Quagga mussels detected again in Idaho, but numbers are down as Ag Department makes eradication push

TWIN FALLS (Idaho Capital Sun) — Idaho state officials say they are continuing to push for full eradication of quagga mussels after again detecting the invasive species in the Snake River near Twin Falls earlier this month. 

Although quagga mussels and larvae are small, state officials and conservationists say they cause giant problems because of how rapidly they reproduce. A mussel infestation can clog pipes used for irrigation and drinking water, which could have serious effects on the state’s economy, agriculture and recreation industries, officials said.

“Our outlook is that not treating this infestation is not an option,” said Sydney Kennedy, a public information officer for the Idaho State Department of Agriculture said in an interview Wednesday. 

Gov. Brad Little agrees.

“Other states have shown us what happens when invasive mussel infestations are allowed to establish before action is taken,” Little said in a written statement. “Those delayed responses have resulted in significant long-term management costs coming directly out of taxpayer pockets. Idaho has set the standard by responding aggressively. Our responsible strategy on the Snake River is proving to be effective, and we will continue making decisions that give us the best opportunity to eliminate quagga mussels to protect Idaho’s waters and the people who depend on them.” 

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More than 1,700 different users, including municipal drinking water systems, depend on water from the Snake River, according to the state. 

Officials at the nonprofit Idaho Conservation League call the detection and presence of quagga mussels a no-win situation and one of the primary threats to the health of the Snake River.  

This month’s detection marks four consecutive years of finding quagga mussels in the Snake River near Twin Falls.   

In September 2023, state officials announced for the first time they had detected invasive quagga mussel larvae in water samples. 

Since 2023, the portion of the river where quagga mussels have been detected has shrunk, the Idaho State Department of Agriculture said. 

Fewer quagga mussels detected so far this year in Snake River

State officials say the number of quagga mussels detected this year is down by 77% compared to last year. 

As of Aug. 14, the Idaho State Department of Agriculture has detected eight quagga mussel larvae, which are known as veligers, and seven adult quagga mussels.  

“We know what is at stake if invasive mussels gain a foothold in Idaho,” Idaho State Department of Agriculture Director Chanel Tewalt said in a written statement. “Our previous treatments have proven to be effective, and this level of containment is unprecedented among known quagga mussel infestations. No other state has a proven playbook for how to accomplish this work. Idaho is charting new territory with our eradication effort.”  

Efforts to reach Tewalt for an interview were unsuccessful. 

Going back to last year, Kennedy said the department has been able to reduce the infested portion of the Snake River that is affected, contained the mussels to that smaller area and detected fewer mussels overall.

“That is at the forefront of our minds, noting our copper treatments are effective, and so we need to continue to push on,” Kennedy said.

Officials at the Idaho Conservation League say that the state cannot afford to let quagga mussels spread and prevention is critical. 

“Prevention, however much it costs, is going to cost a whole lot less and be a whole lot less difficult than actually dealing with quagga-infested waters,” Idaho Conservation League Central Idaho Director Josh Johnson said in an interview Monday. 

What will Idaho fight quagga mussels with this year?

State officials announced Wednesday their treatment plan begins Thursday or Friday and is expected to end Sept. 13. 

Idaho State Department of Agriculture officials posted a draft of their plan, the 2026 Quagga Treatment Plan, online. 

The plan calls for two steps of copper-based treatments:

• 16,855 gallons of a copper-sulfate called Earthtec QZ in the Twin Falls pool, and another 20,338 gallons of Earthtec QZ in the Shoshone Falls pool.
• 9,346 gallons of a Natrix, a copper-based chemical state officials have used since 2023 to fight quagga mussels, in the Twin Falls pool and another 11,290 gallons of Natrix in the Shoshone Falls pool.

The plan also shows officials applied for a special local exemption for an algaecide called Cutrine Plus, with the exact quantities and placements pending.

New this year, state officials plan to focus on treating smaller, targeted pools of water and fissures where adult mussels survived – not just a full river treatment like in previous years. An increased use of scuba divers has helped the state pinpoint the smaller nooks and crevices where adult mussels survived previous treatment cycles, Kennedy said. 

The copper chemicals used to kill quagga mussels also kill fish and other life in the Snake River.

In 2023, the first year the state used Natrix to kill quagga mussels, state officials removed six or seven tons of dead fish that floated to the surface of the Snake River.

Johnson, with the Idaho Conservation League, said the state’s plan to eradicate quagga mussels is ambitious, the stakes are high and he and his colleagues are rooting for the plan to work. 

“A lot of people around the country and even around the world are looking at us here in Idaho to see if we pull this off,” Johnson said. 

A treatment plan like this has never been attempted at such a large scale on a moving river before, Johnson said.  

If the state’s plan works, it would be a huge success, Johnson said. 

However, Johnson said the state can’ make dumping copper chemicals into the Snake River an ongoing, annual event. 

“With all that said about how important it is to eradicate quagga mussels, we also can’t just poison the river annually forever,” Johnson said. “I think the most important thing going forward is objectively assessing how well treatments are working.”

To step up the fight against quagga mussels, Johnson said he would like to see the state expand its mandatory inspection and hot wash decontamination stations to all popular bodies of water.

“The only way to know for sure that quaggas aren’t going into a water body is to make every boat get a hot wash decontamination,” Johnson said. “It is expensive, but I think it is something we should think about how much that costs versus the cost of having quagga mussels in places like Payette Lake or Lake Pend Oreille.”

Idaho Capital Sun is part of States Newsroom, a nonprofit news network supported by grants and a coalition of donors as a 501c(3) public charity. Idaho Capital Sun maintains editorial independence. Contact Editor Christina Lords for questions: info@idahocapitalsun.com.