BY MAX SILVERSON
The Star-News
Herbicides to kill invasive Eurasian milfoil are planned to be released in Payette Lake to fight an infestation that has reached a tipping point, the McCall City Council was told on Thursday.
Eurasian milfoil is an invasive species found in several lakes in Idaho, but the plant has become so widespread in Payette Lake that efforts to eradicate it must be increased, according to Jeremy Varley, the chief treatment engineer with the Idaho Department of Agriculture.
Varley gave a presentation to the council last week on the state’s potential eradication efforts for the worsening infestation. Varley gave a similar presentation to Valley County Commissioners last fall and again on Monday before the Big Payette Lake Watershed Advisory Group.
A week-long survey of the lake last year by 15 divers showed a “bathtub ring” of milfoil around the entire shoreline, where the plant was found in every area of water three to 24-feet deep, Varley said.
Varley’s team plans to start with a roughly eight-acre test plot near North Beach in late July as the first step in a multi-year effort. No specific date has been set.
What type of herbicide has yet to be finalized, but Varley’s team can only use specific products that have been approved by the state.
Herbicides under consideration for Payette Lake have no effect on the safety of drinking water or recreators swimming in the lake, Varley told the Big Payette Lake Watershed Advisory Group on Monday.
Officials have been working to remove milfoil from Payette Lake for about 20 years through “mechanical treatments,” which involves sending a diver into the lake to rip the plant out from the roots and suck up the branches with an underwater vacuum.
Removing the plant by hand has been helpful, but “we’ve reached a tipping point,” and more needs to be done, Varley said.
Using herbicide to kill invasive plants in Payette Lake has never been done before on a large scale.
Similar pesticide treatments have been done in several Idaho lakes. Varley highlighted the recent example of an effort in Lake Pend Orielle near Sandpoint.
Milfoil mats in the lake were reduced from hundreds of acres to 48 acres, Varley said.
There are native milfoil species in Payette Lake and across the state, but invasive Eurasian milfoil has no natural predators like fish and invertebrates to keep it in check.
Once it gets into a waterbody it can take over the lake, creating warmer water and degrading water quality, Varley told the council.
“It really is an ecosystem crashing plant,” Varley said.
Milfoil is so difficult to control because of the way it spreads. Any section of its segmented structure that breaks off can start a new plant.
Once the poison is introduced, the plant typically does not grow back in that area unless it is reintroduced by a “hitchhiker” like a small piece that can root the ground and spread.
When a full matting of milfoil grows out of control, it can take create a “ring of inaccessibility” taking over fish habitat and choking out waterways where the plant can tangle up motor boat props, he said.
Herbicides would not eradicate the plant completely from Payette Lake, but the goal is to bring the infestation in check so that mechanical treatments could be used to control milfoil more effectively in the future.
The North Beach test plot was selected because it is far from a drinking water intake and has good water movement, creating a lower chance of recreational contact with high concentrations of the herbicide before it dilutes and spreads, Varley said.
City officials worried that the herbicide could affect McCall’s drinking water, which is taken from the lake near Legacy Beach and Davis Beach in McCall.
Varley said the project would not put dangerous levels of the herbicide into city drinking water and divers would be used to remove milfoil in areas near the drinking water intakes.
Testing sites would be set up to monitor concentrations of the chemical before, during and after spraying to confirm whether drinking water is safe, Varley said.
Sites would be tested every 24 hours following the treatment. The chemical is usually not detectable on a scale of parts per billion within 36 to 50 hours, he said.
City council members expressed concerns about how the herbicide could affect drinking water, but also acknowledged the importance of fighting the invasive plant.
“I don’t think our community wants to see continual chemical treatment, even if it’s every three years,” said Council Member Julia Thrower.
Without treatment, Payette Lake could be looking at some type of algae bloom in the near future and “non-treatment is pretty much bury your head in the sand,” said Council Member Lyle Nelson.
The pesticide is released underwater by a specialized sprayer attached to a boat “to make sure we’re delivering it at the lowest rate that is effective against that plant and as targeted as possible,” Varley said.
After the herbicide is used, each site is monitored for effectiveness and water quality to confirm whether the poisons have dissolved and if concentrations are safe, he said.
The herbicide targets Eurasian milfoil specifically, and does not kill other plans or native milfoil species. Fish and invertebrates are also not affected, Varley said.
Treating the test area near North Beach would cost about $40,000 and treating the whole lake would cost about $3 million, Varley said.
“Frankly, there is more milfoil in Payette Lake than our budget can take care of in a single year,” he said.
This year, the Idaho State Department of Agriculture has a budget of $1 million for all Milfoil treatments across the entire state, he said.
