What is Eurasian Watermilfoil (EWM, milfoil)?
Eurasian watermilfoil is a non-native aquatic plant that is present in most U.S. states and much of Canada. This plant is known for its rapid growth and ability to spread, which can lead to significant problems within a lake. Milfoil forms dense beds that can seriously impair the recreational use of a lake, reduce the availability of fish spawning grounds, outcompete beneficial native plants, and otherwise alter a lake’s natural environment.
The growth and spread of Eurasian watermilfoil is a threat to all our lakes and ponds. Once Eurasian watermilfoil has infested a lake and becomes established, it can be impossible to eradicate it. Lake managers can only seek to control it by integrating the most effective, economically feasible, and environmentally sound methods available.
Of note, Shadow Lake, which discovered milfoil plants in 2011, was able to eradicate an incipient population after eight years of intense efforts. Prevention and early detection of milfoil introduction to a lake through programs like the Vermont Public Access Greeter Program and the Vermont Invasive Patrollers (VIP) Program are critically important to prevent the spread. After milfoil was newly discovered in Shadow Lake, their lake association got to work using numerous methods over many years to eradicate their small population of milfoil before it became established and spread throughout the lake.
Currently, approximately 99 of the 800+ lakes and ponds in Vermont are infested with milfoil. A list of confirmed aquatic invasive species in Vermont lakes can be viewed here (updated May 2021).
Eurasian watermilfoil is listed on the Vermont Noxious Weed Quarantine list.
What is the Impact of Milfoil on a Lake?
Eurasian watermilfoil is not native to North America but originates from Europe, Asia and northern Africa. As an introduced aquatic plant species to this continent, Eurasian watermilfoil has no natural controls (insects, bacteria, fungi) to keep its growth in check. Milfoil stems can reach the surface in up to 20 feet of water, growing up from the lake bottom each year from a fibrous root system. Milfoil grows and spreads extremely quickly, forming dense surface mats. Eurasian watermilfoil will grow readily in many types of water bodies, as well as in almost any bottom type: silty, sandy, or rocky.
The presence of Eurasian watermilfoil often brings a change in the natural lake environment. Over time, it outcompetes and suppresses beneficial native aquatic plants, severely reducing natural plant diversity within a lake. Since its growth is typically dense, milfoil beds are poor spawning areas for fish. Although many aquatic plants serve as valuable food sources for wildlife, waterfowl, fish, and insects, Eurasian watermilfoil is rarely used for food. Dense surface mats of milfoil can also impede recreational activities like fishing, boating, kayaking, and swimming.
Eurasian watermilfoil reproduces almost exclusively by the breaking off of stem pieces which can drift away, sink, develop roots, and grow into new plants. A stem fragment just a few inches long can start a new plant. This fragmentation occurs both naturally and as a result of human activity. Within a lake, wind and waves may break plants loose, allowing them to drift into new locations and become established. Boating activity through dense milfoil beds also contributes to the fragmenting and spread of milfoil plants.
“Eurasian water-milfoil competes aggressively to displace and reduce the diversity of native aquatic plants. It elongates from shoots initiated in the fall, beginning spring growth earlier than other aquatic plants. Tolerant of low water temperatures, it quickly grows to the surface, forming dense canopies that overtop and shade the surrounding vegetation (Madsen et al. 1991). Canopy formation and light reduction, are significant factors in the decline of native plant abundance and diversity observed when Eurasian water-milfoil invades healthy plant communities (Smith and Barko 1990; Madsen 1994).
Eurasian water-milfoil has less value as a food source for waterfowl than the native plants it replaces (Aiken et al. 1979). And although fish may initially experience a favorable edge effect, the characteristics of Eurasian water-milfoil’s overabundant growth negate any short-term benefits it may provide fish in healthy waters. At high densities, its foliage supports a lower abundance and diversity of invertebrates, organisms that serve as fish food (Keast 1984). Dense cover allows high survival rates of young fish, however, larger predator fish lose foraging space and are less efficient at obtaining their prey (Lillie and Budd 1992; Engel 1995). Madsen et al. (1995) found growth and vigor of a warm-water fishery reduced by dense Eurasian water-milfoil cover.
The growth and senescence of thick vegetation degrades water quality and depletes dissolved oxygen levels (Honnell 1992; Engel 1995). Typical dense beds restrict swimming, fishing and boating, clog water intakes and result in decaying mats that foul lakeside beaches.”
From the Vermont Department of Fish and Wildlife:
“Despite a variety of treatment methods, Eurasian watermilfoil is nearly impossible to eradicate once it has invaded. Current control efforts include benthic barriers, mechanical harvesting, diver-operated suction harvesting, biological control using watermilfoil weevils, and chemical treatment. Due to the high costs and continuous effort required, the best management option for milfoil is spread prevention. Eurasian watermilfoil was originally brought to North America through the aquarium trade and was introduced to lakes and ponds through aquarium dumping. The plant species is on the Vermont Noxious Weed Quarantine list, making it illegal to buy, plant, or transport milfoil in the state.
