Updated decontamination guidance for New Zealand mudsnails

By Ellen Voss, Climate Resilience Director for the River Alliance of Wisconsin


Loyal readers of Wisconsin Trout know about the threat New Zealand mudsnails (NZMS) pose to Wisconsin’s trout streams. Since they first appeared in our state waters back in 2011, NZMS have continued their slow crawl to new water bodies, aided largely by unknowing anglers and paddlers accidentally moving them around on their boots, waders and canoes.


While the risks of spread and ecological impact remain, the good news is that a recent study out of Michigan offers specific, and easy, guidance for how you can decontaminate your boots and waders when you’re out fishing, especially if you’ve visited one of the known infestation hot spots in the state.


In a 2022 study published in the journal North American Fisheries Management, Jeremy Geist and his coauthors determined that spraying gear with Formula 409 was 100 percent effective at killing mudsnails, outperforming bleach and even Virkon Aquatic in lab trials. The paper also provided a detailed protocol outlining the steps.


How to limit new introductions to uninvaded systems


If you’ve fished a known or suspected mudsnail-infested stream, please do the following:

  • LOOK: Visually inspect wading and fishing gear and remove any snails you see*.
  • BRUSH: Use a stiff-bristled brush and/or wipe off organisms, debris, and organic material** from your wading and fishing gear.
  • SPRAY: With the angler positioned out of and away from surface waters, spray the wading and fishing gear liberally with Formula 409 (as prepared by the manufacturer), completely covering all material that was in contact with the water body.
  • WAIT: Let Formula 409 remain on the fishing and wading gear for at least 10 minutes.
  • RINSE: With the angler positioned out of and away from surface waters, rinse the fishing and wading gear with clean water to remove residual Formula 409***.


Remember that these snails are about the size of a grain of rice, so look carefully, especially under fabric flaps, in Velcro, boot grooves, and crevices. Formula 409 is less effective when mud is present. Direct contact with the chemical reagent is necessary for it to have maximum effectiveness, so remove as much mud as possible before spraying. A freshwater rinse will remove residual Formula 409 that could be harmful to native stream organisms. An old milk jug or spray bottle filled with tap water from your house works well for this step.


Currently known locations of mudsnails in Wisconsin, as of December, 2023:


  • Vermont Creek, Dane County, 2023; tributary of Black Earth Creek
  • Garfoot Creek, Dane County, 2023; tributary of Black Earth Creek
  • Turtle Creek, border of Rock and Walworth counties, 2022; first known warmer-water fishery creek, though still a well-recreated state fishery area with good public accesses.
  • Fryes Feeder, Dane County, 2022; tributary to Mt. Vernon Creek
  • Elvers Creek, Dane County, 2021
  • Token Creek (Dane County, 2021)
  • Black Earth Creek
  • Badger Mill Creek
  • Badfish Creek
  • Brewery Creek
  • Strutt Creek
  • Mt. Vernon Creek
  • Rowan Creek


A key finding of the study was that these preventative measures only work if the protocols are followed correctly and anglers are willing to do them. It takes all of us working together to keep our trout streams as good as they can be, and a few minutes of decon at the end of your fishing adventure will do wonders for protecting our priceless streams.


To learn more, check out Geist et. al, 2022: Coupling Gear Decontamination Trials and Angler Surveys to Minimize Spread of Invasive New Zealand Mud Snails Potamopyrgus antipodarum


Coming soon: In collaboration with Rock River Coalition, efforts are underway to create a short, sharable video highlighting these easy steps for anglers.