Overall, certified pesticide applicators do an excellent job keeping pesticides on target to protect crop yield and the aesthetics of various landscape settings. But in recent years, there have been increasing reports of what is believed to be herbicide damage on trees found particularly on state and private lands bordering agricultural fields. Additionally, there have been several mature trees severely injured or killed in southern Illinois and herbicide drift is strongly suspected to have played a role. In multiple cases, testing from Illinois Department of Natural Resources (IDNR) and Illinois Department of Agriculture (IDA) has confirmed the presence of the herbicides dicamba and/or 2,4-D in damaged foliage samples.
How are trees being affected?
Trees are showing injury symptoms of or like that of plant growth regulator herbicides. These PGR herbicides mimic various growth regulating compounds found in plants and include dicamba, 2,4-D, triclopyr, MCPP, MCPA, clopyralid, fluroxypyr, and picloram. Various formulations of these are available for use by both private and commercial applicators and are frequently used in commercial agriculture and turfgrass maintenance. Typical injury symptoms of these growth regulator herbicides include leaves that are cupped, twisted, puckered, or curled. While some plant species (such as redbud) are known to be very sensitive to dicamba and 2,4-D, less is known about the sensitivity of other species. These herbicides can be toxic to broadleaved plants at ultra-low concentrations; 1/800 of the labeled rate can damage grapes. We also know that drift can travel long distances from the application site.
While it is true that often healthy, mature trees that inadvertently receive small amounts of pesticide drift in a single event will often grow out of the injury symptoms, the herbicide should not be on the trees in the first place. Even if an injured tree isn’t killed right away, the adverse effects could be long lasting. Multiple drift events can have cumulative, detrimental effects by causing prolonged stress to the plant.
Chronic exposure combined with predisposing stress factors such as drought, drainage problems, soil compaction, or pest problems may lead to tree decline or death. According to the Illinois Forestry Association’s (IFA) issue page on Off-Target Herbicide Drift, “Foliage damage appears to have contributed to a decline in health and increased mortality of many of our tree and plant species including, but not limited to oaks, elms, maples, redbud, honey locust, hickories, flowering dogwood, and sycamores.” Their members have documented damage in multiple counties.
Why is this an issue now?
IDA inspectors and University of Illinois specialists believe the environmental conditions experienced in recent years (wet springs followed by hot, dry summers) have acted as an additional stress to trees. Adding herbicide injury to already stressed trees may just be the tipping point.
Dicamba and 2,4-D are now applied to MANY more acres than previously due to their use on tolerant soybean varieties. Also, early spring applications coincide with development of trees and other ornamentals. Since drift can’t often be seen with the naked eye, the applicator may not even know it is occurring. Therefore, drift prevention is key.
Multiple groups have been tracking herbicide injury to trees for the past several years. Recent editions of “Illinois Forest Health Highlights” written annually by Dr. Fredric Miller, Illinois Forest Health Specialist, have included a section on herbicide drift damage. Additionally, a statewide survey was conducted to ascertain how extensive the problem is and to better understand contributing factors. The 2022 report can be found here.
In 2022, the Prairie Rivers Network released their report, “Herbicide Drift and Chemical Trespass on Natural Landscapes & Habitats” for the Illinois Nature Preserves Commission. The intent was to provide a summary of legislative and monitoring efforts related to the current issues of herbicide drift. The report documented off-target impacts to natural areas and focused on 2,4-D and dicamba. The report mentioned there are five groups surveying, monitoring, and reporting tree damage from herbicide exposure. Oaks, redbuds, and sycamores are three species frequently showing damage, but 83 were reported with herbicide damage. Their website features pictures of injury symptoms and encourages citizen reporting to help document potential herbicide injury to trees and other broadleaf plants. Guidelines are provided for their online reporting system.
Most recently, the May issue of Outdoor Illinois featured the article, “Herbicide Drift Threatens Habitat Quality”. This public awareness piece describes what herbicide damage looks like and encourages readers to look for it.
Injury in larger trees may not be noticed due to tree size and location. Trees often go overlooked as background scenery by those who aren’t interested or maybe those who aren’t involved in forestry or landscaping. While trees in general may not generate the income that a grain crop can, they provide many benefits. Trees are essential to our ecosystem.
What is being done and what are the obstacles?
To help keep these herbicides on target, state specific restrictions have been put in place and labels have been revised. But more attention to this issue is needed as proposed legislation lingers.
IDA is tracking and investigating herbicide misuse complaints on trees through all formal pesticide misuse complaints they receive that result in investigations. Misuse would include any label violation or misapplication, such as application drift onto not-target areas. In 2022, of 384 total misuse complaints 119 were determined to be dicamba related and 155 were tree related (including 41 complaints received from IDNR). All complaints in Illinois are forwarded to the United States Environmental Protection Agency (US EPA). To help stress the importance of preventing unintended herbicide injury on trees, stricter penalties are being considered for misuse cases where trees are affected.
