Oak leaves with cupped edges and discoloration consistent with dicamba exposure.

The cupped edges and leaf discoloration on an oak tree in Arkansas are consistent with dicamba exposure. Photo: Andrea Morales

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The Weedkiller Dicamba Is Poisoning Wildlife Habitat. Will the EPA Finally Act?

Blamed for destroying crops and fraying community ties, the widely used herbicide also poses a threat to the plants birds need, experts say.

Midday, mid-June, mercury in the mid-90s. On the Arkansas Northeastern College campus in Blytheville, Dan Scheiman peers through binoculars at a big oak. Robins and cardinals flit nearby, but it’s not a bird that’s caught his eye. Something is wrong with the tree.

He marches over for a closer look. The leaves are curling inward, an abnormality called cupping. “Top to bottom,” he observes, circling the trunk. “All around.” Scheiman, plants for birds program manager for Audubon Delta, Audubon’s regional office, can’t be certain of the culprit without sending a sample to a lab. But he has no doubt: Cupping is a classic sign of exposure to the herbicide dicamba.

Though used for decades to combat weeds on farms and lawns, dicamba took off six years ago with the introduction of crops genetically engineered to tolerate being sprayed with the chemical while surrounding weeds die. The EPA figures roughly two-thirds of U.S. soybeans and three-quarters of cotton by acreage are dicamba-tolerant.

Trouble is, dicamba won’t stay put. Particles of any weedkiller can drift on the wind, but dicamba travels from its target without so much as a breeze. When temperatures are high enough—the exact threshold is uncertain—it evaporates, rises, and roams ghostlike across the landscape. It can become airborne days after it’s sprayed and drift for miles. While exposure to the vapor hasn’t been proven to significantly sicken humans or birds, it injures or kills broadleaf plants that people and wildlife depend on, from soybeans to strawberries to sweetgum.

Many farmers, scientists, and advocates say dicamba’s damage to crops, ecosystems, and rural communities is among the worst things ever to befall American agriculture. Growers who rely on the weedkiller, meanwhile, say they need it to protect their yields from stubborn nemeses no longer fazed by glyphosate, another herbicide. No place has seen more conflict than the Arkansas Delta—part of the fertile plain flanking the Mississippi River—with its abundance of soybeans and cotton, broiling summers, and frequent temperature inversions that suspend dicamba vapor in the air. The chemical even had a part in a 2016 murder there: One farmer was convicted of shooting and killing another who blamed him for damaging his soybeans.

When symptoms appeared across the region, Scheiman grew concerned about the impact on birds. In 2019 he launched a community science project to document damage in public places, hoping to convince state regulators to crack down. On this June day Scheiman finds many of the same species looking just as ill as before: gnarled mulberry at a wildlife refuge and Important Bird Area; a maple, its leaf points twisted like arthritic fingers, casting weaker shade on the headstones at nearby Dogwood Cemetery.

But today he’s surveying his old haunts out of curiosity. He scrapped the monitoring program last year when it became obvious his findings wouldn’t persuade the body that sets Arkansas’s pesticide rules: “There’s no use in me applying science and advocacy anymore, because they’re not going to listen.” In the battle over dicamba here, he says, chemical manufacturers and their supporters have won. “The federal level is where change has to come from.”

Many environmentalists and farmers are fed up with the EPA for failing to stop the ongoing destruction. In parts of the country dicamba is inescapable in summer; scientists found it in the air on 32 of 39 days they sampled at a University of Arkansas research station. Fields and forests are stewing in the low-dose poison summer after summer. “What’s been happening these past few years is this gigantic, uncontrolled experiment,” Scheiman says. If we don’t call it off soon, he and others fear, we may learn too late that the results are catastrophic.

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o farmers, weeds are landbound leeches bleeding them of time and money. “No other single feature of farming requires such universal and unceasing attention,” lamented a 1918 government publication. That was a few decades before chemicals largely replaced cultivation and hand tools for weed control, but weeds continue to plague growers today. “Weeds are the number one pest in agriculture and the number one input that we spend money on, other than fertilizer,” says Kevin Bradley, a University of Missouri weed scientist.

