The European weevil feeds only on an invasive aquatic plant now found in 23 states, but researchers say building enough insects for field releases will take a year or two.

The U.S. Department of Agriculture has authorized permits to release a small European weevil, Bagous nodulosus, to fight flowering rush, an invasive aquatic plant that clogs irrigation ditches, crowds out native wetland plants and hampers boating and fishing. The decision was published Oct. 16 in the Federal Register, where USDA's Animal and Plant Health Inspection Service (APHIS) announced a final environmental assessment and a finding of no significant impact.

The notice matters most to irrigators, lakefront property owners and weed managers in the Northwest and Northern Plains, where flowering rush has spread along rivers and reservoirs. APHIS said the plant now grows in 23 states and nine Canadian provinces along the border and is still expanding.

Expectations are modest by design — Washington State University Extension weed biocontrol specialist Jennifer Andreas told Capital Press that the weevils are unlikely to wipe out flowering rush but should stunt it enough to give native plants a chance to recover.

Federal Notice Clears the Way for Permits

APHIS first released a draft assessment in September 2024 and received 21 public comments, 20 of them in favor of release, according to the notice. The agency said its finding showed the release would not significantly affect the human environment, so a full environmental impact statement was not required. APHIS Administrator Kelly Moore signed the notice Sept. 30.

Capital Press reported that the agency's Oct. 15 announcement confirmed a finding USDA had reached in 2024. Andreas and colleagues petitioned USDA in 2022, and Northwest weed experts had been building the case for importing the weevil since 2012.

Releases will not happen overnight — Swiss-origin weevils are imported to a USDA Agricultural Research Service center in Sidney, Montana, and second-generation insects have gone to WSU's research center in Puyallup and to the Montana Biocontrol Project in Missoula. WSU received 22 weevils in July and is raising them in 300-gallon (about 1,136-liter) tanks. "This coming year will really be a year of learning," Andreas said.

A Pink-Flowered Invader in Ditches, Lakes and Rivers

Flowering rush is native to Europe and Asia and reached North America by 1900, possibly through ballast water, packing material and the ornamental pond trade, according to APHIS. Its umbrella-shaped clusters of pink flowers made it popular in water gardens. Underwater, its rhizomes and fragments spread easily, and hand pulling or cutting often leaves enough behind to regrow.

APHIS said the plant displaces native species, impedes water flow, alters habitat and nutrient cycling, and damages fishing, hunting, and boating. Capital Press reported that herbicides have limited effect on the submerged plant and that it provides habitat for an invasive snail that hosts the parasite behind swimmer's itch, an itchy rash people can get after wading in infested shallows.

In Washington, flowering rush is a Class A noxious weed, and buying or possessing it is prohibited, according to the Washington State Department of Ecology. The agency's 2021 summary listed known populations at Silver Lake in Whatcom County, wetlands on Joint Base Lewis-McChord, the Yakima, Columbia, and Pend Oreille rivers, and the Spokane River down to Lake Roosevelt. Plants can survive in water 20 feet (about 6 meters) deep or more.

Proving a Weevil Will Stick to One Plant

Classical biological control means bringing in a natural enemy from a pest's home range to restore some of the pressure it lost when it moved. The risk is that a released insect could turn on native plants, so regulators require years of feeding and egg-laying tests on related and look-alike species before approval.

Flowering rush is the only species in its plant family, which made the search for a specialist easier. APHIS wrote in the notice that "Host-specificity testing indicates B. nodulosus is unlikely to attack non-target species." Research in Switzerland showed the weevil sticks with flowering rush, Andreas said.

The weevil attacks early and keeps at it. Adults become active in April, and larvae feed inside the plant from April to September, which APHIS said should limit the plant's ability to bounce back. Females lay eggs in the underwater plant, and the larvae eat the leaves, according to Capital Press. Agricultural Research Service scientists in Sidney have described it as the first biocontrol agent for flowering rush in the United States, in an ARS research feature.

Suppression on a Multiyear Timeline

Several questions remain open. Researchers do not yet know how well the weevils will survive Northwest winters, how fast they will build populations after release, or how much damage they will do in cold, deep water. Andreas said the insects were settling in for winter and were hard to assess. WSU expects more weevils from USDA in the spring.

For households and farms, nothing changes immediately. Lake residents, boaters and irrigators should keep cleaning, draining and drying boats and equipment, avoid planting or moving flowering rush and report suspected patches to their county noxious weed board. Releasing weevils requires a federal permit, so private releases are not an option.

The cost of doing nothing falls on irrigation districts, which pay to clear ditches, and on shoreline owners who lose water access. Biocontrol will not replace herbicides, mechanical removal or prevention, but it could lower long-term control costs if the weevil establishes. More guidance on federal efforts is available through USDA's invasive species resources. Nature World News will report on first field releases and early results.

What Readers Want to Know

What did USDA approve? APHIS authorized permits for field release of the weevil Bagous nodulosus to control flowering rush in the continental United States, per an Oct. 16 Federal Register notice.

Where is flowering rush a problem? APHIS says it grows in 23 states and nine Canadian provinces near the border. In Washington, it occurs in Whatcom and Pierce counties and along the Columbia, Yakima, Pend Oreille and Spokane rivers.

Will the weevil eat native plants? APHIS says host-specificity testing indicates it is unlikely to attack non-target species. Flowering rush is the only member of its plant family.

When will weevils be released? Labs in Washington and Montana are rearing them now. WSU expects to build up populations over the next year or two before wider releases.

Is flowering rush linked to swimmer's itch? Capital Press reported that the plant provides habitat for an invasive snail that hosts the parasite causing swimmer's itch.

What can property owners do now? Clean, drain, and dry boats and gear; never plant or move flowering rush, and report suspected patches to your county noxious weed board.