Abstract

Abstract Paraquat (1,1' -dimethyl-4,4'-bipyridinium) and its dichloride salt (1,1', dimethyl-4, 4'-bipyridinium dichloride) are broad-spectrum contact plant killers and herbage desiccants that were introduced commercially during the past 25 years. Today, they rank among the most widely used herbicides globally and are frequently used in combination with other herbicides. The recommended paraquat field application rates for terrestrial weed control usually range between 0.28 and 1.12 kg/ha (0.25 and 1.0 lb/acre), and for aquatic weed control the range is 0.1 -2.0 mg/L. Target plant species are unable to metabolize paraquat and tend to contain elevated residues; paraquat-resistant strains of terrestrial flora, whose numbers are increasing, require greater concentrations for control and may contain proportionately greater residues. Paraquat from decayed flora is usually adsorbed to soils and sediments. Paraquat in surface soils generally photodecomposes in several weeks, but paraquat in subsurface soils and sediments may remain bound--and biologically unavailable--for many years without significant degradation. Paraquat is not significantly accumulated by earthworms and other species of soil invertebrates and is usually excreted rapidly by higher animals; however, delayed toxic effects--including death of birds and mammals--are common. At concentrations below the recommended application rate, paraquat is embryotoxic to developing eggs of migratory waterfowl (0.056 kg/ha) and adversely affects sensitive species of freshwater algae and macrophytes (250 µg/L), larvae of crustaceans (0.9-5.0 µg/L), and frog tadpoles and carp (500 µg/L). Sensitive species of birds are negatively affected at daily dose rates of 10 mg/kg body weight or when fed diets containing 20 mg/kg ration or drinking water containing 40 mg/L. Adverse effects in sensitive mammals were observed at dietary levels of 85 -100 mg/kg ration and higher or 100 mg/L in drinking water. Acute oral LD50 values for sensitive species of birds were near 200 mg/kg body weight and, for mammals, 22 -35 mg/kg body weight. Humans are among the more sensitive species, and numerous human poisonings have resulted from accidental or intentional ingestion of a concentrated paraquat formulation. The biochemical mechanism of paraquat toxicity is due to the cyclic oxidation and reduction in tissues, leading to production of superoxide anion and other free radicals and eventually the highly destructive hydrogen peroxide. The lung is the organ most severely affected in paraquat poisoning, due largely to the preferential accumulation of paraquat in lung alveolar cells. Although many organs are affected by paraquat, death is usually due to progressive pulmonary fibrosis. At present, there is no completely successful treatment for paraquat-induced lung toxicity. More information is needed in several areas in order to establish effective criteria for the protection of sensitive species of fish and wildlife against paraquat. These include: flux rates of paraquat from soil into terrestrial food chains; biomagnification potential of paraquat in aquatic food chains, with special reference to plants, plant detritus, amphibians, and reptiles, toxicokinetics of mixtures of paraquat and other herbicides applied concomitantly; and the implications of the high sensitivities of crustacean larvae and waterfowl embryos to paraquat. (Eisler, R. 1990. Paraquat hazards to fish, wildlife, and invertebrates: a synoptic review. U.S. Fish and Wildlife Service Biological Report 85 (1.22). 28 pp. 3 Paraquat (1,1'-dimethyl-4,4'-bipyridinium) is one of the most widely used herbicidal chemicals in the world and is now available in more than 130 countries (Kimbrough