Cobbossee Lake to get herbicide treatment to combat Eurasian water milfoil

WINTHROP — Winthrop residents want Eurasian water milfoil eradicated from Cobbossee Lake.

Residents expressed support Wednesday at a public hearing for using an aquatic herbicidal treatment to kill the invasive plant. The Maine Department of Environmental Protection would administer the treatment this summer.

Eurasian water milfoil was discovered in the lake in 2018, scattered across an acre between the north shore and Island Park.

The infestation is fairly new and not well established, John McPhedran, biologist for the Maine DEP’s Invasive Aquatic Plants Program, said at the hearing. He expects the combination of hand removal, with the herbicide, will produce good results.

“We still feel the plant is isolated to this part of the lake,” McPhedran said, “which means we have the opportunity to limit the spread through the lake and other waters.

“We are proposing to use an herbicide because we are not able to get all of the plants with a manual removal,” he added.

Because the infestation is not well established, the individual plants are hard for divers to find and remove.

“I’ve seen (invasive milfoil) on other bodies of water where it will grow up and mat,” lake resident Adam Douin said. “Once it does that, you might as well sell your place; you’re not going to use your boat.

“We have a chance at eradication,” he added. “We have a golden opportunity.”

The treatment would cover about 4 surface acres on the north shore and around Island Park. The lake is 5,516 acres in size.

The herbicide that’s been selected is ProcellaCOR EC. The active ingredient in it is florpyrauxifen-benzyl, McPhedran said, a synthetic plant hormone that makes the plant cells grow larger than they should so the plant dies.

He said Eurasian water milfoil is susceptible to this herbicide, though there also might be some temporary effects on a few native species, such as watershield. Not all plants will be susceptible to the herbicide.

The discharge of the herbicide will take one day to complete.

While no swimming restrictions will be in place, the DEP advises lakegoers to stay out of the area during treatment. Boats also should stay out of the area the day of the treatment, because watercraft will be discharging the herbicide.

Also, the DEP advises residents in the area not use lake water for hydroponic, greenhouse or nursery irrigation without contacting the DEP to confirm the herbicide has dissipated. Residential and non-agricultural irrigation for residential landscape plants and homeowner gardens should be avoided for three days after treatment.

Eurasian water milfoil spreads by autofragmentation, when pieces of the plant break off and new plants form.

Preventive measures are the best way to keep invasive milfoil species out of lake bodies. DEP recommends that boaters:

• Clean off any organic debris watercraft, trailer, motor and other equipment, and discard that material in the trash or on high, dry ground so it cannot wash back into the water.

• Drain all water from a boat, an engine, and other equipment, away from the water.

• If rinsing with hot-high pressure water is not an option, dry anything that came into contact with the water in the sun for five days.

How Eurasian water milfoil spread into the lake isn’t known.

Only a handful of herbicidal treatments have been undertaken in Maine waters, McPhedran said, including at Salmon Lake in Belgrade in 2009, which was successful at removing Eurasian water milfoil.

Eurasian water milfoil exists in two other ponds in the state, McPhedran said, and there are six species of milfoil that are native to Maine.

Toni Pied, of Friends of the Cobbossee Watershed, said Eurasian water milfoil can be identified from other types of the invasive plant because the pinwheel-shaped leaves are spread farther apart than those of native milfoil.

The Eurasian variety is not the only invasive milfoil plant clogging Maine waters. Annabessacook Lake in Monmouth and Great Pond in Belgrade have been affected by variable-leaf milfoil.

The next step for the DEP is applying for a Division of Water Quality Management permit to discharge the herbicide in the lake. Part of that application requires written acknowledgement from organizations providing public water supply on the lake, which includes two summer camps — Camp Cobbossee and Camp Kippewa — and one campground, Birches Campground.

Once it has the permit in hand, DEP officials expects to notify nearby landowners that the treatment will take place.