Landscapers switched to Imprelis this year to control weeds because it was claimed to be safer for the environment than predecessors, The Detroit Free Press reports. The Environmental Protection Agency approved it last year.
Amy Frankmann, head of the Michigan Nursery and Landscape Association, said she has not seen such widespread tree death since the emerald ash borer ravaged ash trees.
"I'd say this is right up there as far as the significance and losses," Frankmann said.
"The customers are calling: 'My trees are dying, what's up?' " said Mark Underwood, a Michigan lawn care specialist. "We've never experienced anything like this."
The EPA held a teleconference Wednesday with agriculture officials from several states to gather information about Imprelis' effect on trees.
DuPont spokeswoman Kate Childress said: "We're taking this seriously. We're very committed to understanding the circumstances."