Because milfoil fragments can easily hitch a ride on boats and equipment, practicing the Clean, Drain, Dry spread prevention methods is very important. Drain all water from your boat, canoe, kayak, and other vessels and any equipment used in the water. Clean vessels and gear and remove all plant fragments and dispose of properly. Dry all damp areas of boats and vessels, such as livewells and bilges, with a towel and let air dry in the sun for at least five days before using in another waterbody. If this is not possible, rinse equipment with hot, high-pressure water.”
According to the Lake Champlain Basin Program (LCBP), milfoil can:
- Decrease native plant diversity by out-competing native aquatic plants
- Decrease light penetration, habitat complexity and oxygenation
- Increase sedimentation, nutrient loading, and accelerate eutrophication
- Affect pH and temperature levels
Prevention and Early Detection Are Extremely Important
Lakes that are milfoil-free should participate in Vermont DEC early detection programs like the Vermont Public Access Greeter Program and the Vermont Invasive Patrollers (VIP) program. The costs to prevent new infestations of aquatic invasive species (AIS), and to detect them early, are significantly less than what it costs to control an established milfoil population.
From the Vermont DEC, about the Vermont Public Access Greeter Program:
“Aquatic invasive species are spread by overland transport of watercraft, trailers, and fishing and recreational equipment. The most effective way to prevent spread is through education and equipment inspections designed to catch invasive species ‘hitching a ride’ from one waterbody to another. Preventing the spread of aquatic invasive species is far more effective and economically sensible than eradicating invasive species once they are established. With support from Vermont DEC, Public Access Greeters educate lake visitors about invasive species, provide courtesy watercraft inspections, and STOP introductions.
Since 2002, the Vermont Public Access Greeter Program has expanded operation to access points at 32 lakes and ponds statewide, and those numbers are increasing annually. The total number of inspected watercrafts has increased since the program’s inception, with 404 inspected in 2002 to 31,052 in 2022. In 2022, Greeters intercepted and removed 530 instances of aquatic invasive species. Of these AIS intercepts, 451 were Eurasian watermilfoil.”
From the Vermont DEC, about the Vermont Invasive Patrollers (VIP) Program:
“The Vermont Invasive Patrollers (VIP) program was established by the Vermont Department of Environmental Conservation in 2007 to focus on early detection of all known and potential aquatic invasive species (AIS). VIPs are trained to identify both aquatic invasive plants and animals that are either established in Vermont or in nearby states and pose the greatest threat to Vermont’s water bodies.
Through hands-on workshops, lakeshore residents and lake users learn what aquatic invasive species are, how to prevent the spread of AIS, and how to identify the species that pose the greatest threats to Vermont lakes and ponds. By enlisting the help of a trained network of volunteers, Vermont DEC staff are much more likely to learn of new AIS infestations early and as a result, may have more management options at their disposal.”
Milfoil Control Methods
Lakes with established populations of milfoil have several tools available to keep it under control. Some of these options require an Aquatic Nuisance Control (ANC) permit from the Vermont Department of Environmental Conservation. When dealing with a well-established infestation of milfoil, the goal is control. While eradication is possible (see Shadow Lake above), it is certainly not a high possibility unless it is detected early and acted upon immediately, and adequate resources exist for long-term control.
Hand-Pulling & Fragment Cleanup: Milfoil can be hand-pulled at any time during the season and does not require a permit as long as no mechanical methods are used. When hand-pulling, it is very important to be sure to remove the whole plant, including the roots, and to also remove any plant fragments that may occur during the removal. While fragments can occur any time, milfoil plants naturally become brittle in the fall and fragment more easily. If diving or snorkeling when hand-pulling, a mesh bag should be used to collect the milfoil to be able to bring it to the surface, remove it from the lake and reduce escaping fragments. It is also beneficial to remove floating milfoil fragments from the waterbody so they do not grow into new plants.
Diver-Assisted Suction Harvesting (DASH): DASH crews hand-pull milfoil by the roots from the lakebed and place the plant into a suction tube. This tube pulls the plant up to the surface to a boat with a collection basin where a team member will load the milfoil into buckets for transport from the lake. In areas where milfoil is present among native plants, divers are selective and only remove the milfoil while leaving native plants alone. DASH require a permit from the Vermont DEC and typically can’t begin before July 1st to protect fish spawning.
Photo credit: NYS Parks Department
Benthic Barriers: Benthic barriers are mats (which can be made of various materials) that are weighted and placed on the lakebed to prevent sunlight from reaching covered plants. These bottom barriers can be effective but are not selective. All plants under the mat will die, and the mats can also have a negative impact on invertebrates and slow moving organisms like mussels. The use of benthic barriers requires a permit from the Vermont DEC, and they may be installed on July 1st and must be removed on October 1st.
Herbicides: Aquatic herbicides are another tool that can be used to control milfoil. Over the past several years, the herbicide ProcellaCOR has been successful in controlling milfoil in lakes in Vermont and around the country as part of an integrated control plan. Vermont DEC has a rigorous process for applicants seeking to obtain an Aquatic Nuisance Control (ANC) permit to use an herbicide in waters of Vermont.
