Frogs are changing genders — and it ain’t just to Halloween getup. The most popular weed-killer in the U.S. is causing sex changes in frogs, according to a summary of a new study published today in Nature. The study, led by Tyrone Hayes, a biologist with the University of California at Berkeley, contains the first field evidence that atrazine, an herbicide used since the 1950s and once thought benign, is causing developmental defects in a common Midwestern frog. In one watershed researched, 92 percent of the male frogs had been feminized in water with just 0.1 parts per billion of the chemical. The U.S. EPA allows 3 parts per billion in drinking water. In 1999, an estimated 80 million pounds of atrazine were applied to U.S. crops, golf courses, and lawns. In an unusual move, scientists on an industry-financed panel assembled to issue a critique of a longer version of Hayes’ study appearing in the journal Environmental Health Perspectives.