Scientists trying to break AIS stranglehold on Tahoe waterways

After three years of intensely studying how best to remove aquatic invasive species from the Tahoe Keys, including using herbicides, much was learned, but no sure-fire treatment plan floated to the top.

The main take-aways are several treatments work, but no one method will solve the problem. It’s going to take a combination of solutions to kill the plants.

“We want to contain the infestation to protect the rest of the lake,” said Laura Patten, natural resource director with the League to Save Lake Tahoe. “We want to stop fragments from escaping into the lake.”

Eurasian milfoil is an aquatic invasive species at Lake Tahoe that people are trying to control. (Image: Kathryn Reed)

Scientists and others now have a trove of data from which to move forward. They hope to development a management plan this fall that will look at what is possible, what is most strategic, and what can be effective.

Early next year (date to be determined) a public science symposium will be conducted that summarizes all the results from the testing, includes outcomes from this fall’s planning sessions, as well as discussions taking place on the side. The goal is to create an action plan that includes ways to pay for it.

It’s going to cost big bucks. The three-year project was funded with $10 million from the federal government via the Lake Tahoe Restoration Act, $5 million from the Tahoe Keys Property Owners Association, and $200,000 from the League.

Those involved with the Keys’ project recently hosted a webinar with 88 people listening in.

Called the Control Methods Test or CMT, testing occurred in 2022, 2023 and 2024. It was a collaborative approach with the League, Tahoe Regional Planning Agency, Sierra Ecosystems Associates, and TKPOA the main players.

The Tahoe Keys is the priority because the 170 acres of lagoons contain the largest infestation of aquatic invasive species at Lake Tahoe. Curlyleaf pondweed, Eurasian watermilfoil and coontail are the unwanted plants.

“Why do we control these? They have severe impacts to Lake Tahoe; threatening it’s famed clarity, impairing native habitats and outcompeting our native species (that provide) that delicate food web that Tahoe is famous for. All of that threatens Tahoe’s $5 billion recreation and tourism economy,” Dennis Zabaglo, TRPA’s AIS program manager, said.

An unanticipated scenario arose when in 2022 the water level was low, exposing a large swath of shoreline. The next two years were wetter, with that shoreline then under water.

This revelation means applications will need to be flexible and adaptive.

Scientists believe a 75 percent reduction in AIS will achieve desired water quality levels as well as the ability for native plants to flourish. It’s also a realistic target for non-chemical uses.

2022 was the only year two herbicides (Endothall and Triclopyr) were used.

Other methods included laminar flow aeration (LFA), diver assisted suction harvesting (DASH) , bottom barriers, and ultraviolet light. LFA is the only method that had zero effect on target species, while DASH is the only that gets rid of the whole plant.

“Some methods only work in mid-channel, while others work best in the near-shore,” Lars Anderson, AIS biologist and consultant for TKPOA, said. “Native plant regrowth occurred with every method applied and increased in the herbicide sites.”

Forty-six acres were treated each year, which included a 7-acre control site. Applications were used at various sites, including an herbicide-only, and herbicide-UV combo in 2022. The following two years the herbicides were not used and the other methods increased in acreage.

“We had different responses from the different species by the different methods we used. No one method is going to be the solution to the whole problem,” Anderson said. “There are going to be a multiple range of methods that are going to be integrated to make this work.”

Note: A version of this story first appeared in the Tahoe Mountain News.