Dye study may lead to use of herbicides in Lake Tahoe

By Kathryn Reed

One day the Lahontan Water Quality Control Board is probably going to have make the decision to either leave the invasive non-native Eurasian milfoil plant in Lake Tahoe or eradicate it with herbicides.

The mantra for handling invasive aquatic species, be it plant or animal, is prevention, control, eradication. Prevention, in this case, is not an option because it’s been here for years. Control seems sketchy as the plant continues to clog the canals in the Tahoe Keys in South Lake Tahoe and is growing in other locales around Lake Tahoe. Eradication would be the next step.

Channels in the Tahoe Keys in South Lake Tahoe are full of milfoil. Photo/Kathryn Reed

For now, though, Lahontan is allowing a study to go forward this summer involving a dye that is designed to duplicate how an herbicide would react if it were injected into Lake Tahoe.

“Part of why we are doing this is to see how long it will stay in a localized area,” Lars Anderson, lead scientist with the U.S. Department of Agriculture’s Weed Science Program, told the board Wednesday. (The water board met April 14 at Lake Tahoe Community College.)

Dan Sussman, environmental scientist with the water agency, said how the dye breaks down would help determine what herbicides could be used in the future.

The agency is not advocating herbicides be used, but wants the studies in place to know what options are available. Studies like the one Anderson will conduct this summer could then be part of environmental documents that would be put together on a case-by-case basis for eradicating the various invasive species in Tahoe.

Herbicides are not permitted in Lake Tahoe. However, a revision to the document that dictates what’s allowed per Lahontan is likely to permit herbicide use. The amendment is expected to be before the board in November. The document will be available to the public this summer.

The fluorescent red dye will be put into the lake in June and late summer in about 2 to 3 feet of water. Instruments on boats will be used to collect data in real time about the spread of the dye. It’s expected that the dye will not be able to be seen more than eight hours after it enters the lake.

The board and Executive Director Harold Singer voiced concerns about letting the public know what is going on to minimize any potential outcry. Officials are meeting with the Tahoe Keys Homeowners Association later this month as part of that outreach. Information will also be posted at the marina and other locations where the dye may be used – like in Emerald Bay, Lakeside Marina and Incline Village.

The problem with milfoil is that it can overtake a lake. It easily spreads from lake to lake via boat trailers. The feather-like leaves are distinctive. The plant is popular in home aquariums.

It can interfere with swimmers, boats and clog in-take systems for drinking water. Dealing with those issues as well as eliminating the weeds is expensive.

In the past, harvesters have been used to get rid of milfoil. Sussman told Lake Tahoe News there are harvesters on the market now that pull out the entire plant instead of mowing it.

Bottom barriers are also a tool. These have been used more often for eradicating clams. However, Sussman said they are working on invasive plants because they block out the light.

The dye (Rhodamine WT) tracer study that will begin in two months is one of several pilot programs being conducted around the lake to look into how best to rid Lake Tahoe of invasive plant and animal species.