Can anyone believe it is possible to lay down such a barrage of poisons on the surface of the earth without making it unfit for all life? They should not be called 'insecticides,' but 'biocides.'"
--Rachel Carson, Silent Spring
Here is another story too small to attract media attention. Researchers have found that chlorothalonil, the most commonly used pesticide to control fungus and mold formation, is highly toxic to frogs. We use 14 million pounds of chlorothalonil per year in the United States, much of it sprayed on peanut, potato, and tomato plants. About 10% of it is sprayed on golf courses to control fungus growth on the turf.
The study authored by Taegon McMahon and colleagues ("The fungicide chlorothalonil is nonlinearly associated with corticosterone levels, immunity, and mortality in amphibians") was published in the peer-reviewed journal Environmental Health Perspectives. In a series of experiments, researchers tested the effects of chlorothalonil exposure on four species of frogs (Southern leopard frog, Cuban treefrog, squirrel treefrog, and green treefrog) at doses ranging from 0.01% to 200% of the expected environmental concentration (EEC). Here is the bottom line:
Chlorothalonil killed nearly every amphibian at the ~EEC and, at concentrations to which humans are commonly exposed, it increased mortality and was associated with elevated corticosterone levels and changes in immune cells.
The EEC is a standard formula applied to agricultural chemicals (pdf), which reflects the expected concentration in surface waters from runoff from typical field applications. At 100% of this level, nearly all of the tadpoles and adult frogs died within 24 hours of exposure.
The researchers do a very nice job of summarizing the basic issues.
Amphibians are arguably the “poster child” of the present extinction crisis (Wake and Vredenburg 2008) with over 32% of species threatened and at least 43% experiencing population declines (Stuart et al. 2004). Chemical pollution is a concern for both the health of amphibians and humans. It is considered the second greatest threat (behind habitat loss) to aquatic and amphibious species in the US and has been linked to amphibian declines and disease (Davidson et al. 2002; Rohr et al. 2008a). Similarly, contaminants have been linked to mortality and disease in humans (Dietert et al. 2010). Importantly, many vital systems of amphibians, such as endocrine and immune systems, are similar to those in humans (Hayes 2005), and a genome analysis revealed that the amphibian Xenopus tropicalis has >1,700 genes with human disease associations (Hellsten et al. 2010). Thus, in addition to being of conservation concern, amphibians might be an underutilized model taxon for studying stressor effects on human health.
Let me rephrase that a smidgeon. Frogs and other amphibians are the poster children of what is shaping up to be the 6th major extinction event of our planet's history. Thanks to habitat destruction, chemical pollution, and climate change, human activity is largely responsible for the disappearance of our amphibian friends. Our careless use of chemicals may not only be killing amphibians. There is enough biological similarity at the intracellular and organ system levels between amphibians and humans to raise suspicions of whether we are poisoning ourselves.
The researchers go on to point out that chlorothalonil is known to be "very highly toxic" to fish and other marine life, but has not been extensively tested in amphibians. As an aside, chlorothalonil is one of 6 pesticides thought to be having a significant negative impact on endangered pacific salmon. Chlorothalonil breaks down rapidly, but its primary metabolite has a broader spectrum of biological toxicity.
The study examined immune response metabolites, liver cell counts, and short-term mortality in frog and tadpole populations living in artificial pond-like environments containing water, leaf litter, and food. The experiments provide unequivocal evidence of toxicity for chlorothalonil. Concentrations at or above the EEC (164μg/L) resulted in death within 24 hours of exposure. Lower concentrations were associated with liver and immune responses indicative of biological stress.
In aquatic ecosystems, the results suggest that amphibians most directly exposed to runoff from field applications chlorothalonil are likely to die. Those farther removed in time and space from runoff points are likely to experience some physiological stress from lower levels of exposure, potentially increasing susceptibility to disease and reducing reproductive fitness.
Here is summary of chlorothalonil use in agricultural applications by state in 2002, courtesy of the US Geological Survey's Pesticide National Synthesis Project. Chlorothalonil runoff in the heavy application areas (shown in red and orange) is likely to pose a significant threat to aquatic life, including amphibians.
The St. Petersburg Times was one of the few media outlets to cover the story, probably because the research team was from University of South Florida. That story contained the reaction of one of the major manufacturers of chlorothalonil to the study. The response was predictable.
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.
The statement that EEC levels are not "found in the real world" is false. I found two studies (a, b) with a brief search that demonstrate chlorothalonil levels in surface water runoff from agricultural applications comparable to the EEC levels used in Taegon McMahon's study. I bet I could find dozens of studies with a more extensive search. The EEC level used by the EPA comes from standard plot field study applications.
Rachel Carson wrote Silent Spring in 1962 as awareness of the environmental impact of pesticides was growing. Carson focused on dichlorodiphenyltrichloroethane (aka DDT) which nearly wiped out many raptor species, including the bald eagle, and breakdown products of the pesticide were found throughout the marine food web across the globe. DDT was also found to be moderately toxic to humans, producing genetic mutations and disrupting endocrine and neurological function.
Fast forward nearly fifty years and we are looking at another organochlorine pesticide that is widely used, highly toxic to wildlife, and potentially hazardous to human health. We also now have a political environment that promotes the idea that deregulation is the key to future prosperity. What can possibly go wrong?