Effects of Herbicides and the Chytrid Fungus Batrachochytrium dendrobatidis on the growth, development and survival of Larval American Toads (Anaxyrus americanus)

Pesticides and pathogens adversely affect amphibian health, but their interactive effects are not well known. We assessed independent and combined effects of two agricultural herbicides and the fungal pathogen Batracho-chytrium dendrobatidis (Bd) on the growth, development and survival of larval American toads (Anaxyrus amer-icanus). Wild-caught tadpoles were exposed to four concentrations of atrazine (0.18, 1.8, 18.0, 180 mu g/L) or glyphosate (7, 70, 700, 7000 mu g a.e./L), respectively contained in Aatrex (R) Liquid 480 (Syngenta) or Vision (R) Silviculture Herbicide (Monsanto) for 14 days, followed by two doses of Bd. At day 14, atrazine had not affected survival, but it non-monotonically affected growth. Exposure to the highest concentration of glyphosate caused 100% mortality within 4 days, while lower doses had an increasing monotonic effect on growth. At day 65, tadpole survival was unaffected by atrazine and the lower doses of glyphosate. Neither herbicide demonstrated an interaction effect with Bd on survival, but exposure to Bd increased survival among both herbicide-exposed and herbicide-control tadpoles. At day 60, tadpoles exposed to the highest concentration of atrazine remained smaller than controls, indicating longer-term effects of atrazine on growth, but effects of glyphosate on growth disappeared. Growth was unaffected by any herbicide-fungal interaction but was positively affected by exposure to Bd following exposure to atrazine. Atrazine exhibited a slowing and non-monotonic effect on Gosner devel-opmental stage, while exposure to Bd tended to speed up development and act antagonistically toward the observed effect of atrazine. Overall, atrazine, glyphosate and Bd all showed a potential to modulate larval toad growth and development.

Automated summary

Pesticides and pathogens have negative effects on amphibians, and their combined effects are not well understood. This study examined the individual and combined effects of two herbicides and a fungal pathogen on the growth, development, and survival of American toad larvae. The results showed that high concentrations of glyphosate caused 100% mortality, while lower doses had a monotonic effect on growth. The herbicides did not interact with the fungal pathogen on survival, but exposure to the pathogen increased survival. Atrazine had long-term effects on growth, while glyphosate's effects disappeared. Overall, both herbicides and the fungal pathogen can modulate larval toad growth and development.