Wednesday, March 3, 2010

Atrazine: more controversy and sexually confused frogs

Ooh! It’s not every day I get to see an aquatox issue covered in the news. CNN is carrying the headline “Weed killer 'castrates' male frogs, study says” and before even clicking the link, I knew it was going to be about atrazine. Hmm, this isn’t exactly news to us. Atrazine and endocrine disruption/teratogenic effects have been studied in frogs and fish for over a decade now. Amphibians seem to especially sensitive. So what’s this all about now?

Skimming through the CNN article there is this nice gem,

“Syngenta, a Swiss company that is the largest manufacturer of atrazine, has challenged the validity of Hayes' study.

"We haven't seen these kinds of responses that Dr. Hayes reports," said Keith Solomon, an environmental toxicologist at the University of Guelph in Ontario, Canada, who has served as a consultant to Syngenta. "Some of these studies are poorly conducted and are entirely inconsistent."”

Uh oh, looks like another spat between Hayes and Solomon, who have a pretty good history of butting heads. Solomon says the Hayes’ science is poorly designed, and Hayes says Solomon only finds negative results when his studies are funded by Syngenta. Anyways, ignore the drama for now, let’s see what this study says to warrant its publication in PNAS.

Actually, I guess a little background is in order first. Atrazine, the most commonly used herbicide in the US, with over 80 million pounds applied on the ground yearly. Yikes! It is also the most commonly detected pesticide as a groundwater contaminant. The EU has gone as far to ban its use and while, as mentioned above, the effects are slightly controversial, the new administration’s EPA has ordered a review of atrazine’s environmental impact.

Hayes’ new study started out with a 100% male group of African clawed frogs (Xenopus laevis) and exposed them to 2.5 parts per billion (pbb) atrazine from development through as far as 3 years. So while most previous amphibian tox studies looked at developmental disruption, this new study examines how chronic atrazine exposure can lead to detrimental adult reproductive function.

90% of the males were morphologically male when examined. Wait, only 90% of males were males? The other 10% had become morphologically female. They exhibited a pronounced cloacal labia, and upon dissection, they possessed ovaries and a lack of male features. Figure 1 shows the difference in cloaca in unexposed males compared to a “female.”

Furthermore, those females who were not dissected were allowed to live and ended up reproducing with males (Figure 2), even producing viable eggs which were reared to adulthood! And yet, based on chromosome analyses, these “females” were still genetically male. Ack! It’s quite extraordinary that the degree of hermaphroditism observed led to complete feminization.

Of the 90% of that remained males, what of them? Well, two-thirds of them had become essentially, chemically castrated. This was due to for example, lower testosterone production and underdeveloped testes. And compared to unexposed males, atrazine exposed males had a lower libido and were outcompeted by unexposed males in breeding. The comparison in Figure 3 showing a control frog’s gonads compared to an exposed frog’s gonads.

The results from this study do seem pretty jarring, especially considering the low concentration they were exposed to, which in some areas is environmentally relevant, meaning they are equivalent to a chronic exposure organisms are already experiencing in the wild. It would be interesting to see what the critics have to say regarding this study and whether these results really are replicable. Another nail in the coffin for atrazine? Could be. I eagerly await the EPA’s finding regarding their ongoing critical review of atrazine’s environmental impact.


Hayes, T. B., V. Khoury, A. Narayan, M. Nazir, A. Park, T. Brown, L. Adame, E. Chan, D. Buchholz, T. Stueve, and S. Gallipeau. Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis). Proceedings of the National Academy of Sciences. PNAS published online before print March 1, 2010, doi:10.1073/pnas.0909519107


  1. Atrazine is the single most widely used herbicide in sweet corn, applied to fields before crop emergence, after crop emergence, or at both times. Manufacturers of many of the other herbicides recommend tank-mixing with atrazine to increase their products' effectiveness.

  2. Atrazine is commonly used to kill weeds on highway and railroad right-of-ways or swales. After atrazine is applied, it will remain in the soil for several days to several months.

  3. Life without Atrazine would complicate weed management in corn, especially for sweet corn growers. A study at the University of Illinois looked at 175 sweet corn fields in the Midwest to find out just how important this 50-year-old, broad-spectrum herbicide is in sweet corn grown for processing.