STOCKBRIDGE — Call it The Case of the Mysterious Vanishing Eurasian Water Milfoil.

For at least half a century, Stockbridge Bowl, the state-owned Great Pond also known as Lake Mahkeenac, has been bedeviled by those highly invasive weeds.

A tug of war between the Stockbridge Bowl Association, advocating an herbicide treatment to clear up the lake, and the town's Conservation Commission, firmly opposed to chemical cures, yielded a surprising outcome this month.

The weeds can't be found in large numbers.

Foresight Land Services, on behalf of the town, commissioned a study on the status of invasive plants, and of rare snails that inhabit the shoreline and remain protected by the state.

The report would have cleared the way for the first limited test this month of a fluridone herbicide application to wipe out the milfoil on a small portion of the lake's southern end. That was to be the first step in a treatment plan approved by Berkshire Superior Court Judge John Agostini last year and sanctioned by the state Department of Environmental Protection, despite opposition from the town's Conservation Commission and other herbicide skeptics.

But, last week, the study by Thomas W. Coote, director of the Berkshire Environmental Research Center, appeared to catch all sides by surprise.

Coote's report says "no extensive beds of invasive plants were found" during the July 28 and July 31 samples along the lake's perimeter and the channel at the southern end. The studies included water depths of 8 to 15 feet, as well as less than 8 feet.

"Our elusive enemy, Eurasian Water Milfoil, has evaded capture and destruction by disappearing this summer," Stockbridge Bowl Association President Richard Seltzer said in a report to the association's board.

"After our hard-fought battle with the Stockbridge Conservation Commission and our decisive victory in Superior Court to get permission to use an herbicide in the lake, it feels strange to have the target of our herbicide virtually disappear this summer," Seltzer said.

But, the situation might be temporary.

Kenneth Wagner, a lake-management biologist who advises the association, said the disappearance of invasive milfoil weeds has been noted in other lakes, most recently Lake Garfield in Monterey.

He told the group that insects that eat milfoil, such as the milfoil weevil, can gain in numbers and thin the weed down nearly to its roots. But, after those insects become prey for fish, their population falls.

"The milfoil has always come back, usually within two years," Wagner said.

The 'conundrum'

Select Board member Patrick White says the missing milfoil presents "a conundrum."

"How do we proceed with a multiyear ecological restoration in terms of the proposed herbicide treatment where the problem we are trying to address may not present itself from one year to the next?" he asked.

White, who also sits on the Conservation Commission, voiced hope that the town and the Stockbridge Bowl Association can set aside their differences.

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"We all need to work together via the Stockbridge Bowl Stewardship Commission to identify the issues facing Lake Mahkeenac and develop workable solutions that address the long-term health of this Great Pond," he said.

Selectwoman Roxanne McCaffrey said the town hired Foresight Engineering to prepare a "Notice of Intent" for the DEP for weed harvesting only. That took longer than anticipated, she said.

That harvesting cannot happen if significant beds of the weed cannot be located. The DEP allows such harvesting only if milfoil and curly-leaf pondweed, individually or collectively, make up more than 50 percent of the lake's "plant community."

At a recent Conservation Commission meeting, Chairman Ron Brouker said: "The Stockbridge Bowl Association seems to be in a situation where they need more Eurasian milfoil to grow, and infest the lake more, so they can go into the lake and kill it all off. Very strange position to be in."

Coote's report cited only a "very small patch of milfoil at the north end" as well as a widespread, but short, weedy invasive plant called Najas minor. On the main lake, the study found only native plants in scattered locations, including the shore of the town beach. In shallow waters, native plants were found, some growing densely around docks.

Extensive beds of milfoil were not spotted in the lake or the channel, although there were individual plants around the perimeter of the lake and higher densities in the channel.

Most of the milfoil found was not reaching the surface. The weeds were sparse and relatively short, the survey found, though, in some areas of the channel, milfoil was reaching the surface.

Sharp contrast

Seltzer, the Stockbridge Bowl Association president, says that in a recent tour of the lake with a biologist, they found "very little" aquatic vegetation such as milfoil growing near the shoreline. That finding was in sharp contrast to the prevalence of milfoil identified in a survey two years ago.

"If the amount of milfoil found this week were typical of every year, no one would be spending time and money seeking to eradicate milfoil in our lake," Seltzer said.

Now, he believes that plans for herbicide testing in two areas of the lake where milfoil was plentiful in the summers of 2018 and 2019 would be pointless.

"Next year may produce a very different picture," he said. "In past years, milfoil has been prevalent in different places, sometimes around almost the entire perimeter of the lake, sometimes dense in some parts and less dense in others.

"This is the first time I've seen so little milfoil anywhere," he said.

Seltzer questioned the DEP's ruling that the town would only be permitted to harvest milfoil and curly-leaf pondweed.

"I know of no other lake or pond in the Commonwealth that is restricted from harvesting long-leaf pondweed or water lilies," he said, adding that it's not true that the lake is substantially clear of weeds this year.

The Stockbridge Bowl Association board's meeting Sunday is expected to include a full discussion on future steps.

Clarence Fanto can be reached at cfanto@yahoo.com, on Twitter @BE_cfanto or at 413-637-2551.