Mortality, Spatial Avoidance and Swimming Behavior of Bullfrog Tadpoles (Lithobates catesbeianus) Exposed to the Herbicide Diuron

Diuron is a widely used pesticide for weed control in Brazil, Australia, the USA, and EU; therefore, environmental consequences are of concern. The aim of the present study was to determine the potential toxic effects of diuron (as formulation Diuron Nortox® 500 SC) on tadpoles of the bullfrog Lithobates catesbeianus, considering three endpoints: lethality, swimming behavior, and avoidance (ability to escape from contaminant). Secondly, it was intended to compare the sensitivity of tadpoles with to fish and other amphibian species by applying species sensitive distribution (SSD). Five diuron concentrations (20, 30, 35, 40, and 45 mg a.i./L) were tested in triplicate to evaluate mortality, swimming speed, and distance traveled after 96 h of exposure to the herbicide. Two avoidance tests were conducted using a multicompartmented system (n = 6) containing three tadpoles per compartment. Dilution gradients were established at 0; 5; 25; 50; 75; and 100% for both systems, corresponding to 0; 0.5; 2.5; 5; 7.5; and 10 mg a.i./L in an experiment, taking into account the results of the acute toxicity test and 0; 5; 25; 50; 100; and 150 μg a.i./L for another experiment, considering reported environmentally relevant concentrations in Brazilian water bodies. The acute toxicity tests (n = 3) with diuron revealed an 96 h-LC₅₀ of 31 ± 3.7 mg a.i./L. The decreased maximum speed was only observed in tadpoles exposed to 30 mg a.i./L (0.15 ± 0.04 m/s) when compared to the control (0.28 ± 0.06 m/s). In the avoidance tests, concentrations of 2.5 and 5 mg a.i./L caused the escape behavior of L. catesbeianus tadpoles. However, tadpoles were also able to avoid diuron contamination at environmental relevance, especially in concentrations of 25 and 50 μg a.i./L. As expected, avoidance and effects on swimming behavior ocurred at lower diuron concentrations than mortality. Sublethal diuron concentrations may thus lead to the displacement of organisms to more favorable areas when they are available and the ability to swim is not affected.