Herbicide Drift Threatens Habitat Quality

Native landscapes across the Midwest and southeast face an urgent, though often overlooked threat. The damage is often unrecognized, even by those who steward and recreate in these areas—our naturalists, hunters, anglers and outdoor enthusiasts. The next time you are in your yard, on a hike, or out hunting or fishing, look closely at the trees and plants around you. Do they appear healthy, or does something seem amiss? Do you see stunted, folded, curled or cupped leaves? You might first notice these deformations on a redbud. Then you realize that all the redbuds seem to be ailing. Next you notice similar damage afflicting a sycamore, as well as oaks. Even ground layer plants appear oddly twisted, cupped or discolored. The leaves are not flat, smooth and full as they should be. Perhaps it has been some time since you really looked into the tree canopy above. Is it full; or can you see the sky through the gaps in the branches? Do the trees still shade the ground as they once did? Do dead or dying trees seem more common than in years past?

If you observe these symptoms in multiple species while hiking, hunting, or fishing this year, you may be detecting injuries caused by drifting herbicides. The symptoms of herbicide drift can be observed as early as the first part of May. Many trees are exposed to drift during early spring herbicide applications before many plants have leafed out. The timing of early spring herbicide applications in lawns, golf courses, and farm fields often coincides with the sensitive stages of bud swell and leaf emergence in many trees, particularly oaks. And with the increase in spraying herbicide-tolerant crops during growing months, and the rising popularity of lawn care herbicide applications, symptoms are popping up all summer long.

Some herbicides sprayed onto crops and lawns don’t just drift a few feet when they are applied, but can travel long distances. They can move onto your property from your neighbor’s lawn; they can run off fields and lawns during rains and contaminate streams and drinking water; drift can move long distances, even damaging entire tracks of forested land. Some herbicides are known to volatilize (think evaporate) off their intended target, move in the air – sometimes for miles – and land where they harm wild plants, insects and other animals, and directly and indirectly harm the people of Illinois.

What’s at risk? Studies tell us that plants exposed to herbicides can produce fewer flowers and those flowers are often visited by pollinators less frequently. Exposure to herbicides can lower the nutritional value of forage and can also impact plant reproduction. This can translate into fewer food resources for wildlife, which can lead to changes in animal behavior and serious consequences for food webs.

Depending on the severity of the exposure, most plants and trees can recover from a single drift event. However, multiple drift events in a single year, or repeated events year after year, can have cumulative detrimental effects to trees and plants. Herbicide exposure causes stress, and chronic and/or repeated events weaken the plant, therefore making them more susceptible to diseases and pests. This decline in the health and the dieback and death of trees is extremely worrisome. Oaks, in particular, appear to be especially affected by early season applications of herbicides. Declines in the health or the loss of oaks, which are a keystone species in many of our forests, could mean drastic changes for wildlife that rely on acorns for food or the hundreds of species of moths and butterflies that use oaks as host plants for their caterpillars.

Because this is truly a national problem and declines in the health of pollinator habitat and forested lands are occurring in many states, Prairie Rivers Network partnered with the Xerces Society for Invertebrate Conservation and the National Wildlife Federation to author the 2020 report Drifting Towards Disaster: How Dicamba Herbicides are Harming Cultivated and Wild Landscapes. The report provides an overview of the potential impacts to wildlife and resources they depend on. Additionally, in 2022 Prairie Rivers Network created a report for the Illinois Nature Preserves Commission that provides background on the issue of herbicide drift and highlights the known and potential threats to some of our state’s most ecologically diverse and important habitats, our Nature Preserves, Land and Water Reserves, and qualifying sites.

Prairie Rivers Network’s monitoring efforts have documented declines in tree health and even increases in tree death in areas where the symptoms of drift have been documented for multiple years in a row. We have been working with and telling the stories of conservationists, landowners, and outdoor enthusiasts who have witnessed the trees and plants they love suffer injury year after year. Landscapes are changing due to this invisible threat. Many people are deeply concerned about the future of our trees and other native plants, and the wildlife that depend on them. You can see videos of these stories here.

Prairie Rivers Network has been monitoring areas across the state for symptoms of off-target herbicide drift since 2018. Through our Tree and Plant Health Monitoring Program, we have identified symptoms of growth regulator herbicide exposure (e.g. dicamba and 2,4-D) in over 85 species of trees, 78 species of forbs (annual and perennial forbs and herbaceous vines), and 44 species of shrubs and woody vines.

Injuries from herbicide drift are sometimes attributed to the misuse of a herbicide, meaning that the injury occurred because the user did not correctly follow the label guidelines. But misuse is not the only, or even the predominant way, that drift occurs. Signs are not only found on the edges of fields, or in well-manicured lawns and parks. Some widely used herbicides are highly volatile, and can travel long distances in the atmosphere. Eventually these herbicides may land on non-target plants and cause serious injury. Injuries from volatilization (or vapor drift) are widespread and often occur far from application sites.

Most injuries caused by herbicide drift go unreported, or signs may be misattributed to other stressors, such as disease, weather or pests. And, indeed, identifying signs is made more difficult if an injured plant also shows signs of disease or pests. In part, this is due to lack of awareness of the prevalence of herbicide injury and its visible signs. And, because these signs have been so widespread in recent years, injured foliage may be thought to look “normal.”

As part of Prairie Rivers Networks’ annual monitoring program leaf samples from areas of concern are analyzed for herbicide residues. Some locations are sampled multiple times a year. Results indicate that multiple herbicide exposures are occurring at some locations throughout the growing season. The full scope of the injuries these plants are sustaining from repeated exposures is poorly understood, but declines in vigor, health, flower, seed and nut production are evident in many parts of the state.

Illinois ranks 46th in the U.S. in percentage of public lands. Society has radically transformed the landscape, with much loss to our outdoor heritage, including fewer places to hunt, fish and be outside in areas that are wild. Illinois was once home to vast swaths of forest, prairies and wetlands, but now only remnants exist in small pockets around the state.

The decline in the health of these areas, and the loss of the diversity in trees and plants that inhabit them, means a loss of habitat for wildlife and the resources they depend on. And let us not forget that this type of chemical trespass also threatens the rights of private landowners. Drifting herbicides, particularly volatile herbicides, do not respect property lines when they damage your trees, your gardens, your homes and our public lands.

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Kim Erndt-Pitcher is the Senior Habitat and Agriculture Programs Specialist at Prairie Rivers Network – a statewide conservation organization based in Champaign.

Martin Kemper is a retired Illinois Department of Natural Resources biologist and a volunteer for Prairie Rivers Network who co-leads the monitoring program.