Beetle picked to battle Flathead’s flowering rush invasion

A Missoula-based invasive weed workgroup is eying the prowess of a European weevil to nibble down flowering rush at Flathead Lake, where the exotic aquatic perennial thrives.

An invasive ornamental plant of Eurasian origin, flowering rush was first found in the West’s largest natural freshwater lake in the early 1960s, with little overall success in thwarting its spread.

Melissa Maggio, project coordinator for the Montana Biological Weed Control Coordination Project, said Thursday the group is now looking for partners in releasing one of the plant’s natural-born enemies there: Bagous nodulosus, a thrush-loving weevil native to Europe.

The release of the beetles would be in and around Flathead Lake, pending site surveys.

With semi-aquatic, overwintering adults, the small brown bug’s larvae develop in the plant’s leaves and rhizomes — the adventitious rootstocks that have helped flowering rush expand and flourish — and then swim from plant to plant.

Maggio discussed the potential release during the Upper Columbia Conservation Commission’s winter meeting. A petition to release the weevil is expected to be submitted in April.

The plan will first go through the Technical Advisory Group for Biological Control Agents of Weeds, an independent voluntary national committee known simply as TAG.

If approved by TAG, the petition would then be forwarded to the USDA Animal and Plant Health Inspection Service and U.S. Fish and Wildlife Service for final review.

APHIS would ultimately publish a 30-day notice in the Federal Register for public comment on a resulting environmental assessment concerning the proposed beetle release.

Maggio said pre-release monitoring for the weevil is otherwise planned for 2023, once ideal sites are selected during surveys planned for this year.

The group is hopeful that release of the weevil will garner broad support during the review process.

“So, you know, we have to wait to see if we get approval,” Maggio said Thursday. “But we feel pretty confident that (the weevil is) going to move forward. It’s a very solid biocontrol agent.

“By that, I mean it’s host-specific, and it’s impactful,” she added.

Flowering rush was first collected at the northern margin of Flathead Lake in 1962, according to the Montana Field Guide.

Growing in water depths of up to 20 feet, the plant is now also found throughout Sanders, Lake and Flathead counties; in the upper and lower forks of the Flathead River; and in the Clark Fork River, according to the state guide.

Its steadfast invasion within the Upper Columbia River Basin also offers the plant prime downstream access to water bodies through to the Pacific Ocean.

One plus, Maggio said, is that flowering rush remains the lone species within its botanical family, making it a prime target for biocontrol tactics.

This lone plant species and the prescribed weevil have coevolved alongside one another, which should limit the weevil from potentially targeting other plant species.

“So they really develop a specialty to that plant,” Maggio said.

She said overall that 18 insects and six fungal pathogens are known to develop specifically on flowering rush, Bagous nodulosus — known commonly as the Flowering Rush Weevil by the United Kingdom Species Inventory — being chief among them.

Ahead of final approval, the Flathead’s planned weevils will be reared this year at the USDA Agricultural Research Service facility in Sidney, Montana.

Although not foolproof, biocontrol measures entail the deliberate release of plant species’ natural enemies, namely insects.

It’s not meant to eradicate a plant species, Maggio said, but rather “bring back the balance” and reduce a plant’s abundance or spread.

Biological weed control dates in Montana to 1948 with the release of Chrysolina beetles on St. John's Wort by state entomologist George Roemhilt, according to the Montana Biological Weed Control Coordination Project.

The practice in Montana was later expanded by the state, Montana State University and USDA, as well as the Bureau of Indian Affairs and Bureau of Land Management.

Biological control activities grew significantly in the 1990s but have since waned.

Insects have been released in Montana to help control toadflaxes, leafy spurge, the musk and Canadian thistles, puncturevine and spotted knapweed, among other noxious plants, according to the project.

The Montana Biological Weed Control Coordination Project was created in 2013. Staffed by Missoula County, the group is funded through contributions and grants.

For the Flathead weevil initiative, Maggio said, the group so far has been working with Washington State University and a private consultant.

During its winter meeting, the Upper Columbia Conservation Commission appeared to support the effort for further consideration but did not yet formally approve its joining the project.