Specialty Crops at Risk from Dicamba and 2,4-D Drift PART 1 OF 5 An Overview of Dicamba and 2,4-D Drift Issues Dicamba and 2,4-D drift Changes in Dicamba and 2,4-D Use Dicamba and 2,4-D are post-emergence herbicides that have been damage has captured national used for many decades to selectively control broadleaf weeds in attention in recent years. corn, hay fields or pastures, small grains such as wheat, and turf. They are also used to destroy existing broadleaf weeds prior to Could your farm be at risk? planting agronomic crops. And if so, what should you Starting in 2016, the use of these herbicides changed. Over- know to prevent, prepare, dependence on glyphosate has resulted in widespread glyphosate- resistant broadleaf weeds. To improve management of these and respond? In this fact weeds, new soybean varieties tolerant to over-the-top applications of dicamba (Xtend soybean) or 2,4-D (Enlist soybean) were sheet—the first in a series— commercialized in 2016 and 2019, respectively. Widespread we explore how dicamba adoption of dicamba- and 2,4-D-resistant soybeans has increased use of these herbicides during the months of May, June, and July. and 2,4-D drift has become (USDA 2019). a threat to specialty crop Dicamba and 2,4-D have been effective in controlling glyphosate- resistant broadleaf weeds, but their expanded use has increased producers. the risk of drift damage to high-value fruit and vegetable crops, landscape plants, and soybeans that do not carry the trait for dicamba or 2,4-D resistance. In addition to being more phytotoxic than glyphosate for many specialty crops, both herbicides are highly prone to drift. Off-target movement of dicamba, in particular, continues to be well documented in Missouri (Bradley 2017, 2018), Illinois (Illinois DOA), and Indiana (Office of Indiana State Chemist 2019), despite efforts to reduce drift though improved formulations, training, and label restrictions. An Overview of Dicamba and 2,4-D Drift Issues | 1 John D. Byrd, Mississippi State University, Bugwood.org Mississippi State University, John D. Byrd, not. The lack of vertical air movement means spray droplets can become suspended near warm air the ground surface in colder, denser air. cooler air Air can move horizontally, like a fog, onto neighboring fields, carrying these spray droplets along. Morning sunlight or stronger winds usually mix the air layers and bring an end to inversion conditions. But applications made during an inversion late in the day may have several hours to move off target. Dicamba and 2,4-D can also move off-target as a gas. They are among the relatively small FIGURE 1 | In normal While concentrations of herbicides in drift number of pesticides that are highly prone daytime conditions (left), are typically low, several crops have shown to volatilize (turn into a gas). A spike in air warmer air rises from the extreme sensitivity to dicamba and/or 2,4-D. temperature can cause these herbicides to earth’s surface, while For example, damage has been documented cooler air from higher in turn into a gas even after they have been on grapes exposed to 2,4-D and dicamba the atmosphere sinks, successfully applied to target surfaces. As concentrations as low as 1/800th of the causing a vertical mixing temperature inversions form, pesticide vapors labeled rate. For high-value sensitive specialty of air. In the absence of lingering in the atmosphere can be pulled crops, a small amount of drift can result in substantial horizontal back down to the surface where they can huge losses. Notable lawsuits from producers winds, herbicide droplets cause damage. will hit their target on the of honey, peaches, grapes, and tomatoes have ground or vaporize and recently brought this issue to the attention of The applicator is legally responsible for dissipate into upper air specialty crop growers nationwide. checking both current and predicted weather levels. conditions, following all label restrictions, But during a temperature How Drift Happens and knowing where neighboring sensitive inversion (right), the air crops are located. temperature close to the Drift is the movement of pesticide from the Off-target plant damage may also result from ground is cooler than target field onto nearby sensitive plants. the air column above it Most drift consists of spray droplets moving inadequate cleaning of sprayer equipment, and vertical mixing of downwind during an application. This type contaminated runoff water, and sometimes air does not occur. In of drift (referred to as droplet, physical, from contaminated grass clippings or these conditions, small primary, or particle drift) can be minimized compost. pesticide droplets stay by following the label recommendations for suspended in the denser spray equipment, procedures, and weather How Dicamba and 2,4-D Drift cold air, settling close to conditions. Damages Plants the surface. But the air can still move horizontally, In addition to