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Registration date : 2009-07-08

PostSubject: Milfoil Prevention Chemical   Tue Sep 21, 2010 12:45 pm

Chemical effective in killing milfoil before it grows
By DAVID BROOKS Staff Writer

When more than a ton of herbicide was applied in Lake Potanipo last week, the Brookline water body became one of more than two dozen throughout the state where the battle against variable milfoil has involved such chemicals.

“We’re trying hard not to think of chemicals as the first line of defense, and just throw herbicide in there, but sometimes that turns out to be the only practical route,” said Bob Wolff, administrator of the groundwater program in the state Division of Pesticide Control.

Wolff said alternative methods like installing barriers, using mechanical harvesters or even uprooting the aquatic invasive weed are part of long-term management plans, but they have their limitations.

“It all sounds great until you put a diver underwater and try to weed 80 acres – particularly when you pull up one of these things, it raises so much silt you can’t see,” he said.

No matter the method, officials always prefer to prevent invasive plants from arriving, rather than drive them out after they’re here.

A number of programs have begun in recent years to try to keep plants from spreading. The most obvious are lake hosts that inspect boats as they are pulling from lakes or ponds. They try to make sure that no bits of weed are stuck on a propeller or trailer, to be carried as a “seed” to another water body.

Variable milfoil, which arrived in the U.S. decades ago from Asia, grows so thick that it can choke out native species, ruining the ecosystem for other plants and animals and make a pond unsuitable for swimming or boating. It is by far the most prevalent aquatic invasive species in New Hampshire, found in 64 public ponds and lakes, including a number in the Nashua region, ranging from Flints Pond in Hollis to Otternic Pond in Hudson to Horseshoe Pond in Merrimack.

Six other species of invasive water plants have been targeted by the state, including the slimy algae called “rock snot” in some North Country brooks to water chestnut, found only in the Nashua River. Eurasian milfoil, a related species, is also invasive in New Hampshire but is not as aggressive.

None of these is anywhere near as widespread as variable milfoil, however.

Herbicide has long been used in controlling variable milfoil. These days, the state prefers a chemical called 2,4-D, which was applied in Potanipo.

Another herbicide, diquat dibromide, has largely been supplanted, Wolff said.

“It was a quick fix but didn’t kill to the roots. It was sort of like cutting the grass – it knocked down the growing milfoil (in) the summer, but it would start to regrow by the fall,” he said.

The drawback to 2,4-D is that its potency requires more restrictions on the water body after each application.

Under a series of regulations developed by the federal Environmental Protection Administration and state Department of Environmental Services, swimming, drinking and even use for irrigation is often forbidden for at least 24 hours, and often longer.

“It depends on analysis of samples – not some number of days (since the application) – to establish how much of the herbicide is present, and what is safe,” Wolff said.

The application for Lake Potanipo received permission to apply 100 pounds per acre, covering up to 46 acres of contaminated lake bottom areas. The product can be applied in doses of up to 200 pounds per acre. The maximum amount allowed in each case is established by the Division of Pesticide Control’s licensing procedure.

The state is assessing more data to determine what happens to the herbicide over the long term and how such factors as water temperature and depth affect this. Potanipo is part of that regimen, Wolff said.

David Brooks can be reached at 594-5831 or dbrooks@nashuatelegraph.com.
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