By LISA W. FODERARO

SCHROON, N.Y. — Nosing into a shallow bay on Schroon Lake, Steve LaMere peered over the side of his pontoon boat. He was on an unusual reconnaissance mission, looking for signs of an aggressive aquatic invader, a stringy herb from Eurasia that once adorned American aquariums but has no business here in the Adirondacks — or anywhere in North America.

“I’m a fifth-generation Adirondacker, and I take this stuff personally,” said Mr. LaMere, a certified lake manager and ecologist who is on the front lines of the fight against invasive species. “The Adirondack Park is where I’ve drawn my line in the sand. There’s no reason we can’t get it under control.”

The plant he was after, Eurasian watermilfoil, is not new. First found in the United States in the 1940s in a pond in Washington, D.C., it has since spread to almost every corner of the country, endangering swimmers, boaters and other aquatic plants. Since the 1970s, its growth — along with that of many other invasive plants and animals — has exploded.

Like other invasive species, Eurasian watermilfoil is spread from continent to continent by ballast water from ships, and locally by recreational boaters and fishermen who unwittingly introduce plant fragments to clean lakes from infested ones. Until recently, it was an infrequent sight in the Adirondack Park in upstate New York, where many of the more than 3,000 mountain lakes and ponds are relatively inaccessible.

But as the park’s popularity has risen, so has the presence of Eurasian watermilfoil. By far the most widespread of a half-dozen nonnative aquatic plants in the park, Eurasian watermilfoil is now in more than 45 Adirondack lakes, including giants like Lake George and Saranac Lake. It threatens their biodiversity by muscling out native plants and can grow so thick that it becomes entangled in boat propellers and the limbs of swimmers.

While some places greet invasive species with a defeatist shrug, the Adirondacks is girding for war. In the past few years, the Adirondack Park Invasive Plant Program, which is a partner of the Adirondack chapter of the Nature Conservancy, has trained about 230 residents in the detection of aquatic invaders.

“We tried to recruit anyone who valued the water of the Adirondacks, whether shore owners or paddlers,” said Hilary Oles, the coordinator of the program.

Several lakes, including Schroon, have begun educational programs aimed at prevention, stationing volunteers at boat launches. Along with posted signs, the volunteers urge boaters to remove watermilfoil from their boats, as well as zebra mussels — another nonnative scourge.

Hundreds of thousands of dollars are spent each year to remove watermilfoil. In July and August, teams of scuba divers descend to hand-harvest plants, which can grow up to 15 feet tall. Where the watermilfoil is too dense for that approach (scientists have found as many as 300 stems per square meter), divers fasten huge sheets of plastic, called benthic barriers, to the lake bottom to blot out the sun.

Another method, known as biocontrol, uses nature — in the form of insects and fish — to fight nature. At Augur Lake, where Mr. LaMere was hired to combat its Eurasian watermilfoil infestation, hundreds of sterile grass carp were released several years ago to eat the plants. For now, the watermilfoil, which had cloaked 10 percent of the lake, is still there, but is less of a nuisance.

“We realized that benthic barriers weren’t realistic,” Mr. LaMere said. “Grass carp love to eat aquatic vegetation. The only Catch-22 is they prefer our succulent native vegetation over fibrous exotics. So you run the risk of denuding the bottom of the lake.”

There are also chemical weapons, but so far the Adirondack Park Agency, which regulates all land use in the six-million-acre park, has not permitted their use against watermilfoil.

The assault on Eurasian watermilfoil, as well as other invasive aquatic plants like curly leaf pondweed and purple loosestrife, comes as New York State is devoting new resources to the problem. In 2005, an Invasive Species Task Force appointed by former Gov. George E. Pataki issued a 146-page report, with a dozen recommendations and a call for the state to budget from $5 million to $10 million annually to address the issue.

The task force was led by two state agencies, the Department of Environmental Conservation and the Department of Agriculture and Markets. It also included representatives from 14 agencies and organizations, including the state’s Department of Transportation, Cornell University, the Nature Conservancy and the New York State Nursery and Landscape Association.

With its many airports, shipping ports and canals, New York has long been a gateway for invasive species. The report noted that while about a third of the state’s plants are not native, only a portion of those — perhaps 10 to 15 percent — are considered invasive, in that they are harmful to the economy, the environment or human health.

To illustrate how quickly invasive species spread, the report said that since the task force convened in 2004, at least six new ones have arrived in the state. They include the European crane fly and Brazilian elodea, a popular aquarium plant discovered last year.

Steve Sanford, chief of the Department of Environmental Conservation’s Bureau of Habitat, said that $5 million was allocated in the current budget to prevent and manage invasive species. Some of that money will help carry out one of the task force’s main recommendations, the creation of eight regional partnerships with nonprofit groups around the state.

One is the Adirondack Park Invasive Plant Program, which Mr. Sanford called a model for thwarting nuisance species in the park. “From an ecological perspective, the Adirondacks are much less invaded than the rest of the state,” Mr. Sanford said. “You haven’t had a whole lot of agriculture or manipulation of the land.”

The task force also gave a boost to a fledgling grant program in which the Department of Environmental Conservation shares the costs of eradicating invasive plants and animals with nonprofit groups and local governments. Last year, the Town of Schroon was awarded $26,000 to tackle Eurasian watermilfoil and curly-leaf pondweed over three years.

But the state is looking to the federal government for a more aggressive response to invasive species, Mr. Sanford said, “something comparable to the Clean Water Act.”

For now, the state funds are allowing lake managers to broaden the scope of their work. In Lake George, where the watermilfoil was discovered in 1985, scientists have identified 157 infested sites. All but 30 of those are largely free of the plant, the result of years of relentless hand-harvesting and the use of barriers.

Of those 30 sites, however, 17 have severe infestations, and because of the limited financing until now, the Lake George Park Commission, which oversees management of invasive species in the lake, has been able to address only a small fraction of them.

Michael P. White, the commission’s executive director, said the additional state money would mean more divers this summer. “We have two locations we have been working with the benthic mats where we’ll return, and we’ll add four more locations,” he said. “The state funding will allow us to expand our operations more on a scale that’s appropriate to the challenge.”

While some lakes in New York are choked with Eurasian watermilfoil, the early efforts on Lake George paid off. Of 1,800 acres of lake bottom where watermilfoil could conceivably take root (generally the shallower fringes), only about 10 to 12 acres have dense growth.

“Lake George has one of the most incredible assemblages of native plants, with more than 50 species of aquatic plants,” Mr. White said. “It’s a genuine environmental catastrophe to see these plant communities being ravaged.”

Once it establishes itself, Eurasian watermilfoil, which spreads when fragments of the plant break off and take root, is almost impossible to wipe out. “It becomes a maintenance effort,” said Lawrence Eichler, a research scientist with the Darrin Fresh Water Institute in Bolton Landing. on Lake George. “It really is underwater gardening.”

Some Adirondackers, like Helen D. Wildman, president of the Paradox Lake Association, harbor no illusions that pristine lakes will stay that way. “People say with great confidence that Paradox Lake will never have any milfoil,” she said. “But it will if we let our guard down.”