A watershed moment in the war on weeds came in 1996, when agrochemical giant Monsanto introduced Roundup Ready seeds, genetically engineered to withstand its glyphosate-based herbicide. Amazed by its efficacy, some growers sold their tilling equipment and went all-in on Roundup. A decade later, some 90 percent of U.S. soybean acres were glyphosate-tolerant. Despite Monsanto’s assurances that weeds wouldn’t evolve resistance to this chemical, within two years farmers found rigid ryegrass that refused to die. Today at least 56 weed species are glyphosate-resistant. It’s a textbook example of the herbicide treadmill, Bradley says: “We use one thing too much and get resistance to it and move on to the next.”

In this case, dicamba came next. Farmers have used the weedkiller, which mimics a growth hormone to drive plant death, since 1967; for decades they mainly sprayed fields before spring planting, since dicamba kills cotton and soybeans. (It’s also used on a small scale to manage invasive species for ecological restoration.) Then, beginning in late 2015, Monsanto rolled out seeds engineered to tolerate dicamba.

But the EPA allowed Monsanto to put the horse before the cart, permitting farmers to buy Xtend brand seeds before it approved new formulations for spraying “over the top” of crops after they emerged. In 2016 some growers protected their investments by illegally spraying older, more volatile dicamba products over their now-tolerant crops. That summer regulators received reports of crop damage on thousands of acres in several states.

Later, amid a Missouri peach grower’s successful lawsuit, court documents revealed that Monsanto not only expected the devastation, it also leveraged it as a sales pitch, telling farmers to plant Xtend seeds to protect their crops from dicamba drift. In 2020 a jury found the company and its collaborator, BASF, had been negligent and engaged in a civil conspiracy; the two sides are still battling over monetary damages. That same year Bayer, which acquired Monsanto in 2018, agreed to pay $300 million to settle soybean growers’ claims. Bayer now faces a lawsuit from a honey producer who alleges drifting dicamba so diminished pollen and nectar availability that he decided to move his operation out of Arkansas. And 57 Texas vineyard owners are suing Bayer and BASF for at least $560 million, claiming crop losses topping 95 percent. The companies dispute these claims.

Dicamba use skyrocketed once the EPA registered over-the-top products from Monsanto, BASF, and DuPont beginning in late 2016, allowing farmers to legally spray it in the heat of summer. The companies and the EPA claimed these lower-volatility formulations would solve the vapor-drift problem, but state agriculture departments were soon deluged with 2,700 reports of injury to 3.6 million acres of crops.

This unprecedented destruction began to tear at the fabric of rural life. Unlike the liquid particles of herbicides sprayed on a windy day, vaporized dicamba tends to move unpredictably and leave its mark uniformly across entire fields, making it difficult to pinpoint the source. Crop insurance won’t cover the losses, so farmers are left seething. “There’s more and more tension between neighbors,” Scheiman says.

The EPA added some restrictions in late 2017, prohibiting use on windy days, for example, and requiring user training. But weed scientists say those measures targeted physical drift and did nothing to address the real problem: Dicamba is simply too volatile to spray over crops without risking off-target injury. Court records show Monsanto scientists struggled to prevent it from vaporizing and drifting in their tests, and the company didn’t make its herbicide available for independent testing before it went to market, which Bradley calls “a red flag.” (Bayer tells Audubon it “stands fully behind” its XtendiMax formula.)

Complaints kept piling up, but in late 2018 the Trump EPA extended its registration of over-the-top dicamba for two more years. Its inspector general later reported that political appointees improperly influenced that decision. In 2020 the Ninth U.S. Circuit Court of Appeals ruled that, by downplaying the impact on farmers and the environment, the approval violated federal pesticide law. Yet four months later the agency re-registered dicamba through 2025. For the first time it set cutoff dates: June 30 for soybeans; July 30 for cotton. “We have reached a resolution that is good for our farmers and our environment,” said Andrew Wheeler, then EPA administrator.

The 2021 growing season proved otherwise. The EPA received claims of nearly 3,500 incidents. The damage reached up to 20 miles from the nearest treated field and into 242 counties where endangered species live. It reached fruit trees, peanuts, and garden vegetables. It reached across yards and parks into the heart of a national wildlife refuge in Arkansas that provides some of the continent’s most important waterfowl winter habitat. And it reached far, far beyond what the data suggested. The EPA estimates that only 1 in 25 such incidents are ever reported.
 

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any of those incident reports came from Scheiman. A Long Islander by birth, he’s adapted well to the South. He’d rather be too hot than too cold, and Arkansas obliges. But if Scheiman mostly feels at home after 17 years in Little Rock, amid pro-dicamba farmers he sometimes detects a chill even in summer. In certain towns he leaves his Audubon cap in the car.

