The battle against Eurasian watermilfoil in Beaver Lake continues.

The latest development was a Wednesday evening closure of the lake by the Montana Department of Agriculture.

The closure came after ag department Weed Coordinator Dave Burch talked to people in Flathead County who told him the milfoil had grown so much this spring that it was tall enough to get caught on boats. The 40-by-50 patch of weed discovered last fall has spread to a larger area and single weeds have been found at multiple sites on the lake, Burch said. All of those factors played in his decision to issue a temporary quarantine to close the lake. The closure is to all watercraft, but not to anglers or swimmers, Burch said.

He expects the closure to be lifted in about a week.

A diver-operated dredge is now slated to be at Beaver Lake, north of Whitefish, on Wednesday. The dredge essentially sucks the weeds out of the lake bottom.

A working group which has been meeting for several months to figure out a plan of attack to get rid of the milfoil in the lake had determined last week that they would begin with diver dredging, but the expectation of that group was the dredging couldn’t occur for about another week.

The ag department’s quarantine action and hasty approval of various contracts to begin the work allows for it to start Wednesday, Burch said.

Personnel with the Montana Department of Natural Resources and Conservation notified lease holders on the lake of the closure on Thursday, Burch said. Some sort of structure is to be placed at the public boat launch soon, to prevent any boats from launching until the quarantine is lifted, he said.

After the dredging occurs, the lake will be surveyed again to make sure all the milfoil has been removed, Burch said. Once the weed is gone, the lake will be opened to watercraft.

RECENT INSPECTIONS of the lake by members of the group trying to address how to control the aquatic invasive species showed that mats placed on top of the weeds last October failed to kill the weeds as hoped.

A diver inspected the area twice last week, marking the fourth diving inspection of the lake bottom this spring. All four times divers found the mats had shifted and weeds were sprouting up in the areas between mats. Weeds were also squirming out around the edges of the mats and through some small holes in the seams between mats, Flathead County weed department employee Gordon Jewett said. Additional mats were placed on top of weeds last week.

Raking inspections also have been conducted on the lake this spring, in which a rake is tossed out from someone in a boat to see if more patches have been established or to see if stray, single weeds have popped up around the lake.

EURASIAN WATERMILFOIL was found in Beaver Lake last October, a few feet from the public boat ramp. The boat ramp was closed for several months to prevent the weed from spreading. It was reopened after the ice came off the lake this spring.

The mats, which are designed to block sunlight from the weeds and kill the patch, have remained on the lake bottom since they were placed on top of the patch on Oct. 31.

But, working group members acknowledged the mats aren’t killing the weeds.

Deciding what to do next was a slow process for group members, who represent Flathead County, Montana Fish, Wildlife and Parks, DNRC, the ag department, the Whitefish Lake Institute, the Flathead Lakers and the Flathead Basin Commission.

After two meetings in the last few weeks, more inspections of the lake and a decision by Fish, Wildlife and Parks to allow a multiple-pronged approach to killing the weeds, working group members last week agreed to a plan. It calls for leaving the barrier mats in place until a diver-operated dredge can arrive at the lake to suck the weeds out. That will occur Wednesday.

The plan also allows an herbicide to be applied to patches of milfoil to kill the weeds. Mats may continue to be used, as needed, under the plan.

Burch agreed that if the dredging isn’t as successful as expected, the herbicide treatment will occur later.

The timing is right to pull milfoil now, according to Erik Hanson, a biologist and consultant to the work group, because the weeds won’t break apart like they do once water temperatures cool down and the weed stops growing for the season. That was the case last October when the group tried to pull weeds out of the lake bottom.

ALTHOUGH DNRC Stillwater Unit Manager Brian Manning, Fish, Wildlife and Parks Regional Supervisor Jim Satterfield and Whitefish City Council Member Frank Sweeney all supported applying an herbicide as soon as possible, other working group members wanted to pull weeds first, evaluate the results and then perhaps apply herbicide or place more mats in the lake. In the end, those options won out and were supported by Burch.

“I think it can all be taken out by diver dredging,” Hanson said, “but it will take coming back to check” and possibly dredge a second time.

And, he said, even if an herbicide is used, divers would have to check on results and remove dead weeds.

“It seems to me we need to knock it down with herbicides,” Sweeney said. “We need to deal with it now.”

Manning suggested the diver dredge be used on individual plants and the patch be treated with a chemical.

“I’ve been advocating herbicide on the mats all winter,” Satterfield said. “If another patch emerges, we may have to apply a chemical a second time.”

“I don’t think we have anything to lose in putting a double whammy on that patch,” Satterfield said. He was hopeful, though, that a one-time chemical application would be adequate.

Others weren’t as optimistic, including Celestine Duncan, a weed specialist who is a consultant to the DNRC.

“I’ve never seen 100 percent results with a chemical, but I’ve never treated over mats,” Duncan said.

Heidi Sedivy, who works with Hanson, said herbicide used on milfoil in the Noxon area has resulted in a kill rate of between 60 and 90 percent of the weeds.

Hanson pointed out that herbicides usually have to be applied more than once to kill milfoil. Pulling the weed should be a permanent solution.

If herbicides are used, short-term closures of the lake are possible in order to avoid diluting the herbicide. If the herbicide is pushed away from its target by activity in the water, it may not kill the weeds.

The herbicides are safe for people to be around.

“You can swim and even drink the water” after the herbicides are approved, Sedivy said.

“Any closure associated with herbicides is not because of a threat to people, it’s to avoid spread,” Flathead County Weed Supervisor Jed Fisher said.

IN ADDITION TO treating Beaver Lake to kill the milfoil, the plan calls for installing a barrier at the outlet of Beaver Lake to prevent milfoil fragments from reaching Whitefish Lake. That barrier has been ordered, Burch said.

That spread is a serious concern to members of the Whitefish Lake Institute and the city of Whitefish. Institute members have repeatedly surveyed the area between the two lakes but have found no milfoil moving toward Whitefish Lake.

Manning shares the concern for the weed spreading. In the scope of work for the milfoil eradication effort, he wrote that the purpose of the project is to “protect the aquatic habitat in Beaver Lake and minimize the opportunity for transfer of (milfoil) to other nearby waters.”

The scope of work also provides for monitoring for a few months after all work is completed.

Eurasian watermilfoil is a noxious aquatic weed. It grows as water temperatures rise in the summer. Milfoil spreads quickly and easily and left untreated can render a body of water unusable. The weeds can damage boat motors and grow in thick mats that affects fish habitat which can kill fish. The noxious weed also kills native species in a lake.

Milfoil floats easily through a lake and small pieces of the weed can become established quickly.

If it’s stuck to a boat that is then taken to another body of water, it infests that second body of water. That’s why people are encouraged to wash their boats after each use.

Reporter Shelley Ridenour may be reached at 758-4439 or sridenour@dailyinterlake.com.