By Cat Lazaroff

BERKELEY, California, April 16, 2002 (ENS) - Atrazine, the top selling weed killer in the United States, disrupts the sexual development of frogs at concentrations 30 times lower than levels allowed by the U.S. Environmental Protection Agency. The researchers who uncovered the problem join environmentalists in expressing concern about heavy use of the herbicide on corn, soybeans and other crops in the U.S. Midwest and around the world.

Researcher and study author Tyrone Hayes (Photo courtesy UC Berkeley)

The herbicide also lowers levels of the male hormone testosterone in sexually mature male frogs by a factor of 10, to levels lower than those found in normal female frogs.

As Hayes later discovered, many atrazine contaminated ponds in the Midwest contain native leopard frogs with the same abnormalities.

"Atrazine exposed frogs don't have normal reproductive systems," Hayes said. "The males have ovaries in their testes and much smaller vocal organs," which are essential in calling potential mates.

In an article in today's issue of the "Proceedings of the National Academy of Sciences," Hayes and his colleagues note that it is unclear whether these abnormalities lead to reduced fertility. Hayes now is trying to determine how the abnormalities affect the frogs' ability to produce offspring.

"The use of atrazine in the environment is basically an uncontrolled experiment - there seems to be no atrazine free environment," Hayes said. "Because it is so widespread, aquatic environments are at risk."

More than 60 million pounds of atrazine were applied last year in the United States alone. Manufacturer Syngenta estimates that farmers use the herbicide to control weeds on about two-thirds of all U.S. farm acres planted with corn and sorghum. On average, atrazine improves corn yield by just over four percent.

An African clawed frog, Xenopus laevis, the type used in Hayes' laboratory studies. (Photo courtesy UC Berkeley)

Aquatic species, however, swim and breed in atrazine contaminated field runoff. Though previous studies showed deformities and abnormalities in adult amphibians only at very high doses, no one had looked in detail at hormone levels in frogs or at effects on tadpoles, the larval stage of frogs.

The findings come at a time when the Environmental Protection Agency (EPA) is reevaluating allowable levels of atrazine in drinking water, which stand today at three parts per billion (ppb). The EPA has drafted new criteria for the protection of aquatic life, limiting four day average exposures to 12 ppb.

Hayes found hermaphroditism in frogs at exposure levels as low as 0.1 ppb. Levels of 40 ppb of atrazine have been measured in rain and spring water in parts of the Midwest, while atrazine in agricultural runoff can be present at several parts per million.

The herbicide also contaminates drinking water supplies in many communities in the Midwest, leading some environmental groups to voice concern about its effect on children, infants and the developing fetus. France, Germany, Italy, Sweden and Norway are among the nations that have already banned the use of atrazine.

Abnormal gonads in a male Xenopus frog, the result of exposure to the herbicide atrazine. The frog has become a hermaphrodite, carrying both male (testes) and female (ovaries) sex organs. (Photo by Tyrone Hayes/UC Berkeley, courtesy Proceedings of the National Academy of Sciences)

Though Hayes initially received funds from Syngenta for the studies, all the current published studies were conducted independent of Syngenta.

For his laboratory tests, Hayes used the African clawed frog, Xenopus laevis, a popular research subject that, like many frogs, is very sensitive to hormones that mimic the effect of their own sex hormones. If raised in a pond with estrogen, for example, all Xenopus tadpoles turn into females. In the presence of androgens, frogs grow larger voice boxes, or larynges.

In laboratory experiments at various concentrations of atrazine, the researchers found that exposure to the chemical affected the sexual development of frogs at concentrations of 0.1 ppb and higher. That is 30 times lower than the EPA's allowable limit of three ppb in drinking water and 120 times lower than the proposed chronic exposure limit for aquatic life, 12 ppb.

At these concentrations, as many as 16 percent of the animals had more than the normal numbers of gonads - including one animal with six testes - or had both male and female organs: testes and ovaries. None of the control animals, which were not exposed to atrazine, had such abnormalities.

"The current data raise new concerns for amphibians with regard to atrazine," the researchers wrote in their paper. "If such effects occur in the wild, exposed animals would suffer impaired reproductive function."

Atrazine has been in use for 40 years in some 80 countries around the globe. The authors of the report note that the herbicide's effect on sexual development in male frogs could be one of many factors in the global decline of amphibians, he added.

"This is very important and elegant work," said Theo Colborn, PhD, a senior scientist at the World Wildlife Fund and an internationally recognized expert on endocrine disrupting chemicals. "Tyrone's work demonstrates the need to do research on the safety of chemicals in the field where the animals live and at the levels to which they are exposed."

"The changes he found in the gonads were not discovered with the traditional high dose atrazine experiments used in the past," Colborn explained. "In addition, microscopic examination of the internal organs of the frogs is required to detect the hidden effects from low dose exposure."

To date, atrazine's effects on mammals and amphibians have been tested only at large doses, not at doses commonly found in the environment.

"The effective doses in the current study," Hayes and his colleagues write, "demonstrate the sensitivity of amphibians relative to other taxa, validate the use of amphibians as sensitive environmental monitors/sentinels, and raise real concern for amphibians in the wild."

The agriculture industry argues that without herbicides like atrazine, crops can be choked by weeds. The field on the left has been treated with an herbicide; the field on the right is untreated. (Photo by Doug Buhler, courtesy Agricultural Research Service)

But the effects of atrazine on frogs could be a sign that the herbicide is subtly affecting human sex hormones, Hayes said, interfering with androgens such as testosterone, which control male sex characteristics.

"Atrazine is obviously affecting frogs," Hayes said. "We have shown serious effects on their sexual development. We need to ask the questions, 'What are the environmental costs of using atrazine? What diversity have we lost?'"

Recent findings by the U.S. Geological Survey document that atrazine is found in U.S. surface waters, fog and rainwater. Millions of Americans drink tap water laced with atrazine, which is at peak levels in the spring, when corn farmers apply tens of millions of pounds of the chemical on their fields.

"This rigorous scientific study reinforces what we and other scientists have been saying for years: atrazine is a dangerous pesticide," said Dr. Jennifer Sass, a senior scientist at the Natural Resources Defense Council (NRDC), one of several conservation groups that have called for a U.S. ban on atrazine. "The fact that doses of atrazine - at a fraction of the federal tap water standard - disrupted frog reproductive organ development has frightening implications for humans, especially pre-pubescent children."

The NRDC argues that farmers have access to affordable, safer alternatives to atrazine.

"Farmers have found that modern cultivation practices allow them to slash the amount of pesticides they apply to their fields - or dispense with them altogether - without cutting production," said Jon Devine, an NRDC senior attorney. For example, Devine said, Iowa farmers discovered they can plant their corn crops in elevated ridges and remove weeds mechanically, increasing their profits and eliminating the need for atrazine.