A herbicide recently accused of killing trees will be recalled and discontinued, DuPont announced this week.

The herbicide, called Imprelis (aminocyclopyrachlor), targets broad-leafed plants including dandelions and seemed particularly promising because it was not very toxic to mammals. It is a member of a new class of herbicides called pyrimidine carboxylic acids, which act by mimicking the plant hormone auxin. Many other herbicides on the market also mimic auxin, but have not shown the same arboreal toxicity.

The US Environmental Protection Agency (EPA) approved the compound for use by turf specialists last autumn. DuPont, based in Wilmington, Delaware, says that it carried out over 400 field trials of Imprelis, yet only a few months after Imprelis hit the market, complaints – and lawsuits – about dead trees from Iowa to New Jersey came rolling in. Hardest hit are the stately and fast-growing Norway spruce and white pine trees. In some Pennsylvania housing developments where Imprelis was sprayed, nearly every Norway spruce and white pine tree is gone, says Peter Landschoot, a turf grass specialist at Pennsylvania State University.

How could DuPont – and the EPA – have missed such a dramatic effect? Landschoot says that a DuPont representative showed him data from tests on about 20 different types of trees, but neither Norway spruce nor white pines were on the list of tested species. And even if sensitive trees were included in some of the field trials, other experimental conditions may have masked the effect. “When a company has an herbicide like this, they’ll screen it against as many plants as possible,” says Landschoot. “But you can never duplicate every condition that’s going to be out there in the field.”



Indeed, Landschoot says that he has seen healthy trees flanked by poisoned neighbors, and whole stands of white pines that were unaffected by nearby Imprelis spraying. The reason for the chemical’s inconsistent effects is unclear. DuPont spokeswoman Kate Childress says that the company is now reviewing its data on the compound.

Some of those data may soon be made available to the public. DuPont has submitted thousands of documents, many of which were initially classified as ‘confidential business information’, to the EPA. In a 3 August letter to the company, Abraham Ferdas, EPA director of the Land and Chemicals Division, urged DuPont to make those documents publicly available. “EPA is concerned about the sweeping nature of DuPont’s assertion of confidentiality and is evaluating whether these studies warrant such claims under the law,” he wrote. Childress says the company has since lifted the ‘confidential’ status of those papers and it is now up to the EPA to provide them to the public.

One thing is already clear: the other pyrimidine carboxylic acid herbicides in the pipeline will face added scrutiny, says Bert Cregg of Michigan State University’s forestry department. “It’s certainly going to change the approaches as to how these things are looked at and tested,” he says. “That’s for sure.”

Photo: Imprelis damage caught by John E. Kaminski, via Flickr