EURASIAN WATERMILFOIL

Myriophyllum spicatum

Name and Family

Eurasian watermilfoil is a member of the watermilfoil (Haloragaceae) family.

History

Eurasian watermilfoil is native to Eurasia and northern Africa. It is thought to have been accidentally introduced to North America through the aquarium trade, or in ballast water from shipping in the 1940s. The first record in Canada was from a herbarium specimen collected in 1961 at Rondeau Provincial Park in Morpeth, ON. It has since spread throughout the Great Lakes and inland lakes in southern and eastern Ontario, and can also be found in Quebec, New Brunswick, Manitoba, and British Columbia. It has become one of the most widespread and problematic aquatic invasive species in North America due to its competitive growth habits and ability to spread vegetatively by plant fragments.

While the Eurasian watermilfoil was previously identified in one site on PEI (Schooner Pond), subsequent investigations have been unsuccessful in finding the plant. It may have initially been misreported, or populations at the site may have died back. At present, there are no known populations of Eurasian watermilfoil found on PEI.

Identification, Biology, and Life Cycle

Identification

Eurasian watermilfoil can be identified through the following characteristics:

• Typically found in water depths between 1 – 4 m, up to a depth of 10 m. Once the plant reaches the water surface it branches extensively, forming dense canopies.
• The stem is a leafy shoot and can be long and spaghetti-like. The tip is usually reddish and thin, getting thicker below the flowers. 
• Leaves are arranged in whorls (circles) of 3-6 (usually 4) around the stem. Leaves are feather-like, with greater than 12 threadlike divisions per leaf (usually 12-20). Leaves are limp when out of water. 
• Flowers bloom in late July or early August. They grow on terminal spikes above the water and have a pink-red hue. 

Biology

• Eurasian watermilfoil can reproduce both sexually through seed and asexually through vegetative reproduction, fragmentation of the plant, and stolon formation. The ability for parts of the plant to break off and be transported to new areas where it can reestablish as a new plant has led to its nickname “Zombie Plant”. 
• Stem fragmentation is the most notable means of dispersal over long distances and the main method by which the plant colonizes new habitats. Recreational boating is the primary way stems can fragment and spread to new waterbodies. 
• Eurasian watermilfoil is most commonly found in slow or stagnant waters, such as ponds, marshes, lakes with little disturbance, or along sheltered portions of streams (i.e. coves, inlets). It can rapidly colonize areas that have experienced some form of disturbance since disturbance can result in fragmentation and spread. 
• Eurasian watermilfoil can also hybridize with the native milfoil species Northern watermilfoil, Myriophyllum sibiricum, leading to a more aggressive invasive species. This hybrid milfoil expresses a trait called hybrid vigor, which is the expression of superior qualities of both parents.

Life Cycle

• In the Great Lakes region, Eurasian watermilfoil begins to break dormancy in early spring and initiate growth as water temperatures approach 15^oC.
• It can be established by April, well before other aquatic macrophytes. This longer growing season gives it a competitive advantage. 
• Flowering spikes emerge above the water line once the plant has reached the surface, typically from June to July. 
• Fruits develop later in July or August and persist until September. The plant reaches its peak growth and biomass shortly after flowering. At this point milfoil stems begin to branch and form dense clusters, blocking available sunlight to other submerged plants. 
• Soon after peak biomass is reached in late summer to early fall, the plant begins a natural process called autofragmentation. This is a process whereby small branches begin to develop roots, break away from the parent plant, and float until they lose buoyancy. Rooted fragments then sink and root in the sediment, ultimately forming new plants. 
• Towards the end of the growing season, some plants die back to their root crowns, while others overwinter intact in an evergreen form. Unlike other species of watermilfoil, Eurasian watermilfoil lacks specialized structures called turions. Turions are overwintering buds that store starch to be used for growth. In Eurasian watermilfoil, reserves are stored in overwintering roots and shoots.

Potential Lookalikes

• Eurasian watermilfoil can be confused for Northern (aka Siberian) watermilfoil, M. sibiricum, a native species. In comparison, Northern watermilfoil has fewer threadlike leaf divisions (less than 12) with the presence of turions (overwintering buds). Northern watermilfoil leaves remain rigid when removed from water while Eurasian watermilfoil’s leaves fall limp. However, these two species may hybridize where there is a range overlap, complicating identification. If you are unsure, the most reliable method of identifying any milfoil species (i.e. confirming Eurasian vs. hybrid watermilfoil) is through molecular testing using the plant’s DNA.
• Other potential lookalikes:
  • Variable-leaved watermilfoil, Myriophyllum heterophyllum (S1 – critically imperiled)
  • Andean watermilfoil, Myriophyllum quitense (S1 – critically imperiled)
  • Slender watermilfoil, Myriophyllum tenellum (S1- critically imperiled)
  • Whorled watermilfoil, Myriophyllum verticillatum (SH – possibly extirpated)
  • Common hornwort (aka coontail), Ceratophyllum demersum (S4 – apparently secure)
  • Prickly hornwort, Ceratophyllum echinatum (S1 – critically imperiled)
  • Bladderworts, Utricularalria spp. (S1 – critically imperiled, to S4 – apparently secure)

