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Habitat for Habitat

It wasn't a perfect weapon, but the herbicide glyphosate-trade name Aquamaster, an aquatic version of Roundup - used to be the best one in the Invasive Spartina Project's arsenal. But glyphosate had serious limitations in treating infestations of Atlantic cordgrass (Spartina alterniflora). According to the ISP's Erik Grijalva, the herbicide easily binds to sediment in a marsh and becomes inactive. Effective treatment requires coating the entire plant (rarely possible) and a 6- to 12-hour post-application period without tidal inundation.

In the 2005-2006 treatment season, the ISP switched to imazapyr (Habitat), recently registered for use in California. A recent report from Leson and Associates that summarizes laboratory and field studies describes imazapyr as both more effective and less hazardous than glyphosate. Among the report's findings: imazapyr degrades rapidly in water and inundated soil, leaving no detectable residue after two months; maximum planned application rates are not toxic to mammals, birds, or bottom-dwelling organisms; and exposure risks to workers applying the herbicide and to the general public are minimal.

The downside: accidental spraying of non-target plants carries a higher likelihood of damage than with glyphosate. Laboratory tests also indicated a slight risk to fish at highest concentrations. But Grijalva considers it "extremely unlikely that there is a risk to fish based on the rates that will be used in an actual application."

Fact sheets from the Pesticide Action Network of North America note adverse effects to laboratory animals from imazapyr, including an increased incidence of cancer in rats that the U.S. Environmental Protection Agency considers statistically insignificant. It therefore considers the herbicide noncarcinogenic.

Despite imazapyr's promise, cordgrass may still be a tough adversary. Some weed species, including perennial ryegrass and rigid ryegrass, have evolved resistance to the terrestrial version of the chemical, marketed as Arsenal and Chopper.

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