COVID-19: Essential Information


Attack of the weevils: tiny insects safely control the pesky aquatic weed milfoil

August 29, 2005

MIDDLEBURY, Vt. - On Saturday, Aug. 27, Middlebury College Professor of Biology Sallie Sheldon pulled on her wetsuit and dove into the waters of Fairfield Pond in Fairfield as part of an effort to control the pond's infestation of Eurasian watermilfoil. Her weapons: aquatic weevils, tiny beetles that feast on the invasive plant.

A weevil sits on milfoil. (Middlebury College)

Sheldon and two student researchers, Middlebury College sophomore Julie Erickson and Lincoln resident Jess Lueders-Dumont, who will attend Colby College this fall, spent part of their summer growing the bugs-a milfoil weevil, Euhrychiopsis leconteia, native to North American waters-in tanks in the greenhouse attached to Middlebury's Warner Hall. To introduce the bugs to Fairfield Pond, Sheldon collected milfoil plants from the pond and brought them to Middlebury, where they were placed in the tanks, and weevils were given the chance to deposit their eggs. "Our weevils are egg-laying machines," says Sheldon.

Now covered with weevil eggs, the plants were placed back in the bed of milfoil in Fairfield Pond from whence they came. This technique-harvesting the plants from a specific lake or pond and then putting them back into the same pond-helps insure that no non-native species will make their way into the water. The expectation is that the eggs will soon hatch into weevils-Sheldon says the eggs have a remarkable hatch rate of nearly 90 percent-and that this will begin a cycle that will create enough weevils in the pond to begin controlling the Eurasian watermilfoil invasion.


Professor of Biology Sallie Sheldon places weevils on milfoil in Fairfield Pond. (Sally Ann Collopy)

Sheldon's work in Fairfield has been strongly encouraged by the members of the Fairfield Pond Recreation Association, in particular the husband and wife team of Bruce and Sally Ann Collopy.  The Collopys have been at the forefront of the association's long battle with Eurasian watermilfoil, which in the past has involved everything from hand-to-hand combat-divers manually pulling the weeds out of the bottom-to the construction of bottom barriers to prevent weed growth.

Sheldon expects that she will need to apply more eggs to Fairfield Pond in the spring, but that once established, the weevils will be able to make a serious dent in the weed beds that have bedeviled the users of Fairfield Pond and many other lakes since Eurasian watermilfoil arrived, uninvited, possibly via water from a fish tank, many years ago.


Members of the Fairfield Pond Recreation Association view weevils under microscopes provided by Professor of Biology Sallie Sheldon at Fairfield Pond. (Sally Ann Collopy)

Sheldon has been studying the relationship between weevils and milfoil for more than 15 years. She received grants from the State of Vermont, via the United States Environmental
Protection Agency, for several years in the 1990s, and determined that the weevils are an effective force for controlling milfoil. Sheldon and Middlebury then licensed the weevil growing and distribution process to EnviroScience (, an Ohio-based company. EnviroScience has used the weevils in a program it dubbed "MiddFoil" to help control weeds in more than 60 lakes and ponds in 12 states and one Canadian province.

Sheldon remained determined to translate all this success into a program in Vermont, and she has signed up three Vermont lake associations?at Fairfield Pond, at Eligo Lake in Craftsbury, and at Lake Morey in Fairlee. With permits now in hand from the Vermont Agency of Natural Resources, she expects to have weevil eggs in all three bodies of water by the end of this season, and hopes to follow up with more eggs next spring, once the ice melts and the weeds reappear.

The weevils, Sheldon says, do not damage other plant or animal species. The bugs, about half the size of a grain of rice, are native to North America and the Eurasian watermilfoil is not, so the milfoil has not developed defenses against these particular beetles. The plants do contain compounds that make them unappetizing to Eurasian weevils, but the North American bugs find the Eurasian interlopers tasty.

Sheldon says that the work this summer in the three Vermont lakes, funded by Middlebury College, is designed "to remind Vermonters that this all started here, and that it's a technique that works."