Hundreds of trained volunteer monitors will take to the state's lakes and ponds next weekend to resume the war against increasingly damaging and costly infestations of exotic aquatic plants, especially variable milfoil.

About 475 volunteers and 220 paid lake hosts will be stationed throughout the summer at motorized boat access sites to lakes and ponds as part of the New Hampshire Lakes Association's Lake Host program, said Andrea LaMoreaux, the association's vice president of education and communication.

For the 11th year, they'll be checking each boat entering and leaving for signs of milfoil, fanwort, water chestnuts, zebra mussels, and other “invasives” that attach themselves to boats and engine parts. Lake hosts can spot these plants quickly because of their training.

The hosts have also been trained to politely offer boat owners free inspections of their boats, and they pass along current information on threats posed in each lake and pond. They also teach boat owners how to do their own boat and equipment inspections.

“It's an educational kind of thing,” said Carol Foss, a summer resident of Little Lake Sunapee since she was a child, and a lake monitor since 2007.

“As boats come in, we ask them if they're familiar with these species and offer them complimentary boat inspections,” she said. “We also ask them where they've come from; we have a list of infected water bodies with us. And we watch them as they go out.”

Most boat operators are cooperative and are helpful in the effort, especially when they're given details about the problem, Foss said. “It's a terrible problem. It's incredibly expensive to fight and it can really lower lake values.”

The non-native plant species, which mostly have no predators in the state, cling to boats in one water body and migrate to others, where they occupy waters and grow out of control, often causing thousands of dollars in damage, she said.

Once in place, they are very difficult to stop and remove. Under optimal temperature, light and nutrient conditions, milfoil may grow up to an inch per day.

Invasives such as milfoil pose serious threats to the state's most treasured water bodies, endangering the tourist industry. Several towns have seen property values drop because of milfoil infestations, LaMoreaux said.

Local lake associations and conservationists joined with the lakes association and state environmental officials in the Lake Host program, and it has shown great success.

Since 2002, lake hosts have conducted more than 428,000 courtesy boat inspections and have captured 1,119 pieces of hitchhiking exotic aquatic plants. This summer, 78 local groups will be lake hosting at 97 of the most highly used public boat ramps in the state.

Participating local groups — lake and watershed associations — receive a payroll grant from the lakes association to pay lake hosts. Local groups must match the payroll grant.

Paid hosts, mostly young students and retirees, make between $7.50 and $12 an hour, depending on experience, LaMoreaux said.

But for volunteers, pay isn't the issue, Foss said. “These lakes and these waters ... this is why people come up here, this is why we're here — we have to protect them.”