Nonmonotonic and Monotonic Effects of Pesticides on the Pathogenic Fungus Batrachochytrium dendrobatidis in Culture and

ABSTRACT: Pesticides and the pathogenic fungus Batracho-chytrium dendrobatidis (Bd) co-occur and are implicated in the global decline of amphibians, a highly threatened taxon. Here, we investigated the effects of ecologically relevant concen-trations of chlorothalonil and atrazine, two of the most commonly used, immunomodulatory pesticides in the United States, on tadpole (Osteopilus septentrionalis) survival and Bd growth. Tadpole survival was unaffected by the pesticides but was reduced by Bd. Atrazine monotonically (i.e., consistently increasing or decreasing) reduced Bd in culture and on tadpoles, and every concentration tested (0.0106−106 μg/L) significantly reduced Bd growth compared to controls. Chlorothalonil had a nonmonotonic (i.e., nonlinear) effect on Bd growth both in culture and on tadpoles, where low (0.0176−1.76 μg/L) and high (32−176 μg/L) concentrations inhibited Bd growth significantly more than did intermediate concentrations (8.2−17.6 μg/L) and controls. To our knowledge, this is one of only a handful of studies to document a nonmonotonic dose response of a nonvertebrate (Bd) to a pesticide. Although both pesticides reduced Bd growth on frogs, neither cleared the infection entirely, and because we know little about the long-term effects of the pesticides on hosts, we do not recommend using these chemicals to control Bd