regional and seasonal variation affects the intensity of pesticide use. Disease, weed, and arthropod pest complexes vary with locality, climate, and cultivars grown. For example, in the 1994–1995 growing season, while insecticides were applied to 96% of grape acreage grown in Michigan, they were applied to only 17% of grape acreage in Washington (Economic Research Service 1997). Many of the pesticides traditionally used in tree crop systems raise concerns regarding human and environmental exposure and have been either canceled or substantially restricted (for example, phosalone, ethyl parathion, daminozide, ethion, EBDC, and cyhexatin); positive effects of regulatory action are evident in reduced residue detection (chapter 3). The cancellations, however, have increased reliance on fewer products and raised concerns about the availability and competitiveness of alternatives (CAST 1999) and about increased rates of resistance acquisition. A subject of lingering concern, arising at least in part out of failure to enforce laws, is continuing exposure of applicators and farmworkers to remaining traditional chemical pesticides. Institutional and regulatory barriers constrain adoption of nonchemical alternatives under many circumstances (Brunner 1994).

Annual Cropping Systems

Because of the vast acreage dedicated to annual crops, variability —in regional, seasonal, climatic conditions and in cultivar availability —characterizes production in these agroecosystems. Consequently, pest-management practices for these crops reflect variability, as seen in Table 5-1 and Table 5-2.

Corn is the most widely planted crop in the United States and production is overall chemically intensive; in 1995, herbicide was applied to 98% of corn acreage in 10 states surveyed, amounting to 55,850,000 acres (Economic Research Service 1997). Insecticide use in that year, however, was restricted to 26% of the acreage. Sweet corn, however, which is grown for fresh market, received insecticides in 41% of the acreage in Washington and 82% in Illinois. Soybean crops also are widely planted, in diverse soil types, climate regimes, and biotic communities. Herbicide use after planting is high in soybeans, in general, and rose from 52% of acreage in 1990 to 74% in 1995. The diversity of weeds in the weed seed bank, particularly across the diverse acreages planted in soybean (over 45 million acres in major producing states), presents opportunities for weed species shifts and a challenge to single-strategy management plans (Gunsolus et al., 2000).

In contrast with corn and soybean crops, wheat, although one of the largest field crops, currently demands considerably less pesticide use. In 1994, although wheat constituted 29% of all surveyed acreage, it repre-

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