SEATTLE — For decades, oyster growers in southwest Washington have battled to control native shrimp that burrow in the mudflats and make it hard for oysters to grow.

Now, after getting state approval, a group of shellfish farmers plans to spray a widely-used neurotoxic pesticide on up to 2,000 acres of commercial shellfish beds in Grays Harbor and Willapa Bay. They insist it’s a safe way to keep in check a threat to the area’s multimillion-dollar shellfish industry.

But critics, including the U.S. Fish and Wildlife Service and the National Oceanic and Atmospheric Administration, worry about unintended harm. The plan is premature, they say, with too many unknowns about the pesticide’s effects on other organisms, including those that are a food source for threatened species.

With public and customer concerns rising, one of the big players, Taylor Shellfish Farms, said Friday that it would back away from treating its oyster beds.

In comments to the state, NOAA noted that state Department of Ecology “is clearly aware that imidacloprid is a persistent broad spectrum pesticide that will kill nearly all benthic (sediment-dwelling) organisms on acreage directly treated.”

Research “clearly indicate that effects and damages will not be limited to the treatment sites,” added the U.S. Fish and Wildlife Service.

The Ecology Department approved the permit in April, saying the pesticide is less toxic than one previously used, and which is being phased out, and unlikely to result in significant harm to the environment. The permit issued to Willapa Grays Harbor Shellfish Growers Association, a group of about two dozen growers, requires monitoring to ensure there aren’t significant harmful effects.

“I think we have sufficient information to issue this permit,” said Rich Doenges, a water quality manager.

Helicopter spraying over the two estuaries could begin as early as May 17. Areas can only be sprayed once a year, during daytime low tides. Up to 2,000 acres are allowed a year, so up to 10,000 acres could potentially be sprayed over the five-year permit.

But the ghost and mud shrimp play a role in the ecosystem, NOAA and the U.S. Fish and Wildlife Service said. They urged investigating other ways to grow oysters without chemical control.