wind, temperature inversions Dicamba and 2,4-D are the most common moving these droplets have a critical effect on drift. Temperature members of a group of herbicides that mimic onto neighboring crops. inversions occur commonly during naturally occurring plant growth regulators. Midwestern summers (Bish 2019), typically These herbicides overload a plant’s growth- FIGURE 2 | a) Dicamba forming in the late afternoon or evening and regulating system. Sensitive crops exposed to drift injury causes leaf persisting through 1-2 hours after sunrise. drift levels of dicamba and 2,4-D may show cupping to grapes Temperature inversions occur most often b) 2,4-D damage on grape distorted growth in the leaves, leaf petioles, under light or calm winds, due to a lack of manifests as chlorosis and or stems. Symptoms of drift damage can distortion in the shape of vertical air movement (See Figure 1). Since show up within hours if plants are growing the leaf and veination. inversions are marked by very little wind, quickly, or within a few days under slower it may seem like a good time to spray. It is growing conditions. a b The symptoms and severity of dicamba and 2,4-D drift can vary greatly between species. Watch for cupped leaves (Figure 2a and 3) or leaves that become chlorotic (on leaf tissue or veins), narrowed, or develop parallel veination (Figure 2b), stunted growth (Figure 3), necrosis (tissue death), delayed or uneven flowering/ fruit set, (Figure 4), twisted growth on stems and petioles, adventitious root development (Figure 5), and reduced yield. Doug Doohan, Ohio State An Overview of Dicamba and 2,4-D Drift Issues | 2 Mark Loux, Ohio State Doug Doohan, Ohio State Lindsey Orphan, Southern Illinois University Carbondale FIGURE 3 | Dicamba FIGURE 4 | Uneven and aborted fruit set on injured FIGURE 5 | Adventitious roots on pumpkin after damage on non- grapes. exposure to a full dose of dicamba. genetically-modified soybeans includes stunted growth, cupped We’ve talked about dicamba and 2,4-D by conventional agronomic crops, you may and crinkled leaves. together, but they are two distinct chemicals. indeed be at risk. Additional publications will A plant’s response will not necessarily be the compare sensitivities among various common same for both. For example, snap beans and specialty crops, answer frequently asked lima beans are more sensitive to dicamba questions, share proactive tips to protect and than to 2,4-D. In grapes, low concentrations prepare your operation against drift damage, of both herbicides can cause stem twisting and review steps to take if you believe drift We’ve and leaf droop, but dicamba more typically damage has occurred. talked about causes cupped leaves in grapes, while narrow leaves with parallel veins (strapping) is a Helpful Resources dicamba symptom of 2,4-D damage in grapes (Figure 2). Other species may show different signs of Drift Mechanics and Dicamba, 2,4-D and 2,4-D damage. There is too much variation between History together, species to make broad generalizations. The Deal with Dicamba University of California IPM has a searchable National Agricultural Law Center but they are photo gallery of common crops injured by Detailed information on dicamba legal developments. sub-lethal doses of herbicides. University nationalaglawcenter.org/the-deal-with- two distinct of Missouri also has an excellent collection dicamba-part-one/ chemicals. of photos from dicamba and 2,4-D drift Air Temperature Inversions Causes, sensitivity trials which includes a variety of Characteristics and Potential Effects on A plant’s woody and herbaceous specialty crops. (See Pesticide Spray Drift Resources.) North Dakota State Online Publication AE1705 response (Revised Oct. 2019) While tools like these can be helpful, most Detailed information about temperature inversions will not growers will need to consult a professional for written for applicators. ag.ndsu.edu/publications/ necessarily be a final diagnosis. If a state pesticide regulator crops/air-temperature-inversions-causes- cannot be consulted quickly, an extension characteristics-and-potential-effects-on- the same for or university employee, crop consultant, or pesticide-spray-drift farmer advocacy organization may be able to Why are temperature inversions both. suggest professionals or laboratories that can important when deciding on whether we help. See our related fact sheet on responding should spray? to and documenting suspected drift damage. University of Minnesota Extension Crops Short video from 2017 which provides a good It is important to review these steps before description of temperature inversions. youtu.be/ drift damage happens. jG10vCT1POg Five Things We’ve Learned about Dicamba Evaluating Your Risk University of Missouri This fact sheet series is designed to help you Recent findings on temperature inversions, pH effects on consider your level of risk and take action to volatilization, and plant drift injury.