Martin Kidston

(Missoula Current) In an effort to eradicate an invasive plant in Flathead Lake, Missoula County will look to Switzerland and become the first in the U.S to farm and introduce a weevil suited for the task.

Bryce Christiaens, manager of the county's weed district, said flowering rush is slowly taking hold in waterways across western Montana. While the plant creates pleasing pink flowers, it chokes out native plant species and impacts water quality, eventually hurting fish and wildlife.

It also impacts recreation by forming dense stands that restrict access to open water.

“We don't have any flowering rush in Missoula County right now, but it's fairly invasive in western Montana, mostly starting in Flathead Lake where it was first reported,” Christiaens said. “From there, we've seen significant movement down the Columbia River system.”

With management tools lacking, the county has turned to its partners in Switzerland to develop a bio-control agent for flowering rush. The resulting weevil has been petitioned for approval at both the USDA and Canadian regulators.

Christiaens said Canada has already granted approval and will begin releases in British Columbia this year. Approval should be granted soon in the U.S.

“We're hopeful we'll get approval for this weevil this summer, which will allow us to do releases next year,” Christiaens said. “The Montana bio-control is housed in our office, so they've taken the lead in making sure this weevil gets to Montana and the first releases are in Flathead Lake.”

Missoula County will conduct pre-release monitoring of Flathead in partnership with the Confederated Salish and Kootenai Tribe. It's also working with NorthWestern Energy to release the weevil into Thompson Reservoir.

The county plans to farm the weevil locally.

“We may also do mass rearing at our new facility once we have approval. We'll be developing these pond systems to mass rear these weevils,” Christiaens said. “This will be the first in the U.S. It's fairly unique.”

Flowering Rush
Flowering Rush

Like the fragrant water lily, flowering rush was introduced into North America as an ornamental plant, as some favored its clustered pink flowers. But the plant out-competes native plants and has become detrimental to the ecosystem.

Christiaens said flowering rush, or Butomus umbrellatus, is the only plant species in its family. That helped speed the testing process since there was no need to test closely related host plants.

The weevils lay their eggs on the stems of the plan in the water, and the eggs typically stay in the host plants. But the weevils may be more complicated, Christiaens said.

“This particular weevil, once those eggs hatch, the larvae have the potential to self-select their host plants. They'll swim away from where their eggs are laid and select another plant. It's made it harder to manage them in a broader system, which is why we'll be doing the mass rearing,” Christiaens said.

Fragrant Water Lily Also a Problem

Missoula County is also battling the fragrant water lily, which has crowded the lakes and ponds in the Swan and Clearwater system. The county is working with both the Lolo and Flathead national forests, the Army Corps of Engineers and Montana Fish, Wildlife and Parks to create a strategy that will likely include the use of herbicide.

Christiaens said the current strategy will target the upper-most lily populations in both in the Swan and Clearwater system, including hand-pulls in Holland Lake. The partners completed a table exercise earlier this month to gather the information needed before beginning treatment.

“When we put in herbicide trials within the Clearwater chain of lakes, we've taken pre-treatment water samples to make sure we can speak clearly to the potential herbicide already in that system,” Christiaens said. “When we did this work in Seeley and Salmon lakes, we picked up significant levels of herbicides we weren't using.”

County officials said they'll work with various community councils about the project before it begins.

“We wanted to provide them with a full picture of what the population looks like, our suite of tools, and how we think those tools will interact or move throughout the system,” Christiaens said.