2.5 parts per billion and it turns them into females.
http://www.cnn.com/2010/TECH/science/03/01/pesticide.study.frogs/index.html?hpt=Sbin
Researchers found that long-term exposure to low levels of atrazine – 2.5 parts per billion of water – emasculated three-quarters of laboratory frogs and turned one in 10 into females. Scientists believe the pesticide interferes with endocrine hormones, such as estrogen and testosterone.