Atrazine: Bizarre Things You Didn’t Know about This Popular Weed Killer


Berkeley, Calif. -- A widely used pesticide, atrazine, has been found to affect the endocrine systems of frogs, essentially turning them into female frogs, according to a study by University of California-Berkeley biologists.

Atrazine is an endocrine disruptor, interfering with hormones such as estrogen and testosterone. It chemically castrates male frogs, eliminating the testosterone in their systems. These altered frogs are able to mate with unaffected male frogs, but all of the offspring are males.

One of the authors of the study, Tyrone B. Hayes, says a chemical causing this type of sex reversal and skewing sex ratios can be more harmful to a species in the long-run than a chemical that outright kills a population because it slowly degrades the population, producing fewer and fewer females.

Atrazine affected one in 10 male frogs in the experiments. The researchers used the African clawed frog (a common laboratory frog) as their test subject. These findings likely translate to wild frog populations because field studies have shown that atrazine may be one of the causes of worldwide amphibian decline, says Hayes.

The frogs are exposed to the chemical from run-off into ground and surface water. Each year, about 80 million pounds of atrazine are applied to control weeds and increase the crop yield of corn and sorghum.

The Environmental Protection Agency is now reviewing the regulations on using atrazine as a pesticide. The European Union has already banned the use of atrazine, and several U.S. states are looking to do the same.

Dr. Becker's Comments:

If you type the word atrazine into any search engine and quickly scroll down through just the first page of results, it’s easy to see that this widely used pesticide is the subject of much controversy.

Atrazine is a relatively low-cost and effective product, so it’s very popular with U.S. farmers who use it to control weeds and increase crop yields. The chemical is also widely used in forestry and on lawns, gardens, parks and golf courses across the country. Due to its extensive application, according to recent estimates, atrazine has been detected in over 70 percent of U.S. groundwater.

In addition to farmers and farm lobbies, atrazine’s manufacturer, Syngenta, also has an obvious interest in trying to debunk research which points to human health and environmental risks associated with the presence of atrazine in water supplies.

It is worth noting that Dr. Tyrone Hayes, one of the authors of the UC-Berkeley study, initially received funding from Syngenta to test atrazine. When Hayes’ findings revealed the impact of exposure on frogs, Syngenta sent him back to the drawing board to re-run his tests. Hayes ultimately became frustrated with Syngenta, gave up his funding, and ran the tests on his own, with the same results.

Fortunately, after what were reported to be some shady dealings with Syngenta during the re-registration process for atrazine, the U.S. Environmental Protection Agency (EPA) decided late last year to take a fresh look at atrazine toxicity.

A decision is expected by this September. Let’s hope it reflects a sincere effort on the part of our government to understand the implications of a growing body of evidence that shows atrazine exposure to be a health and environmental hazard.

Atrazine and the Sex Lives of Frogs

Atrazine is a potent endocrine, or hormone, disruptor.

The UC-Berkeley research indicates that exposure to atrazine impacted male frog hormones to such an extent that 75 percent of test subjects became chemically castrated, and 10 percent were actually turned into females.

According to Berkeley’s Dr. Hayes:

"These male frogs are missing testosterone and all the things that testosterone controls, including sperm. So their fertility is as low as 10 percent in some cases, and that is only if we isolate those animals and pair them with females. In an environment where they are competing with unexposed animals, they have zero chance of reproducing."

The 10 percent that become female frogs can successfully mate with male frogs, however, all their offspring will be male. Per Hayes:

"When we grow these guys up, depending on the family, we will get anywhere from 10 to 50 percent females. In a population, the genetically male females can decrease or wipe out a population just because they skew sex ratios so badly."

The UC-Berkeley team recently reported their findings in both the Proceedings of the National Academy of Sciences journal and the Journal of Experimental Biology. The researchers concluded that atrazine is a likely contributor to the worldwide decline of amphibian populations.

What’s Happening to Amphibians?

Amphibians -- cold-blooded vertebrates that spend time on land but breed and grow to adulthood in water -- have been in rapid decline across the world since the 1980’s.

An assessment of the problem in 2004 found that 43 percent of species were declining in population, 32 percent were threatened, and anywhere from nine to 122 species had become extinct.

Current estimates by the Global Amphibian Assessment list 427 species as critically endangered.

Causes of the declining populations are believed to include:

  • Disease
  • Habitat destruction and modification
  • Pollution
  • Pesticide use
  • Climate change
  • Increased ultraviolet-B radiation

It is assumed that because the skin of amphibians is so highly permeable, they are more susceptible to toxins in the environment than other species. The effect of this vulnerability is not confined to amphibians – it’s just more immediate.

The concern among many scientists is whatever is happening to decimate the amphibian populations will ultimately take its toll on other species as well.

Other Health Risks of Atrazine

Several recent epidemiological studies indicate that even small amounts of atrazine in drinking water – amounts considered safe by current EPA standards – might be linked to certain human birth defects, including skull and facial malformations, misshapen limbs, premature births and low birth weight.

Research is also currently underway to determine atrazine’s threat as a potential cancer causing agent in humans.

What You Can Do

You can significantly reduce your personal risk of exposure to atrazine, and that of your family including the four-legged members, by taking two very simple precautions:

  1. Avoid using toxic pesticides on your lawn, garden, or anywhere on your property. A partial list of atrazine-containing products can be found here. You can visit for a wide variety of resources on organic lawns and gardens.
  2. Provide only clean, pure drinking water for every member of your household, and consider a filter for all the water you use in your home.


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