Herbicide approved to treat invasive waterlilies on Lake Inez

SEELEY LAKE – The battle against the invasive fragrant waterlily may begin with herbicides mid to later summer on Lake Inez. In April the Missoula County Commissioners approved the Missoula County Weed District's request to apply for a general permit from Montana Department of Environmental Quality for the application of herbicides on Lake Inez.

Missoula County Aquatic Invasive Species Coordinator Chris Mascari said the Weed District will possibly begin application of the herbicide Imazamox for control of waterlilies. Lake Inez is at the northern edge of the population on the Clearwater River and is a priority for the Weed District.

"The natural flow and ebb of these aquatic systems, if we can target that population on Inez, that'll reduce the reproductive potential of the species downstream," Mascari said. "One way that waterlilies can reproduce is send the pods or some rhizomes downstream. Once the waterlilies get attached to the substrate, the rest is history. They can grow into these large monocultures that we see on all the lakes on the Clearwater Chain of Lake system."

Based on information from the Wisconsin Department of Natural Resources fact sheet, Imazamox was registered with EPA in 2008 and is currently marketed for aquatic use as Clearcast™. It is a liquid formulation that is applied to submerged vegetation by broadcast spray or underwater hose application and to emergent or floating leaf vegetation by broadcast spray or foliar application. There is also a granular version.

Imazamox is a systemic herbicide that moves throughout the plant tissue and prevents plants from producing a necessary enzyme, acetolactate synthase (ALS), which is not found in animals. Susceptible plants will stop growing soon after treatment but plant death and decomposition will occur over several weeks.

The Wisconsin DNR fact sheet states laboratory tests using rainbow trout, bluegill and water fleas (Daphnia magna) indicate that Imazamox is not toxic to these species at label application rates. Imazamox is rated practically non-toxic to fish and aquatic invertebrates and does not bioaccumulate in fish.

Treated water may be used immediately following application for fishing, swimming, cooking, bathing and watering livestock, per the DNR fact sheet. If water is to be used as potable water or for irrigation, the tolerance is 50 parts per billion (ppb), and a 24-hour irrigation restriction may apply depending on the water body.

Dissipation studies in lakes indicate a half-life ranging from 4 to 49 days with an average of 17 days. Herbicide breakdown doesn't occur in deep, poorly oxygenated water where there is no light. In this part of a lake, Imazamox will tend to bind to sediment rather than breaking down, with a half-life of approximately two years.

"We conducted some herbicide trials back in 2020 and not only does it give the best control but it's the healthiest for the aquatic environment, meaning that it also facilitates the revegetation of native aquatic vegetation such as bladder warts," Mascari said. "Additionally, the mode of action with this herbicide effects a specific enzyme that is not toxic to other species like fish or macroinvertebrate."

The county plan includes use of Agri-Dex as the surfactant for applying Imazamox. It is a crop oil-based surfactant which is healthy for the environment and doesn't disrupt aquatic life, Mascari said. The herbicide will be applied by hand spraying directly on the plant leaf.

"It is very targeted," Mascari said. "We don't want to harm anything else."

The first step before applying the herbicide is to complete a more detailed survey of the lake, Mascari said. A study was completed in 2020 but the plan is to confirm the invasive waterlily is not entangled with sensitive species or some of our other native species that could potentially be harmed. The survey will include water testing to look for other herbicides already in the lake.

"We don't think there are any entanglements out there from our 2020 survey," Mascari said. "It's good to double check and redundancy is always a key important ingredient in the whole process."

The timeline also depends on the waterlilies' growth patterns. Mascari said it was already coming up in some places.

Steps to potentially use herbicides on Seeley Lake or Salmon Lake will take some time.

"Total eradication of the species, being waterlilies, is near impossible," Mascari said. "With Inez we start at the top and work our way down, that's most beneficial for this system. It is going to take years of work before we an can get down to Seeley or Salmon area."

Clearwater Resource Council proposed cutting the waterlilies on Seeley Lake. However, the method for mechanical removal is still in the works. Cutting is not a one-time thing. There would have be a program of regular cutting over multiple years, if there was a pontoon boat that could do that, said Caryn Miske, CRC executive director.

"We were not able to come up with a pontoon boat," Miske said. "CRC doesn't currently have the funding to purchase even a used one. That is something that is currently on hold."

Jon Haufler CRC president said CRC has not yet developed an opinion or policy regarding the use of herbicides for control of waterlilies at this time.

"It's something we are still looking into as one possible option for lily control," Haufler said.

Mascari stated cutting can be an effective method for small populations. The idea with cutting is to starve the plant of carbohydrates.

"Sometimes it works," Mascari said. "It has to be very small populations and you have to be very persistent, doing it season after season."

While cutting is another treatment tool, Mascari said when dealing with waterlily across the 136 acres within the county, mowing is not an alternative the district has the logistics to tackle.

"Not only does it take a tremendous amount of time, finances and logistics, it potentially spreads the species if you are not dedicated to decontaminating your harvesting device," Mascari said.

Swan Valley Connections has been involved previously in manual removal of waterlilies on Holland Lake with the Flathead National Forest, said Rebecca Ramsey, executive director. Ramsey felt it was an opportunity to try something, but was not sure how effective it was.

"I think that anytime that managers make the decision to use a herbicide or pesticide, any sort of chemical means, they certainly don't do it lightly," Ramsey said. "There are some products that might be able to be used in a more localized way. I can say from my work with native fish, for instance, that no decisions like that are made, especially in a waterway, without heavy consideration by people who are actual experts in their field, not me."

Lake Inez property owner William Campbell said, "From my perspective it would not be a bad thing. They have spread to the point that they interfere with docking our boat. They just keep expanding each year. Last year my wife and a neighbor cut them back enough so that we could dock our boat."