The case for using herbicide in Lake Fairlee – Milfoil then and now

At the January 3rd Selectboard meeting, Nolan Riegler of the Lake Fairlee Association announced that they would be treating the lake with herbicide “in the upcoming year.” The problem? A water plant with bright green whorls of feathery leaves, known as eurasian milfoil. It looks pretty in a glass tank, and it is easy to grow. At one time it was prevalent in the aquarium and ornamental pool industry. But the unintended consequence of introducing this non-native species in the 1940s has been devastating.

It is widely believed that people who grew tired of their aquariums dumped the contents into the nearest body of water. Without the control of insects and diseases from its native homeland, the milfoil, a very adaptable plant, grew explosively. It can put on about a foot a week in hot summer months. It clogged the water, became entangled in boat propellers, and from there was transported to other lakes and ponds. It reproduces readily from fragments.

Milfoil can grow in water up to 25 ft deep before it runs out of sunlight, so it colonizes the near-shore water, blanketing it in a weedy morass. Its detrimental effects include interfering with swimming, boating, and fishing and impacting property values. In spring it starts to grow before native aquatic plants emerge and quickly shades them out, causing a loss of species biodiversity. As native plants disappear, so does the food source they provide for native insects and fish.

In Vermont, milfoil is prevalent in Lake Champlain and has also been documented in over 80 waterbodies, including the Connecticut River. Large sums of money – over $200,000 per year in some locations – may be spent battling it. Milfoil was first noticed in Lake Fairlee in the early 1990s.

At first the Lake Fairlee Association controlled it by employing divers to hand-pull it. Then they added suction harvesting and ecologically questionable “bottom barriers” – large sheets of plastic to smother the weed. In total, these efforts cost over $100,000 per year, of which the State supported 40%.  

Neighboring Lake Morey had fought the same battle for longer and declared the fight lost in 2002 when milfoil growth “became exponential” and impossible to combat in boating and swimming areas. In 2006 an initiative to treat the lake with a herbicide was started, and the herbicide was applied in 2007, with yearly follow-up for a while. According to the Director of Buildings and Grounds at the Aloha Foundation, which owns three parcels on Lake Morey, the clearing of milfoil-overrun areas by the treatment was “stunning.”

The infestation of Lake Fairlee reached a “tipping point” not long after, and following much deliberation, the Lake Fairlee Association recognized that their milfoil control program was no longer adequate. In 2009, with strong public backing, the LFA started the lengthy permit process to treat the lake with herbicide. The actual treatment took place in summer of 2010, with follow-ups.

Even with “stunning” results, one can never get rid of every last bit of milfoil. During treatment, pains are taken to keep herbicide levels in the water as low as possible, and only the badly infested areas are treated.  Now, twelve years later, the milfoil is again getting out of control.

So, what are aquatic herbicides, how safe are they, and how do they work?

Formerly the herbicide of choice was trichlopyr, a chemical that mimics indole acetic acid, a plant growth hormone or auxin. Trichlopyr disrupts hormone action and causes uncontrolled, disorganized plant growth that leads to death. Plants in the dicotyledonous division, like milfoil, are far more sensitive to trichlopyr than plants in the monocotyledonous division that encompasses most native aquatic plants. The herbicide degrades rapidly in water and does not accumulate in the bodies of animals.

A new auxin mimicker, florpyauxufen-benzyl, has been developed. Like trichlopyr, its toxicity favors dicotyledonous plants. It works in a similar way, by causing “non-productive plant tissue growth,” failed nutrient transport, and death. It degrades in water, with a half-life – the time taken for 50% of the chemical to decay – of 1.5-2.5 days. If it gets mixed into sediment, its half-life can be 55 days. However it becomes immobilized by binding to sediment particles. If ingested by an animal, less than half is absorbed, and daily high doses (far exceeding what would be used in a lake) did not harm lab rats or cause cancer. Some growth-stunting effects were seen on aquatic invertebrates. A promising study showed that milfoil was more sensitive to florpyrauxifen-benzyl than to other herbicides. Therefore the herbicide can be applied at lower concentrations with fewer side effects on native plants and invertebrates. It has already been used in other VT lakes, such as Lake St. Catherine.

In order to treat a lake with herbicide, an application must be submitted to the VT Department of Environmental Conservation for an “Aquatic Nuisance Control Individual Permit.” This is no trivial undertaking; the document is 13 pages long, divided into numerous sections. It must comply with state law 10 V.S.A. § 1455.

Among many items, maps showing the density of milfoil growth and treatment areas shall be provided. Also the public must be notified in several different ways 30 days before the treatment date, and residents on the lake outlet up to one mile downstream must be included. The LFA will also be required to provide drinking water “upon request” to those who depend on the lake or its outlet streams for drinking, cooking, etc. for one day after treatment. Herbicide will be added into the water at the mapped sites by a state-licensed expert, who will be supervised by someone from the state. Water samples will be tested 48 hours later for levels of florpyrauxifen-benzyl, and if levels exceed two parts per billion, further testing will be carried out.

At least the LFA is familiar with this process; after all, it’s the second time around. And it is done with the best interests of the lake in mind. In their words, “We are not treating the Eurasian watermilfoil solely for human convenience, but also to bring a better natural balance to our lake.“