Eurasian watermilfoil is an invasive aquatic plant that can cause severe ecological and recreational damage in freshwater systems. This non-native species is highly aggressive and quickly forms dense mats of vegetation that choke out native aquatic life and hinder human activities like boating and swimming. Eradicating this plant is difficult because of its rapid spread. Effective management requires a long-term, multi-pronged approach that combines several different control methods.
Understanding Eurasian Watermilfoil
Eurasian watermilfoil (Myriophyllum spicatum) is characterized by its delicate, feathery leaves, which are typically arranged in whorls of four around the stem. Each leaf usually has 12 to 21 pairs of thin leaflets, a count that helps distinguish it from native milfoil species. When pulled from the water, the leaves of the invasive species tend to collapse limply against the stem.
The plant primarily propagates through fragmentation and runners, not by seed. Even a small segment of stem and leaves can float away and establish a new colony. Once established, the dense surface mats block sunlight from reaching native plants below, reducing biodiversity and lowering water oxygen levels, which harms aquatic organisms.
Manual and Mechanical Removal Methods
Physical removal techniques are well-suited for small, localized infestations or for managing areas around docks and swimming beaches. Hand-pulling allows divers to target the invasive plant while leaving native species undisturbed. However, every single plant fragment must be collected and removed from the water to prevent re-establishment.
Diver-Assisted Suction Harvesting (DASH) enhances this manual process. Divers feed the entire plant, including the root system, into a suction hose connected to a collection vessel on the surface. This technique is effective for removing larger, denser beds and capturing fragments that hand-pulling might miss. Mechanical harvesting using cutting devices is generally discouraged because it can drastically increase the number of fragments, potentially worsening the infestation.
Another physical control is the use of benthic barriers. These are mats laid directly on the lake bottom to cover and smother the milfoil. The barriers prevent the plants from receiving sunlight, killing them within several weeks to a few months. Benthic barriers are most effective in small, specific areas like boat lanes or swimming zones where full coverage can be ensured.
Targeted Herbicide Application
The use of aquatic herbicides is often the most practical option for controlling large-scale or whole-lake infestations of Eurasian watermilfoil. Herbicides such as 2,4-D and triclopyr are commonly used. These are selective, auxin-mimic compounds that target broad-leafed plants like milfoil, causing excessive, unsustainable growth that leads to its death.
For successful treatment, the chemicals must be applied at a concentration and for a duration that achieves the required Concentration Exposure Time (CET) relationship. Low concentrations of 2,4-D, for instance, can be effective if the exposure period is extended, often over 14 days. However, the efficacy of these treatments can be complicated by the presence of hybrid watermilfoil strains, which sometimes exhibit resistance to certain herbicides.
Treatments are typically conducted as either whole-lake applications or as targeted spot treatments for isolated patches. Professional application is a requirement in public waters, necessitating specific state or local permits before any chemical can be introduced. Post-treatment restrictions on water use, such as irrigation or swimming, may be implemented to ensure public safety and environmental protection.
Utilizing Biological Controls
Biological control methods involve introducing or augmenting the plant’s natural enemies. The native Eurasian Watermilfoil Weevil (Euhrychiopsis lecontei) is the most studied biological agent for this purpose. The weevils damage the plant by boring through the stem and consuming the cortex, which reduces buoyancy and limits the plant’s ability to store carbohydrates.
While weevils provide selective, long-term maintenance, their success in achieving initial eradication is variable. For effective control, weevil densities may need to reach between 0.5 to 2 weevils per stem, which can be difficult to maintain due to fish predation and a lack of suitable overwintering habitat. Weevil augmentation is generally considered an integrated management strategy rather than a stand-alone solution for heavy infestations.
Another biological tool is the sterile Grass Carp (Ctenopharyngodon idella). These fish consume milfoil, but their use must be carefully managed. Grass Carp are not selective feeders and may consume desirable native aquatic vegetation alongside the invasive milfoil, potentially disrupting the entire aquatic ecosystem.
Sustaining Control and Preventing Recurrence
Effective, long-term management relies on sustained monitoring and prevention protocols following initial control efforts. Regular surveys detect re-establishment or new infestations early, allowing for immediate, localized treatment before the plant forms extensive mats again. This proactive approach reduces the need for large-scale interventions.
A cornerstone of prevention is the “Clean, Drain, Dry” protocol, which aims to stop the human-assisted spread of milfoil fragments between water bodies.
Clean, Drain, Dry Protocol
- Before leaving a water access, thoroughly clean off any visible plant material from the boat, trailer, and equipment.
- Fully drain all water-holding compartments, such as the bilge and live wells, onto dry land.
- Allow all equipment to completely dry for a minimum of five days, or disinfect it, before launching into a new water body.
Beyond these mechanical steps, managing the surrounding watershed to reduce nutrient runoff, particularly nitrogen and phosphorus, is important, as these nutrients stimulate the aggressive growth of Eurasian watermilfoil.