Federal regulators said Tuesday they will ban a pesticide widely used on California crops such as almonds and alfalfa, saying it imperils aquatic insects that are the food source of fish.
Canceling the registration would prohibit any further sales of products that include the ingredient.
The EPA said it found that the compound breaks down into a more toxic chemical that is harmful to insects that are an important element of the aquatic food chain.
California growers have used the chemical since 2008, and they applied 42,495 pounds of it to 521,140 acres in 2013, the last year for which complete data were available, according to the state Department of Pesticide Regulation.
More than a third of that was applied to almonds: 14,693 pounds on 125,557 acres, according to the department. Growers applied 6,002 pounds of it to 91,828 acres of alfalfa, and 3,684 pounds to 78,348 acres of processing tomatoes, which are used in paste and other products.
Other top crops that used more than 1,000 pounds of the chemical included corn, walnuts, cotton, sunflowers, wine grapes, pistachios and table grapes, according to the department.
A spokesman for Bayer was not immediately available for comment Tuesday.
Last month, Bayer refused a request by the EPA to voluntarily cancel use of the product, saying it strongly disagreed with methods the agency used to study the chemical's effects.
"Denying a product's registration and ignoring its safe use history based on unrealistic theoretical calculations calls into question the EPA's commitment to innovation and sustainable agriculture," Dana Sargent, Bayer vice president of regulatory affairs, said at the time.
The California Almond Board said it hoped Bayer and EPA could find "a fair and science-based process" to assess the chemical's safety.
Environmental groups praised the EPA's decision even as they warned that many chemicals receive conditional approvals before their safety can be fully vetted.
"I'd love to see the EPA build on this decision and seriously consider halting the conditional registration of pesticides," said Nathan Donley, a scientist with the Center for Biological Diversity, an environmental advocacy group.