What it Does to the Environment

Environmental Impacts

• Eurasian watermilfoil forms dense floating mats of vegetation that can outcompete native aquatic plants and alter a waterbody’s natural ecosystem. Dense mats block available light and impact water clarity, and the plant’s extensive root system reduces space and access to nutrients in the sediment for other plant species. As a result, significant declines in native submergent plant species richness and abundance have been observed in infested waterbodies. 
• Dense mats also reduce macroinvertebrate abundance and negatively impact fish populations, by altering water quality, reducing suitable sites for spawning, and reducing dissolved oxygen levels. A reduction in dissolved oxygen levels caused by the decomposition of plant matter can kill fish and contribute to algal blooms. 
• Eurasian watermilfoil mats reduce spawning success for salmonids by covering open gravel areas that make ideal sites for salmonid spawning. Dense mats can also lead to an overpopulation of forage fish, and predatory fish may conversely decrease in numbers as they have more difficulty accessing fish prey hidden in the vegetation.

Economic and Social Impacts

• Dense stands of Eurasian watermilfoil can impact recreational activities such as boating, swimming and angling.
• Thick mats are considered unsightly and decrease the aesthetic value of beachfront properties, creating stagnant conditions ideal for mosquitos. Several studies in the USA have demonstrated that Eurasian watermilfoil can significantly reduce lakefront property values. 
• Excessive growth can also clog industrial and power generation water intakes and restrict the operation of flow metering devices in flood control channels.

Pathways of Spread

Eurasian watermilfoil spreads between waterbodies via plant fragments, which can be transported long distances by boats, boat trailers, fishing gear, and other aquatic equipment. Fragments can also spread naturally by water currents, wind and waterfowl.

Detection and Monitoring

Aquatic invasive species like Eurasian watermilfoil can have lasting impacts on an ecosystem. Therefore, early detection and response (EDR) is critical to slow the spread and enable control efforts. 

The first step is to learn how to recognize Eurasian watermilfoil. Choose a monitoring protocol that is most suitable to the site and resources available. Monitoring might include a visual survey conducted from the lake shoreline, by wading, or by boat. Surveys often use a scale to rank the density of invasion (i.e., sparse, moderate, very dense). Consult resources on aquatic invasive plant monitoring to determine the most appropriate monitoring methods for your site.  

If you have seen Eurasian watermilfoil, you should immediately report it to the PEI Invasive Species Council by email at peiinvasives@gmail.com or via direct message on Facebook or Instagram. Be sure to include photos of the plant and the location at which it was found.

Prevention

To help prevent the spread of Eurasian watermilfoil, avoid boating through infested areas, and remember to clean, drain, and dry all aquatic equipment after each use. Boaters should always drain their bilge pumps and wash their boats thoroughly when moving them from one body of water to another. Remove all plant parts, animals, and mud before moving to a new waterbody. New plants can grow from small stem fragments. 

1. Clean – Remove all plants, animals, and mud from all equipment. Rinse your boat and any equipment that normally gets wet with hot tap water (greater than 50°C) or spray your boat and trailer with a high-pressure water jet.
2. Drain – Ensure that all water has been drained from the watercraft and other equipment. 
3. Dry – Ensure that you allow adequate time for your watercraft to dry completely prior to launching in another waterbody.

Don’t let it loose! Never empty aquarium waste into a natural area. The dumping of aquariums has facilitated the spread of Eurasian watermilfoil and other invasive species such as goldfish and red-eared slider turtles.

Management

General Considerations

Buffer Zone Considerations

• When managing invasive plants in an aquatic setting, you must first obtain a Watercourse, Wetland and Buffer Zone Activity Permit from the PEI Department of Environment, Energy, and Climate Action.
  • It is best to apply at least six weeks before your planned work start date. 
  • The permit can be applied for at the following link: https://www.princeedwardisland.ca/en/service/apply-for-a-watercourse-wetland-and-buffer-zone-activity-permit
• It is unlawful to apply herbicides within 15m of a wetland or watercourse on PEI. 

