Fungicide Toxic to Tadpoles, USF Researcher Shows

Photo courtesy Taegan McMahon

Taegan McMahon, USF doctoral candidate, tests the effect of the fungicide chlorothalonil on tadpoles.

The second-most commonly used fungicide in the nation is lethal to tadpoles even at ten thousand times below the expected environmental concentration, according to research completed by University of South Florida doctoral candidate Taegan McMahon.

It’s possible, pending further research, that chlorothalonil may be the “missing link” that explains plummeting amphibian populations in some parts of the world, she said. “It’s used very heavily in Central America where we’ve seen some of the strongest declines in amphibian populations.”

Another striking result of her research: Tadpoles died in seven hours at the expected environmental concentration of about 1.64 parts per million, or about one drop diluted into 13 gallons. They also showed significant mortality at very low concentrations – less than 0.00001 parts per million — but mortality was actually reduced at middle concentrations.

“We think that it’s directly toxic at high concentrations and that at low concentrations, it is able to sneak under the radar and enter the tadpole’s system, killing it more slowly,” she said. “In the middle concentrations, it appears that they are able to deal with it or block it.”

While the middle concentrations caused less mortality, there is no safe concentration. Even if applied at the middle concentration, dilution will occur naturally in stormwater or streams and ponds, McMahon notes. Chlorothalonil also has a relatively short half-life but breaks down into components that are even more toxic and long-lived.

Typically sold under brand names including Bravo, Echo, and Daconil, chlorothalonil is used by both homeowners and professionals to treat fungus on peanuts, potatoes, tomatoes and lawns or golf courses. “People use pesticides like chlorothalonil on their gardens but have no idea how harmful it can be to themselves or the environment,” she said.

McMahon presented her research findings at the Joint Meeting of Ichthyologists and Herpetologists in Rhode Island earlier this year with travel paid through a grant from Save the Frogs, a California not-for-profit organization which is organizing the International Day of Pesticide Action on Oct. 24 in Washington, DC.

“There are over 18,000 registered pesticides in the United States,” notes Kerry Kriger, founder and executive director of Save the Frogs. “Amphibians have permeable skin that absorbs the pollutants and pesticides that inevitably end up in water bodies where they live and breed. Many of these pesticides weaken amphibians’ immune responses and some are endocrine disruptors that actually change male frogs into females.”

McMahon began this research with her coworkers testing 16 commonly used pesticides. Chlorothalonil showed such an adverse response that additional research was clearly needed, she said. Large-scale studies were completed at the University of Florida’s Gulf Coast Research and Education Center in Wimauma in 800-liter tanks where tadpoles were exposed to chlorothalonil at the expected environmental concentration.

Taking that research back to the lab, she worked through a series of tests to determine which concentrations were safe – and which were not. “I tested the low concentrations because I wanted to look at naturally relevant concentrations – those doses will naturally dilute in the system,” she said. “I wanted to make sure I had a wide range of naturally occurring concentrations covered.”

Although the tests were conclusive, regulatory action limiting the use of chlorothalonil is likely to be years away, McMahon said. “Studies have been showing that atrazine has adverse effects on amphibians and a wide range of other taxa for years, but it’s still one of the most commonly used herbicides.”

For now, she hopes her research persuades people to reconsider the use of toxic chemicals. “Research that shows how people can directly impact their environment and their health – like pesticide use — can be extremely impactful and pertinent.”