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FIELD INVESTIGATIONS There have been many accidental spills of chlorpyrifos, but little quantitative assessment of the environmental effects. One exception is a spill in April 1985 in England (Boreham and Birch 1987). In that instance, a truck overturned, spilling 205 liters of chlorpyrifos into an adjacent stream that drained into the Roding River. A resulting sharp decrease in the number and type of macroinvertebrate benthic organisms in affected parts of the river, compared to unaffected areas, lasted 6 months. In addition, certain chlorpyrifos-resistant benthic organisms were unusually abundant. Chlorpyrifos controls mosquito larvae at applied dosages between 0.028 and 0.056 kg/ha, equivalent to 9 to 18 ug chlorpyrifosjl in 152 mm (6 inches) of water (Marshall and Roberts 1977; Eaton et al. 1985); in 1984 alone, chlorpyrifos was used for this purpose on about 600,000 ha (Odenkirchen 1987). No obvious deleterious effects of chlorpyrifos have yet been noted in mammals, amphibians, or reptiles under field conditions of current use (Table 7). However, at recommended dosage application rates for control of mosquitos and other pestiferous insects (usually 0.028 to 0.056 kg/ha), adverse effects have been documented on survival, reproduction, metabolism, and species diversity of a variety of fishes, terrestrial and aquatic invertebrates, freshwater flora, waterfowl, and horned larks (Eremophila alpestris), and on the marketability of various crops (Mulla et al. 1971, 1973; Hurlbert et al. 1972; Macek et al. 1972; Nelson and Evans 1973; Hoy and Shea 1981; Table 7). We emphasize that the effectiveness of chlorpyrifos under field conditions, like that of other organophosphorus pesticides, is significantly modified by numerous variables such as formulation, route of administration, pond substrate, dose, and water temperature (Macek et al. 1969; Bailey et al. 1970; Rawn et al. 1978; Odenkirchen 1987). 17