Responses of non-target aquatic organisms to aqueous propanil exposure.

Propanil (3',4'-dichloropropionanilide) is one of the world’s most widely used rice herbicides, and it is extensively used in Arkansas, the leading rice producer in the United States (Webster and Gunnel1 1992). On average, the United States has applied approximately five kg/ha/year to about 70-100% of rice hectareacreage for the past two decades (US EPA 1987; Schlenk and Moore 1993). Arkansas, in 1992 alone, applied over 2.7 million kg of propanil (Jackman 1994). It is important to understand the toxicity of such herbicides to non-target aquatic organisms because of the amounts of pesticide field application and the risk of mixture with water exiting fields. During agricultural applications, aerial drift or accidental spills may expose nearby non-target areas such as ponds, rivers, lakes, wetlands, etc. to herbicides. Predictions of possible impacts upon the diverse range of species found in these ecosystems are usually drawn from a somewhat limited number of toxicity tests with standardized organisms. Comparative toxicity tests should use species of different feeding preferences, habitats, physiology, and size to determine a toxicant’s effects (Rodgers et al. 1997). The relative sensitivities of five freshwater aquatic test species to propanil were determined in aqueous laboratory exposures to provide a wider range of response data inclusive of amphibians, insects, and crustacea. Test species utilized in this study were a cladoceran (Ceriodaphnia dubia), an epibenthic amphipod (Hyalella azteca), a larval midge (Chironomus tentans), the fathead minnow (Pimephnles promelas), and an amphibian (Xenopus laevis). Data generated from such comparative toxicity experiments may be used for future assessments of potential effects on non-target organisms following accidental (or intentional) exposures. Comparative slopes that are specific for each test organism can also offer resolution of risks associated with the recent movement toward using more concentrated pesticide products.