HAYDEN LAKE - Eight sites with Eurasian watermilfoil infestations on the east side of Hayden Lake have been selected for treatment with an aquatic herbicide.

That work starts July 16 and is scheduled to be completed July 20, with 72 total acres to receive an application of a liquid triclopyr.

In the past, the herbicide commonly known as 2,4-D was primarily used.

Tom Woolf, aquatic plant program manager for the Idaho State Department of Agriculture, said the triclopyr is more expensive but breaks down faster.

"Water-use restrictions won't be in place as long" with its use compared with 2,4-D, Woolf said.

Some irrigation and drinking water restrictions will be in effect for households using water from the lake. Swimmers are not affected.

Eurasian watermilfoil can out-compete native plants, reducing their diversity. Growth of thick mats of Eurasian watermilfoil degrades water quality, reducing dissolved oxygen levels. It also restricts and impedes recreational activities.

Once established, boats and trailers pick up the plant and take it to other lakes and streams.

Beginning this summer, the Idaho Department of Agriculture is taking responsibility for all control projects within Kootenai County.

Since 1999, Kootenai County has obtained numerous grants and invested a lot of time fighting the battle.

The county's noxious weed department staff members spent 30 percent of their time on the invasive species, according to a press release this week from the Kootenai County commissioners.

The state has contracted with Clean Lakes Inc., which has offices in Coeur d'Alene, to do the herbicide application, the commissioners said.

Woolf said Eurasian watermilfoil in Hayden Lake has crossed with a native milfoil and created a hybrid.

The hybrid is somewhat more difficult to control.

"It's a little more resistant to herbicide," Woolf said. "It just takes a little bit more to knock it down."

Surveys to identify the eight treatment areas were recently conducted.

"Last year's treatments were pretty extensive," he said. The treatments last year covered more than 200 acres.

"We're dealing with less funding this year, so we've had to prioritize," Woolf said.

Some of the areas targeted for treatment are: Henry Point, Skinner Bay, Chicken Point, Mokins Slough, Preston Beach, and O'Rourke Bay.

Because wind and wave action can hamper treatments, Woolf said night treatment might be used in some areas. Any night treatments would occur in open areas where more wind and wave action occurs, he said.

Often divers are used in areas with lower density infestations.

Divers might be used to pull up some plants later this summer, but no diver work is scheduled yet for Hayden Lake.

Eradication has been effective enough at Priest Lake that Woolf said the department hopes to use divers only on the lake this year.

"It's down to where you can just identify individual plants and you can pluck them out," he said.

Information related to the treatments at Hayden Lake will be posted and updated at: www.cleanlake.com.

Originally, Eurasian watermilfoil was sold as an aquarium plant and the practice of dumping aquarium fish and plants in the nearest body of water started the spread of the aggressive noxious weed.

In Kootenai County, infestations have been found in Harrison Slough, Spirit, Cave, Medicine and Black lakes.