Weed wars

As non-native species, the invaders don’t face the parasites and predators that keep native flora in check, and so, when they infect a lake, they usually grow rapidly, crowding out other plant life and destroying fish habitat. Their thick growth can make it difficult to boat and hazardous to swim.

Hill and Christine Guerette of the lake monitoring organization show photographs that are enough to make a water-lover shudder. In one, for example, milfoil has transformed the surface of a Maine pond into a tangled mat of vegetation that obscures the water; it’s as though a green shag carpet has been laid down.

When it comes to battling the invaders, there’s no silver bullet, says Guerette. Sunlight-blocking mats can impede their growth, but they hurt other plant life as well. Divers, sometimes aided by suction tubes, can remove the alien plants. But those labor-intensive methods are expensive. In Vermont, where Lake Fairlee has had a Eurasian milfoil problem for more than 15 years, annual costs for the 457-acre body of water have sometimes exceeded $100,000.

With some invaders, there’s a biological approach. Weevils, for example, like to munch on non-native milfoils. But at $3 to $5 apiece, the little bugs are expensive.

Herbicides can be used, but the chemicals also affect native plant life and can carry risk for people and fish as well. Because of that, Maine considers chemicals a last-resort treatment. Still, both Fairlee and Lake Morey in Vermont have turned to herbicides when divers and other methods have failed to contain milfoil.

Sometimes, the best you can do is to keep whacking away at the weeds with a mechanical harvester in an annual effort to keep them in check. That’s the case with the water chestnut infestation that plagues Lake Champlain.