The Idaho State Department of Agriculture has detected quagga mussels in the Snake River near Twin Falls for the fourth consecutive year, though officials say the infestation is significantly smaller than in previous years — down 77% compared to 2025 — and a new round of chemical treatments is already underway.

What Was Found

As of August 14, monitoring efforts had turned up eight quagga mussel larvae, known as veligers, and seven adult mussels in the affected stretch of the Snake River. While the numbers are markedly lower than in prior years, agriculture officials made clear that pulling back on eradication efforts is not on the table.

“Our outlook is that not treating this infestation is not an option,” Sydney Kennedy, a public information officer for the Idaho State Department of Agriculture, told East Idaho News.

First detected in September 2023, the quagga mussel presence in the Snake River near Twin Falls has now persisted across four monitoring seasons. State officials say the infested portion of the river has shrunk since initial detection, which they attribute in part to chemical treatments applied in the falls of 2023 and 2024.

New Treatment Strategy

The department announced its latest treatment plan this week, with application of copper-based chemicals beginning Thursday or Friday and expected to wrap up by September 13. The approach this year shifts away from broad river-wide treatment in favor of smaller, more targeted interventions.

Crews will focus on specific pools and fissures where adult mussels have survived previous rounds of treatment, identified through increased use of scuba divers. The treatment plan calls for two copper-based pesticides — Earthtec QZ and Natrix — along with a special local exemption application for the algaecide Cutrine Plus.

Planned volumes are substantial: more than 16,800 gallons of Earthtec QZ and over 9,300 gallons of Natrix for the Twin Falls pool, with roughly 20,300 gallons of Earthtec QZ and about 11,290 gallons of Natrix designated for the Shoshone Falls pool.

Trade-Offs: Fish Kills and Water Users

The copper-based chemicals used in the treatment are lethal to fish and other aquatic life, a consequence the department has navigated since the eradication effort began. In 2023, state crews removed six or seven tons of dead fish from the river’s surface following that year’s treatment.

The stakes for inaction are also high. More than 1,700 distinct users draw water from the Snake River in the affected area, including municipal drinking water systems serving communities across southern Idaho. Quagga mussels, an invasive species native to Eastern Europe, are known to clog water intake pipes and infrastructure, causing significant damage and operational costs.

Josh Johnson, the Central Idaho Director for the Idaho Conservation League, underscored the economic argument for continued treatment, telling East Idaho News: “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.”

What’s Next

The department expects this season’s treatment to conclude by September 13. Officials will continue monitoring larval and adult mussel counts to assess the effectiveness of the more targeted approach. The results this fall will likely inform whether the strategy of treating smaller, defined areas proves more efficient than the broader applications used in prior years.

If the 77% reduction in detected mussels holds through the end of the treatment season, it would represent the most significant year-over-year improvement since detection began. Whether that trend can be extended — or whether eradication is achievable at all in a river system as large and complex as the Snake — remains an open question for state agriculture and water management officials heading into the 2027 monitoring season.

The quagga mussel fight is one of several active natural resource policy challenges facing state agencies this year. Idaho officials have also been engaged on a range of other regulatory and legal fronts, including an Attorney General settlement recovering millions in drug pricing overcharges that returned funds to state programs.