Applications to use an herbicide must meet five criteria:
- There is no reasonable non-chemical alternative available
- There is acceptable risk to the non-target environment
- There is negligible risk to public health
- There is a long-range milfoil management plan that has been developed which incorporates a schedule of pesticide minimization
- There is a public benefit to be achieved from the application of a pesticide
These five criteria are required by Vermont law, under statute 10 V.S.A. § 1455.
ProcellaCOR is highly selective to milfoil, but some native plants found in some Vermont lakes may be impacted by a treatment. A recent statistical analysis by the Vermont DEC of lakes treated with ProcellaCOR saw a statistically significant increase in frequency of occurrence of native plants, and a decrease in milfoil.
Recently, there has been a rash of misinformation in newspaper commentaries, and in online comments about the permitting process and ProcellaCOR. Please see the Milfoil FAQ section below for more information.
Other Control Methods: other techniques have been investigated but they have unfortunately not been successful in providing milfoil control. Some of these include:
- Milfoil Weevils: These small aquatic beetles with snouts feed on milfoil, damaging stems. However, maintaining weevil populations at levels in lakes needed to have an impact has not been achieved, and has not resulted in control. Milfoil weevils (Euhrychiopsis lecontei) are native to North America.
- Grass Carp: Sterile triploid grass carp have been stocked on some lakes (not Vermont) for their appetite for aquatic plants. However, they are not selective and will eat many native plants as well as milfoil. This has been an issue at Candlewood Lake in Connecticut where they are now trying to reduce the number of carp because they ate almost all of the lake’s submerged aquatic vegetation. Grass carp (Ctenopharyngodon idella) are native to China and the Soviet Union. In Vermont, it is illegal to stock grass carp in any waters of the state.
- Aeration: Aeration is an in-lake management tool used to increase the concentration of dissolved oxygen in the water. From the DEC: “…there is no evidence to support the use of artificial circulation/aeration systems for the control of macrophyte (aquatic plants) populations in the scientific literature”. You can read more from the Vermont DEC in their report: “Aeration as a Lake Management Tool and its use in Vermont“.
- Mechanical Harvesting: Mechanical harvesting machines have been used for decades in an attempt to control milfoil and nuisance aquatic plants. While this method provides temporary relief, it does not provide control, and can contribute to milfoil spread via fragmentation. From Vermont DEC: “…this method did not provide a satisfactory level of control and may have contributed to its spread via fragmentation. Experience with mechanical harvesting on Rutland County lakes in the 1980s and 1990s showed that harvesting resulted in dense beds of EWM since the aggressive plant is quickest to regrow after cutting.”
Watershed Management (Phosphorus Reduction): Aquatic plant growth is fueled by phosphorus in the water, so working to mitigate sources of phosphorus entering a lake can help to limit excessive plant growth, and improve water quality. Phosphorus can be carried into the lake by runoff from stormwater events and malfunctioning septic systems. Vermont DEC initiatives like Lake Wise, Stormwater Master Plans, Lake Watershed Action Plans, and Wastewater Workshops offer many phosphorus mitigation strategies. You can read more about phosphorus on “The Phosphorus Challenge” page on our website.
Education: Education of all lake users about milfoil can make a big difference in successfully keeping it under control.
Educational messages for lake users and lake property owners should include:
- Before moving vessels between waterbodies, be sure to Clean, Drain, Dry.
- As a general rule, get as much milfoil out of the lake as possible. Let it dry out on land and dispose of it as you would yard waste or use it as compost, well away from the water.
- Avoid boating through milfoil patches which will create fragments.
- If you have milfoil on your prop, don’t just reverse and drive away, remove it from the lake for proper disposal.
- If you have milfoil growing around your dock or in your swimming area, pull it out by the roots and remove it from the lake for proper disposal.
- If you see milfoil floating anywhere in the lake, near your dock, or along your shoreline, remove it from the lake for proper disposal.
The Case for an Accessible Path for Vermont Lake Associations to Use ProcellaCOR To Control Invasive Milfoil
As you may be aware, the Vermont legislature has created an Aquatic Nuisance Control Study Committee with the passing of Act 57 (H.31). From the Act 57 Committee’s website:
“The Aquatic Nuisance Control Study Committee was created to assess the environmental and public health effects of the use of pesticides, chemicals other than pesticides, biological controls, and other controls in comparison to the efficacy of their use in controlling aquatic nuisances recommendations regarding whether and when pesticides, chemicals other than pesticides, or biological controls should be used to control aquatic nuisances in Vermont in a report to the VT General Assembly on or before December 15, 2023.”
FOVLAP has been monitoring and attending the public meetings of the committee, and felt it was necessary to provide the committee with this paper to outline the need for continued access to regulated, safe, and effective use of herbicides as a tool to maintain control of Eurasian watermilfoil in infested waterbodies.
Click here to read the white paper: “The Case for an Accessible Path for Vermont Lake Associations to Use ProcellaCOR to Control Invasive Milfoil”