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.