Milfoil Scientists Rule Out Weevils

Concord — November 6, 2006 — A weevil that has a taste for Eurasian milfoil doesn’t like the species of the invasive aquatic plant found in New Hampshire and Maine lakes.

“While the weevil likes to eat Eurasian milfoil,” said state Limnologist Jody Connor, “we found it would rather die than eat variable milfoil, the type of milfoil commonly found in lakes and ponds throughout New Hampshire and Maine.”

Some towns had hoped the weevil would help them combat the lake weed instead of using herbicides. The weevil is a tiny beetle the size of a sesame seed and feeds exclusively on milfoil. When exposed to Eurasian milfoil, the weevil eats it, causing the plant to lose buoyancy and collapse.

The weevil is native to North America and has been found in New England states, including Connecticut and Vermont. Scientists believe the weevil originally fed on native Northern milfoil but switched to Eurasian milfoil in the 1940s.

State officials say lakes in New Hampshire and Maine have a different water chemistry than in Vermont. The low calcium and high acidity in Maine and New Hampshire water prevents Eurasian milfoil from taking hold.

“The little weevil isn’t a great cure-all,” said Connor. “And I don’t want people to think this is a new idea.”

He said “weevil fanfare” began 20 years ago when the insect was discovered in a Vermont pond eating milfoil.

“It was doing some damage, and we . . . were very excited to have found a potential biological control for milfoil,” he said.

But experiments had disheartening results. The weevil is attracted to Eurasian milfoil by two chemicals released by the plant: uracil and glycerol. Variable milfoil lacks them. Thus, the picky weevil chooses death over such a bland dish.

Connor noted that the weevil doesn’t kill milfoil even where its introduction has helped control the weed.

“The weevil doesn’t kill milfoil. It merely tones it down, but the plant grows back because it doesn’t take out the roots,” Connor said.

Connor said the best way to manage the weed is to use several methods, including herbicides and pulling it out by hand. Meanwhile, the department continues to look for a biological predator.

“A few years ago, (we) received a $1 million grant . . . to study some invertebrates from the Midwest which might do some damage to our variable milfoil,” Connor said. “We’re presently doing some studies, but we won’t have anything definitive for at least another year.”

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