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In 2003, after reviewing 19 lab and field studies on atrazine and frogs, the U.S. Environmental Protection Agency (EPA) asked Syngenta to do additional research-research using a tiered approach that would address uncertainties found in all previous studies.

Syngenta went on to conduct the most rigorous study to date with atrazine and frogs.

The specifics: Two simultaneous and identical studies were conducted to address uncertainties in previous research. The study was conducted in two separate labs in two countries — the U.S. and Germany. It involved 3,200 frogs and the examination of 100,000 tissue sections. Five concentrations were tested, from 0.01 ppb to 100 ppb of atrazine. A positive control study using estradiol (an estrogen) demonstrated the sensitivity of Xenopus laevis (test species) and suitability of the test system. Researchers sought to eliminate confounding factors in methodology that posed a problem in previous research. The study was conducted in strict compliance with Good Laboratory Practices (GLP). Flow-through tanks helped keep atrazine levels — as well as other factors like temperature and pH — constant and help prevent disease and other conditions that may develop in standing water. Flow-through tanks also reduced stress to the frogs caused by repeated water exchanges and handling. All raw data was shared with EPA and its Scientific Advisory Panel. Results: Atrazine had no effect on the growth, development, survival or sexual differentiation. Dosing overlapped the sensitive window for sexual development and metamorphosis. Positive controls showed effects; atrazine-treated frogs did not. EPA concurred with the conclusions: In its 2007 White Paper, EPA stated, "Based on the negative results of these studies, the Agency concludes that it is reasonable to reject the hypothesis formulated in the 2003 SAP that atrazine exposure can affect amphibian gonadal development. The Agency believes at this time, there is no compelling reason to pursue additional testing with regard to the potential effects of atrazine on amphibian gonadal development." Kloas et al. Does Atrazine Influence Larval Development and Sexual Differentiation in Xenopus laevis? ToxSci Advance Access published on November 13, 2008, DOI 10.1093/toxsci/kfn232. Third-Party Research: Oka, with Taisen Iguchi, exposed wild-type X. laevis to atrazine (0.1 - 100 ppb) throughout larval development using a static-renewal exposure protocol nearly identical to that used in Hayes et al. (2002). These authors were unable to replicate the results from Hayes et al. (2002) and concluded that atrazine at all atrazine concentrations tested (0.1 - 100 ppb) had no effects on time to metamorphosis, gonadal aromatase gene expression, hepatic vitellogenin induction, and, most importantly, occurrence of hermaphrodites (testicular oocytes). Oka et al. 2008. Effect of atrazine on metamorphosis and sexual differentiation in Xenopus laevis. Aquat. Toxicol. 87(4):215-226. LaFiandra et al. exposed the North American Gray treefrog species (Hyla versicolor) throughout larval development to 20 and 200 ppb atrazine, and reported no effects of atrazine (at both concentrations) on length, weight, sex ratio, and gonadal development at metamorphosis. LaFiandra et al. 2008. Effects of atrazine on anuran development are altered by the presence of a nonlethal predator, J. Toxicol. Environ. Health, Part A 71:505-511. Storrs and Semlitsch exposed three native North American species (Bufo americanus, Hyla versicolor, and Rana sphenocephala) throughout larval development to estradiol (27 ppb) or atrazine (1, 3, or 30 ppb), and reported that, in all three species across all treatments, only the positive control (estradiol) — and no atrazine treatments — impacted somatic and/or ovarian development at metamorphosis. Storrs SI, Semlitsch RD. 2008. Variation in somatic and ovarian development: Predicting susceptibility. Gen. Comp. Endocrinol. 156(3):524-530 An historical review of museum specimens in Illinois collected from 1930 to approximately 1990 actually shows a decrease in the percentage of frog limb malformations during the period when atrazine was introduced and used as a corn herbicide in the US. Media Articles on Amphibian Research: Barringer ("Hermaphrodite Frogs Found in Suburban Ponds"): Yale professor Dr. David Skelly found fewer hermaphroditic frogs in agricultural, as opposed to suburban, areas, saying, "What we found in most of the agricultural ponds we sampled was no evidence of reproductive deformity." http://www.nytimes.com/2008/04/08/science/08frog.html?_r=3&oref=slogin&oref=slogin&oref=slogin Renner ("Atrazine effects in Xenopus aren't reproducible"): "Amphibian species are undoubtedly in decline, but the failure of recent studies to find that atrazine feminizes X. laevis calls into question the herbicide's role in that decline." http://www.thecre.com/pdf/2008-Atrazine_ACS.pdf

Syngenta Responds Syngenta response to: "Perturbation of Organogenesis by the Herbicide Atrazine in the Amphibian Xenopus laevis" Environmental Health Perspectives, Volume 116, Number 2, February 2008