Frogs and Pesticides

A red-eyed treefrog. More than one-third of all amphibians are endangered. Photo: Peter Lilja/Getty Images

Frogs are regarded as ecologically important members of wetland communities, cherished residents of wild areas, control agents for pest insects (like mosquitoes), a potential source for new drugs, and nature’s early warning system of substantial aquatic contamination. Frog species have been declining worldwide for some time. A number are close to the point of extinction. A recent study suggests that an estimated 200 frog extinctions have occurred (National Academy of Sciences of the United States of America, Oct., 2015). Earth is facing its largest mass extinction since the disappearance of the dinosaurs, with up to half of the world’s 6,000 amphibian species in danger of extinction, conservationists warn. (The Guardian, 2008). Many frog species are known to be extremely vulnerable to pesticides polluting the places where they struggle to survive.

Leopard Frog. Photo: Ross Feldner, RCC

Because frogs rely on their porous skin for hydration and some of it for respiration they are extremely vulnerable to pesticide absorption.

The EPA does not require pesticides to be tested on frogs and does not routinely assess the risks of pesticide contamination to frogs themselves, to their habitats or to their food sources.

Nevertheless, university scientists have conducted toxicity research on frogs and pesticides. These studies have found that: tadpoles are more sensitive to pesticide hazards than are the adults; pesticide mixtures produce unexpected toxicity; species can differ in their sensitivity to pesticides; pesticides can be transported by wind to contaminate pristine environments and reduce frog populations living there.

Amphibians are the best example of the great extinction of species currently under way, as they are the most threatened and rapidly declining vertebrate group. More than a third of all amphibians are included in the IUCN “red list” of endangered species, with loss of habitat, climate change and disease posing the biggest threats. New research suggests the chemicals are playing a significant and previously unknown role in the global decline of amphibians. Widely used pesticides can kill frogs within an hour, new research has revealed, suggesting the chemicals are playing a significant and previously unknown role in the catastrophic global decline of amphibians. (The Guardian, Jan. 24, 2013).

The following examples of research findings indicate frogs’ vulnerabilities:

  • A recent EPA report details extensive harm from the world’s second most commonly used pesticide atrazine. Based on the results from hundreds of toxicity studies on the effects of atrazine on plants and animals, over 20 years of surface water monitoring data, and higher tier aquatic exposure models, this risk assessment concludes that aquatic plant communities are impacted in many areas where atrazine use is heaviest, and there is potential chronic risk to fish, amphibians, and aquatic invertebrates in these same locations. (, May, 2016).
  • Frogs living in remote mountain ponds in the Sierra Nevada are ingesting pesticides used to grow crops 50 to 100 miles away in California’s Central Valley, according to a study by government scientists. Researchers from the U.S. Geological Survey identified 10 distinct chemicals in the frogs’ tissues, including residues of DDT, an insecticide that’s been banned for more than 40 years. (LA Times, July 26, 2013).
  • Scientific literature has shown the negative effects of commonly used pesticides on amphibians. 200 million pounds of pesticides are applied each year in California alone and a whopping two billion pounds across the USA every year. (Center for Biological Diversity).
  • Hermaphrodite frogs have been found in urban, suburban and agricultural ponds but they have not been found in ponds from undeveloped, often forested, areas. The occurrence of the hermaphrodite condition in frogs was believed to be due to contaminants found in the water such as pesticides, flame retardants, and chemicals used to give fragrance to soap and cosmetics. It is noteworthy that hermaphrodite frogs were not found in undeveloped forested areas.
  • The insecticide endosulfan at low levels found in the environment can be deadly for tadpoles. For example 6.4 ppb (parts per billion) a very low concentration caused 86% mortality in a population of leopard frog tadpoles. (Relyea, R., Oecologia, 2009) Note: Endosulfan is highly toxic to birds, fish, bees, crustaceans, mollusks and to mammals as well as to amphibians. (Briggs, S. and Rachel Carson Council, Inc., Basic Guide to Pesticides)
  • California Red-legged Frog. Photo: Gary M. Fellers, USGS.

    A single application of a mixture of 5 insecticides (malathion, carbaryl, chlorpyrifos, diazinon and endosulfan) each of which was present at a non-toxic level resulted in 99% lethality in leopard frog tadpoles. (Relyea, R., “A cocktail of contaminants: how mixtures of pesticides at low concentrations affect aquatic communities,” Oecologia, 159, pp. 1213-9, 2009).

  • An abstract published by NIH discusses the effects of seven pesticide products on juvenile European common frogs (Rana temporaria) in an agricultural over spray scenario. Mortality ranged from 100% after one hour to 40% after seven days at the recommended label rate of currently registered products. The demonstrated toxicity is alarming and a large-scale negative effect of terrestrial pesticide exposure on amphibian populations seems likely. (Sci Rep, Pub Med, US National Library of Medicine National Institutes of Health, 2013).
  • Tyrone Hayes and others have reported that exposures of tadpoles to mixtures of pesticides were associated with immune system suppression (Relyea, R., “A cocktail of contaminants: how mixtures of pesticides at low concentrations affect aquatic communities,” Oecologia, 159, pp. 1213-9, 2009).
  • A federal district court approved a settlement requiring the U.S. Fish and Wildlife Service to better protect California red-legged frogs from seven common pesticides known to be highly toxic to amphibians. (Center for Biological Diversity, Nov. 4, 2013).

Pacific chorus frogs like this one were found to contain traces of 10 agricultural chemicals that were used in farming fields up to 100 miles away, according to a new study. Photo: Devin Edmonds/USGS)

Frog losses and abnormal populations continue to be documented throughout the world. Generally, uncontaminated habitats have fewer problems for frogs. However, sites in pristine locations receiving pesticide contamination through atmospheric transport, can be hazardous to them.

Certain pesticide mixtures have been found to be highly toxic to tadpoles, even though the individual chemicals are present at levels that do not harm them. Such mixtures have produced immune suppression in some cases and fatalities in others.

Suppression of the immune system can result in higher levels of disease in a population. A deadly fungus is spreading to frogs around the world. The hazardous nature of this infection could be enhanced by the immune suppression produced by pesticide mixtures. A variety of pesticide mixtures have been found in U.S. surface water.

Frogs and other amphibians are under threat from chemical hazards, as long as these toxic materials continue to be released into the air, water or soil. For the sake of our frog populations and other wildlife exposed to pesticides, we need to minimize general pesticide use and ban those pesticides, such as endosulfan, found highly toxic to them.

Rachel Carson designated half of her own land at her house in Maryland to be kept wild for the birds and frogs. She would no doubt be distressed by the continuing pesticide threats to frogs and other amphibians.