Before selecting a control method, consider the following

• The size of the infestation, your available resources, and the amount of effort you are willing to expend. 
• Control is most effective using an integrated pest management (IPM) approach that combines education and prevention strategies with multiple control methods. 
• Ensure you also consider the local ecosystem and what other organisms or ecological processes may be affected by management.
• Use containment measures, such as a filter (i.e., floating boom), to capture plant fragments and prevent them from re-rooting. Note that any left-over material will regrow regardless of control type, therefore extra care should be taken to remove all plant pieces. 
• Management approaches can help reduce milfoil biomass in the short term (weeks to a few years) but it is extremely difficult to completely eradicate populations. 
• Control should ideally begin by mid- to late June, when the native seed bank has had a chance to establish and effectively compete with Eurasian watermilfoil. Control can also be timed during carbohydrate storage low points (May – July). Timed control can help maximize treatment efficacy by limiting growth post-treatment, particularly for chemical and manual control. 
• It is important to be cautious of the impacts of control methods on macroinvertebrates and fish species that may be using milfoil and other plants as shelter. Controlling too late in the season should also be avoided as in the fall (September – October), Eurasian watermilfoil becomes brittle and can fragment easily. 

Physical Control

Hand Pulling

• Hand pulling individual plants, small populations, or in isolated water bodies can help prevent spread. Focus on removing all fragments that may break off during removal and facilitate spread. 
• Regularly monitor the site for several years for new plants growing from roots or plant fragments. 
• The plant can be removed by snorkeling in shallow water or scuba diving. This works best when paired with a partner on a raft who can look out for plant fragments, collect pulled plants, and keep watch for hazards. 
• Continue to monitor and remove plant material throughout the summer. This work is best undertaken after fish spawning (July 1^st) and before fall when milfoil becomes more brittle. 

Raking

• Raking can be effective in maintaining open water around docks or swimming areas but is not effective for large areas. 
• Raking can help remove plant material, but it will not eliminate the plant. In most cases, raking will spread the plant via fragments. Use containment measures to prevent spread. 
• In deeper water, raking may be combined with hand pulling in an effort to remove all floating plant fragments. 
• Plants can also be pulled using a rake by guiding the rake along the plant and spinning the rake. This will cause the stems get wrapped around the rake before pulling it up from the water.

Cultural Control

Benthic barriers

• Benthic barriers are covers that are anchored to the bottom sediment of a waterbody. This method blocks sunlight, preventing plants from photosynthesizing and suppressing their growth. 
• When left in the water, mats will accumulate sediment, allowing new plants to root on top of the mat which essentially buries the milfoil. The milfoil then decomposes.
• Consider transplanting species onto the treated area in order to reduce the potential for milfoil or another nuisance species from taking over.
• Benthic barriers can control growth in localized areas, such as around boat launches, docks and/or swimming areas as part of an integrated management strategy. Barriers are more effective if combined with other control methods, such as raking.  
• This method is considered low-efficacy as it is expensive, difficult to install, laborious, non-selective, and requires routine monitoring and maintenance throughout the growing season.
• As a non-selective method, note that all organisms (native plants, benthic invertebrates, soil-dwelling organisms, fish) will be impacted in the covered area. Consider tarping for smaller areas, areas where milfoil is the dominant species, or areas frequented by human activity such as boat launches.
• Consider any regulatory restrictions before installing a benthic barrier and ensure that you operate within these restrictions.  

Biological Control

The North American weevil (Euhrychiopsis lecontei) has been used as a biological control for Eurasian watermilfoil and hybrid watermilfoil. The adults feed on the stems and leaves of the plant, while the larvae bore into and feed on the inside of the stem, thus causing the plant to lose buoyancy and sink. However, for this method to succeed, weevil densities must be high enough to significantly impact milfoil density. This requires continual restocking of beetles to ensure adequate numbers (Borrowman et al. 2014). Unfortunately, past attempts at weevil release in parts of Ontario such as Big Cedar Lake and lakes in the Sudbury area resulted in initial declines in milfoil. but beetle densities were not high enough to cause significant damage, even with augmentation (Cooper et al. 2018).

References and Links

Department of Fisheries and Oceans – Eurasian Watermilfoil

Invasive Species Centre – Eurasian Watermilfoil

Ontario’s Invading Species Awareness Program – Eurasian Watermilfoil

Acknowledgements

This webpage was developed with the financial support of the Department of Fisheries and Oceans Canada through the Aquatic Invasive Species Prevention Fund.

Image Gallery

Featured image credit: Chris Evans, University of Illinois, Bugwood.org