Trees stand alongside the access road to the Sierra-at-Tahoe ski resort in Twin Bridges, Calif.

The U.S. Forest Service is moving ahead with plans to spray the controversial herbicide glyphosate, the main ingredient in Roundup, on thousands of acres devastated by the Caldor Fire, including in the Lake Tahoe Basin. The plans have sparked growing public opposition and have come under scrutiny from local residents, wellness and organic food advocates and environmentalists after an investigation published by Mother Jones this spring showed the federal government is spraying the herbicide at “record levels” across California forests.

Glyphosate is an effective, if potent, weed killer, approved by the U.S. Environmental Protection Agency, and it’s been widely used by the agricultural industry since Monsanto began selling it as Roundup in 1974. The widespread use of glyphosate on crops has long been controversial: It is known to destroy habitat for monarch butterflies and bees. And in 2015, the World Health Organization’s cancer research division said that glyphosate is “probably carcinogenic to humans.” That opened the door for more than 100,000 lawsuits to be filed against Monsanto by people who had been diagnosed with non-Hodgkin’s lymphoma after using Roundup. Now, the Supreme Court is weighing a case about whether Monsanto is liable for not including a cancer warning on its product.

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The agricultural industry isn’t the only industry spraying the chemical. The Mother Jones investigation analyzed millions of spraying reports and discovered that the Forest Service sprayed 266,000 pounds of glyphosate in California forests in 2023, the most recent year that the data was available, which is about five times the amount used two decades ago.

Burned trees in the Eldorado National Forest in California’s Sierra Nevada.

In 2011, the Forest Service conducted a report analyzing glyphosate’s impact on human health and ecological risk. “The preponderance of the available data, however, clearly indicates that the mammalian toxicity of glyphosate is low,” the report states. However, the report relied on research that was heavily influenced by Monsanto, according to a trove of emails and court records reviewed by Mother Jones. 

“Monsanto poisoned the well of public understanding of science,” Naomi Oreskes said in the Mother Jones story. Oreskes is the co-author of the 2010 book “Merchants of Doubt,” which revealed a corporate strategy to influence research on tobacco and climate change. 

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