Group to apply for herbicide trial to fight invasive plants in Tahoe Keys

SOUTH LAKE TAHOE, Calif. – As part of ongoing efforts to evaluate and use numerous tools to fight aquatic invasive plants in the Tahoe Keys, the Tahoe Keys Property Owners Association recently announced its plans to apply for a permit for a small-scale demonstration of aquatic herbicides in 2018.

Since their introduction, aquatic invasive plants (predominantly curly leaf pondweed and Eurasian watermilfoil) have taken over more than 90 percent of the 172-acre Tahoe Keys lagoon system, and are considered the most immediate threat to Lake Tahoe, according to University of Nevada, Reno’s 2015 Implementation Plan for the Control of Aquatic Invasive Species within Lake Tahoe.

“These plants threaten the lake’s ecosystems, the water’s clarity, and our recreation and economy,” said Dr. Lars Anderson, UC Davis aquatic plant expert. “In spite of ongoing efforts, they continue to grow in the Keys, and with the Tahoe Environmental Research Center’s State of the Lake Report showing record-breaking increases in lake temperatures, the threat to Lake Tahoe is greater than ever.”

Herbicides are one of nine invasive plant-fighting methods being used or studied in the Keys. Current methods being evaluated include: improvements to harvesting and fragment collection, scuba-assisted dredging, bottom barrier mats, cultural controls (lake and lagoon friendly landscaping, water quality BMPs), rotovating, dredging, dewatering, nutrient reduction, and biological controls, which are all in various stages of evaluation or implementation.

“This is one evaluation of one method the association is considering in our comprehensive plan to gain control over the invasive plants,” said John Larson, chair of the association’s Water Quality Committee.

If the demonstration is permitted, the association would apply low levels of three herbicides found effective on the same plants in other regions, namely Endothall, Triclopyr and Penoxsulam, at nine test sites in 2018. The test sites cover about 13 acres (eight percent) of the Keys’ 172 acres in dead-end lagoons distant from Lake Tahoe. The process would include a multitude of barriers, monitoring, and safety measures to ensure that these substances would not reach the Lake.

The herbicides, approved by the Federal and California Environmental Protection Agencies, are nontoxic to humans, fish and wildlife, and would be diluted to between 0.02 and 2 parts per million, or about half the maximum concentrations allowed by the EPA.

Comprehensive dye studies conducted in 2011 and again in 2016 have shown that water movement will not carry the herbicides out of the back channel lagoons into Lake Tahoe during the test period. Surface-to-bottom curtains and/or other barriers will be placed at select sites as an extra layer of precaution to isolate the herbicide demonstration areas. Secondary barriers will be ready to be deployed as contingency measures in case trace amounts are detected during the test, ensuring no movement of herbicides into Lake Tahoe.

All of the herbicides break down by light, microbial action and other processes, typically degrading to non-detectable levels within a few days to two weeks.

The association and its members have gone above and beyond agency requirements, with numerous private property owners volunteering to place bottom barrier mats around their properties, installing a boat backup station designed to reduce the spread of weeds, and purchasing the Lake Tallac lagoon in part to better control the infestations. The association has also brought in a variety of new design fragment collection boats for evaluation. A wide range of control options have been studied over the past six years.

The use of herbicides as part of an integrated plan has been endorsed by an array of independent national and regional experts, including Dr. Pat Akers with the Califoria Department of Food and Agriculture, Dr. Joe DiTomaso of UC Davis, Dr. Kurt Getsinger with the U.S. Army Corps of Engineers, Dr. Sudeep Chandra with University of Nevada, Reno and Joel Trumbo with the California Department of Fish and Wildlife.

An exemption to a previous ban on the use of in-water herbicides in the Tahoe Basin has been authorized in recent years by the State Water Resources Control Board and the Federal Environmental Protection Agency. The US Fish and Wildlife Service and others have extensively evaluated the proposed herbicides to confirm their safety for use in environmentally sensitive waters.

Other lakes and bodies of water have successfully used these herbicides to control aquatic invasive plants without harming native species or posing public health risks to drinking water supplies, including Discovery Bay south of the California Delta, Big Bear Lake in Southern California, Clear Lake in Northern California, Loomis Lake in Washington, and numerous areas within the Sacramento-San Joaquin Delta.

“We understand that people have strong feelings about the potential use of herbicides in the Keys. We also understand, however, that to really address the aquatic invasive plant issue not only in the Keys lagoons but for all of Lake Tahoe, we must be willing, as a community, to try a variety of state-of-the art tools to see what combination of options are best for moving forward.” Larson said. “What we are announcing is a test to evaluate another method that could be important for our tool box to gain control of the infestations in the Keys’ lagoons.”

To learn more about the efforts in the Keys to combat aquatic invasive species, the community is invited to attend a public workshop on November 1, 6 :00 p.m. to 8 p.m. at the South Tahoe Public Utility District Building, 1275 Meadow Crest Drive. Find out more at keysweedsmanagement.org.