Beneficial contaminants? Pesticides reduce amphibian pathogen abundance and ameliorate negative pathogen effects on hosts

Background/Question/Methods

The amphibian disease chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), has played a major role in global amphibian declines. This is due in part to its dramatic negative effect on amphibian populations in otherwise pristine environments. However, far less research has been conducted examining the impact of Bd in less pristine contexts. Anthropogenic changes present a variety of environmental perturbations also associated with amphibian declines, though little is known about how these affect the resultant impact of Bd infection.

We conducted laboratory studies to determine the effects of the pesticides Roundup® and Sevin® on Bd both in culture and on gray treefrog (Hyla versicolor) hosts. In culture, we exposed pre-established Bd to either pesticide, or conversely exposed pre-existing pesticides to Bd. We measured the effects of either pesticide on Bd zoospore and zoosporangia production and predicted that the pesticides would reduce Bd abundance. In hosts, we exposed H. versicolor tadpoles (Bd-exposed or unexposed) to Sevin or Roundup and measured tadpole survival and growth upon metamorphosis. We predicted that independent Bd or pesticide exposure would reduce growth and survival. Together, we predicted that pesticides would mitigate the effects of Bd or combine to further reduce amphibian health.

Results/Conclusions

In culture, both Roundup and Sevin significantly inhibited zoospore production. When Bd was applied to pre-existing pesticides, Roundup reduced Bd abundance to a greater degree than Sevin. When either pesticide was applied to pre-established Bd, no differences were observed between the pesticides in their ability to reduce Bd abundance. With zoosporangia, both pesticides significantly reduced Bd abundance with no differences observed between the pesticides, independent of application procedure.

In hosts, relative to control tadpoles, exposure to Bd did not affect H. versicolor growth, while tadpoles exposed to either pesticide were smaller than control tadpoles. In addition, we found a significant Bd x pesticide interaction on survival. Roundup ameliorated the effects of Bd on survival compared to tadpoles exposed to Bd alone. Our results suggest that while pesticides reduce amphibian growth, they may ameliorate the effects of Bd on host survival and kill Bd prior to- or post-host infection.