For Scheiman and other environmentalists, direct impacts aren’t the main worry, though one large study found a link to heightened cancer risk for dicamba applicators, and the EPA recently flagged a potential hazard to honeybees chronically exposed. That’s coming from an agency with a track record of downplaying chemicals’ dangers and an approval process that’s inherently pocked with knowledge gaps, critics say. Human health risks are usually extrapolated from rodent studies; Mallards and a few other species typically stand in for all birds. “We have an incredibly uncertain process in the United States,” says Nathan Donley, Center for Biological Diversity environmental health science director. “Unfortunately, when there’s uncertainty, it often errs on the side of risk.”

Still, the EPA and its critics agree that the bigger concern is what repeated dicamba exposure does to the vegetation upon which entire food chains depend. “It’s the ripple effect, the death by a thousand cuts,” says Lekha Knuffman, National Wildlife Federation senior agriculture program specialist. Even if a plant survives dicamba’s acute effects, it could be weakened and susceptible to diseases or pests. “We just don’t know what the long-term, ecosystem-level implications are, but the signs are not looking good.”

In 2020 Knuffman’s group and others issued a report on those distress signals. One study, it noted, found a native plant called common boneset produced fewer flowers, flowered later, and attracted fewer pollinators after dicamba exposure. Other research showed that painted lady butterfly caterpillars that fed on dicamba-exposed plants were notably smaller. Caterpillars are a staple for birds—it can take up to 9,000 to raise a single Carolina Chickadee brood—so any reduction in their availability or nutritional value is cause for concern. Scientists have also found that dicamba reduces some plants’ seed production, shrinking another key avian food source. The Mississippi Flyway, which more than 325 migratory bird species traverse, passes right through the cropland where dicamba damage is pronounced, the report noted.

An untrained eye might not notice dicamba’s subtle impacts, but tree damage tends to be more obvious. “We see symptoms of injury at most of the places we go,” says Kim Erndt-Pitcher, senior habitat and agriculture programs specialist with Prairie Rivers Network, an Illinois environmental nonprofit. From 2018 to 2021 she and volunteers found signs of injury from growth-regulator herbicides in 188 plant species. Among those that tested positive for dicamba exposure was the state champion post oak, whose branches span 100 feet but have shown signs of dieback in recent years. She says the widespread damage to oaks is especially concerning because they support more than 500 caterpillar species.

Scheiman worries that today’s bare branches and dead limbs may be just the beginning—that the true toll of repeated exposure will reveal itself in time. “In hindsight, over years, you say, ‘There’s fewer trees around here. Where did all the cypress trees go?’ "

He started the monitoring program to prevent such a scenario. For three growing seasons beginning in 2019, he and volunteers made 363 observations of apparent damage in 20 counties, many corroborated by tissue analysis. The afflicted sites included a forest home to the Arkansas Delta’s only known population of endangered Red-cockaded Woodpeckers. They documented oaks whose puckered foliage resembled boxing gloves, sycamores with cupped leaves the size of cereal bowls, and symptoms on Virginia creeper, mulberries, pokeweed, and many other native plants recognized as important food sources for birds.

He took the findings to the Arkansas State Plant Board, which sets pesticide policy—and entered a maelstrom of lawsuits, dirty deeds, and bad blood.
 

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erry Fuller was in Little Rock when he learned his hay was on fire. A farmer and seed dealer in Poplar Grove, Fuller also chaired the Arkansas State Plant Board. He’d traveled to the capital in September 2020 to testify about dicamba on its behalf before a legislative committee. While he was there, someone torched 367 bales. He felt certain it wasn’t a coincidence. A month earlier he’d spoken on the same topic before the same body. When he started two tractors the next morning, he found someone had fed plastic into the oil fill tubes overnight, destroying the engines. Altogether the vandalism cost him some $80,000. No one was caught.

Fuller was no anti-pesticide crusader. He sells dicamba and tolerant seeds at his dealership and thinks farmers should be able to use it except in the heat of summer. But he was appointed to the plant board in 2013 to represent seed growers and, he says, do what he thought was right. When the board received a record number of dicamba-related complaints in 2017, he joined the unanimous vote to ban spraying after April 15: “It’s just a broken product.”