Herbicide injury often isn’t easy to diagnose as disease, insect, and low temperature injury can cause similar symptoms. Additionally, research is needed to determine what factors are contributing to these recent drift issues. IDA staff has noted that a challenging finding is the occurrence of two trees growing side by side but only one with injury symptoms. This makes it very difficult for IDA to pinpoint the source of the injury as patterns play a large role in diagnosis. Herbicide injury cannot be known with absolute certainty without tissue analysis confirming the herbicide or its metabolites is present. Tissue analysis is determined on a case-by-case basis. It is very expensive and not all laboratories use identical analytical techniques. Results can be difficult to interpret with respect to the degree of damage attributable to the herbicide when present. Research documenting the effects of various herbicide concentrations on trees of any species is generally lacking.
Industry groups support the message that applications need to stay on target. For dicamba applications on soybean, temperature and calendar restrictions among other label changes have been put in place to reduce drift via volatilization. It is the responsibility of the applicator to ensure that label directions and any additional restrictions are read, understood, and followed. No one wants herbicide drift to occur. Generally, the negative outcomes of herbicide drift far exceed any positive outcomes.
Unintended injury to trees can result in even larger issues.
Frequent misuse may lead to additional laws and regulations. Last year, HB 4363 Dicamba Ban was introduced which would ban all dicamba use in the state. To bring awareness to the issue and rally support, affected property tours were held during National Pollinator Week. Not only were members of the public Invited, but also reporters, impacted individuals, state and local elected officials, representatives from various conservation and health organizations, and University of Illinois Extension. Tour hosts partnered with Illinois Public Interest Research Group and Environment Illinois on their campaign. This bill died in committed but was then proposed again in January 2023. This ban would include not only uses on corn and soybean but also uses on lawns, athletic turf, pastures, and roadsides. The effects of such a ban are wide reaching. Weed managers are dependent on ALL available tools with the prevalence of herbicide resistance challenges. Simply put, if we want to continue to use these herbicides, we need to be the best stewards possible.
What can applicators do to prevent potential injury to trees?
It is of utmost importance that pesticide applicators take steps to minimize drift. Careless damage to personal property simply cannot be tolerated. IDNR has an online tool available for use by landowners, producers, and pesticide applicators to increase awareness of sensitive areas that include natural areas, Illinois Nature Preserves, state parks, and other sites. This “Natural Resources Awareness Tool for Applicators”, found at https://dnr.illinois.gov/conservation/chemical-drift-awareness-areas.html, was created in response to plant injury reports (especially on oak trees) that could potentially be attributed to drift from applications to nearby agricultural fields. The goal is that pre-application planning can prevent off-target drift.
Specific steps applicators can take include:
- Carefully read and follow all label directions.
- Note the proximity of sensitive areas and vegetation. Online tools such as “FieldCheck” and IDNR’s “Natural Resources Awareness Tool for Applicators” should be used. Applications to soybean are prohibited when the wind is blowing toward any Illinois Nature Preserves Commission site that is adjacent to the proposed field of application. Additionally, soybean applications are prohibited when the wind is blowing toward an adjacent residential area.
- Talk with neighbors to communicate where sensitive plants and areas are. Be courteous, sincere, and respectful.
- Use buffer strips of untreated vegetation or windbreaks.
- If winds are shifting, stop an application and finish later when conditions are favorable.
- Measure wind speed and direction at the boom before applying.
- Do not apply when winds are less than 3 mph or greater than 10 mph.
- Use drift reduction adjuvants and nozzles to reduce the likelihood of drift occurring.
- Check for the presence of an inversion before applying.
- Familiarize yourself with your herbicide’s expected injury symptoms on targeted weeds.
- Follow up after applications and look for injury on nearby non-target trees and other plants. Not looking for it does not mean it’s not there. Recognition is part of the battle.
Lastly, applicators should educate themselves on this issue. IFA provides an excellent summary of the whole situation on their website where they discuss why the damage is occurring now and provide helpful links for landowners and concerned citizens. They too provide a somewhat similar yet different list of ways that applicators can help prevent damage to trees.
For an excellent video that summarizes what to look for when diagnosing herbicide injury on trees with comparisons of other factors that cause similar symptoms, please view, “Recognizing Herbicide Damage to Trees Training” by University of Illinois Extension Forester, Chris Evans. Another video to check out is my webinar, “Safe Use of Herbicide in Natural Settings”.
In summary, the general public is increasingly aware of herbicide drift and damage; damage on trees is being noticed more than ever before. Applicators must be mindful that neighboring landscape trees and timber stands can be affected by their applications. Herbicides are important tools for combatting weeds. Users are reminded to take careful steps to prevent drift from occurring.
ABOUT THE AUTHOR: Michelle Wiesbrook provides subject matter expertise and training in pesticide safety with an emphasis on horticultural weed science. She serves as the Illinois Pesticide Review newsletter editor, collecting and organizing material; and co-coordinates social media information for the PSEP program and ensures its timely publication.