United States and staples in the U.S. diet. Most exotic pests arrived in the United States through human activity and were released, unintentionally or deliberately, before they were recognized as pests. Some well-known exotic pests include

• arthropods such as Mediterranean fruit fly, boll weevil, gypsy moth, pink bollworm, Japanese beetle, European corn borer, sweet potato whitefly, and Russian wheat aphid;

• pathogens such as white pine blister rust, Dutch elm disease, chestnut blight fungus, and potato blight fungus; and

• weeds such as hydrilla, water hyacinth, purple loosestrife, leafy spurge, and kudzu.

Losses to agricultural production caused by exotic plant pests alone equal approximately $28.8 billion per year, and expenditures for their prevention and control equal approximately $3.2 billion per year (Schwalbe, 1993).

The difficulty in identifying an exotic pest species in a timely manner can delay deployment of natural predators. Classical biological control efforts to solve exotic pest problems have not had a high success rate, suggesting an inadequate ecological basis for this approach. A better understanding of exotic pests and their natural predators and parasitoids will increase the potential of biological control as a viable approach for management of these pests.

Soil-borne plant diseases are caused by pathogenic fungi, viruses, viroids, mycoplasmas, bacteria, or nematodes that live in or on the surface of soil. No practical chemical controls are available for many soil-borne diseases, including Pythium, Rhizoctonia, and Phytophthora root rots; Fusarium wilt; root-knot, cyst, and other nematodes; or arthropods such as the Japanese beetle and root weevils. Losses from root diseases cause wheat farmers an estimated $1.5 billion annually (U.S. Department of Agriculture, Agriculture Research Service, 1995). There are two primary reasons why these problems cannot be controlled by current pest management strategies.

1. It is difficult to achieve uniform coverage of the root system with non-