Two years ago some University of South Florida researchers began studying the effects of the most widely used fungicide in the country to see if it might kill more than just fungus.
Turns out it's also a pretty effective frog-icide.
"We were completely surprised to see it basically killed everything," said Taegan McMahon, the lead researcher on the study, which was published this week in a scientific journal called Environmental Health Perspectives. Frogs on farms with treated fields, frogs in ponds on golf courses, frogs in the back yard — the fungicide could be lethal to any of them, the study suggests.
"We don't know what the effect on humans could be," she added. "And we use it heavily in Florida."
The fungicide, chlorothalonil, sold under such names as Bravo, Echo and Daconil, is used to treat farmers' fields, lawns and golf courses and is an ingredient in mold-suppressing paint.
It's part of the same chemical family, organochlorines, as the banned pesticide DDT. It is known to cause severe eye and skin irritation in humans if handled improperly.
Chlorothalonil kills mold and fungi by disrupting the respiratory functions of the cells, explained Jason Rohr, an assistant professor who co-authored the study and heads up USF's Rohr Ecology Lab. At this point the researchers don't know if that's how it kills frogs, too, he said. They just know it's lethal.
"We've previously studied a variety of other pesticides, such as atrazine, as well as herbicides and insecticides," Rohr said. "We haven't seen one with nearly the mortality that we've seen with chlorothalonil."
Frogs and other amphibians play an important role in the food chain, which is why scientists began sounding the alarms in the early 1990s when they discovered many were disappearing. An estimated one-third of the world's 6,300 amphibian species are threatened with extinction, with the blame being pointed at climate change, loss of habitat, chemical use by humans and a spreading fungus — just the sort of thing a fungicide would kill.
However, McMahon said, no one has studied the effects this popular fungicide might have on frogs. For their experiment, the researchers used hundreds of tadpoles from southern leopard frogs, Cuban tree frogs, green tree frogs and squirrel tree frogs, held in jars.
Chlorothalonil's label says not to spray it directly on waterways, so the researchers did not do that. Instead, they used a federally approved formula that calculates how much of a concentration would run off of a farmer's field and wash downstream into a nearby waterway.
It killed nearly 90 percent of the frogs, no matter what species, McMahon said. When they doubled the dose, it killed all of them. Even weaker concentrations harmed the frogs' immune and liver systems and may have altered their stress hormone levels.
Protecting the frogs, Rohr said, might require the government to restrict its use to much farther away from waterways or else ban it entirely, as happened with DDT.
A spokeswoman for Syngenta, the Swiss manufacturer of Bravo and Daconil, challenged the study's findings.
"This study used a model that significantly overstated the potential exposure of amphibians to the fungicide chlorothalonil," said Ann Bryan. "The amount of chlorothalonil was 100 times higher than ever would be found in the real world."
Syngenta reported $11.6 billion in global sales last year.
Studies in Pennsylvania have found far higher concentrations of chlorothalonil in bee pollen. So now there are studies about whether it could be causing a decline in the bee population, Rohr said.
Now that the frog study is done, Rohr said his research group is already studying the effects on other species, such as snails, insects and plankton. Given what has turned up so far, though, he said, "This might be a chemical of concern in the next few years."
The study was done in conjunction with the Archbold Biological Station in Venus and the University of Florida Gulf Coast Research and Education Center in Wimauma.
Craig Pittman can be reached at firstname.lastname@example.org.