Eurasian watermilfoil is an aggressive submerged aquatic plant native to Europe, Asia and North Africa that has become one of the most widely distributed non-native aquatic plant species in North America.

Characteristics

Like other milfoils, Eurasian watermilfoil (referred to as milfoil hereafter) is a submersed, rooted plant that has long, narrow leaves arranged in whorls of 4-5 about the stem that have a feathery look to them. Similar to the native Northern watermilfoil (Myriophyllum sibericum), which generally has 5-9 leaflets, milfoil has 12-21. However, large variations in these numbers can make it easy to misidentify. It differs from Northern watermilfoil as it can grow at great depths of up to 10 m ( 33 ft ) but is generally found at depths of 0.5 to 3.5 m. ( 1.6 to 11.5 ft ). It can grow in a variety of environmental conditions including: contaminated waters, a wide range of temperatures, in still and flowing waters, in clear to turbid waters and can survive in a pH range of 5.4 to 11. Milfoil thrives in disturbed areas and is common in areas where human activity has altered the environment.

Milfoil reproduces both sexually from seed and asexually from stem fragments. The primary method of spread is asexually from stem fragments that become brittle and break apart in the fall. These fragments can float long distances to new areas and settle to become a new plant. Recreational activities (such as boating) and other disturbances can carry stem fragments to other areas or bodies of water to become new plants. Established plants can also spread by producing stolons that come from the root crowns and can establish a new plant up to 1 m away. Although many seeds are produced, many of these lie dormant for a long time and germinate sporadically, leading to the conclusion that sexual reproduction is not the primary method of spread.

Distribution

It is believed that milfoil was introduced to North America in the late 1800s but was not positively identified until 1942 in Washington D.C. by Couch and Nelson. Since that time, Eurasian milfoil has become the most widely distributed invasive aquatic plant species. It has now been confirmed in the provinces of Ontario, Quebec and British Columbia and is confirmed in 45 states in the United States Milfoil is found from Greenland to Florida, demonstrating its adaptability and potential to spread.

Impact

Milfoil grows quickly in the spring and once the stems hit the water surface, they branch in all different directions to produce a dense mat of vegetation and blocks sunlight for plants below the surface. Because of its ability to out compete native plant species a monoculture is created, and aquatic plant diversity can decrease. These thick mats of vegetation can add more phosphorous and nitrogen to the water column and alter water quality by raising pH and temperature and decrease dissolved oxygen making poor habitat for fish, waterfowl and other species. These thick mats can also create stagnant water that may serve as breeding grounds for mosquitos.

Milfoil can interfere with boating, fishing, and swimming when one encounters the dense mats of vegetation that can form. The vegetation can become entangled in boat propellers and boating equipment. Milfoil has also been known to clog pipes in irrigation canals, and water and power generation intakes.

Prevention

As with most invasive species, once milfoil becomes established, it is almost impossible to eliminate from an area. Prevention is the best way to stop the spread of this invasive species. Checking and cleaning your boat, trailer or other recreational equipment for milfoil fragments is essential to prevent it from spreading into other areas. There are many methods of control, but these generally only control milfoil for a season or two before it comes back again. Some of the less harmful control methods include: rotovation (underwater roto-tilling), diver hand-pulling and/or dredging, downdraws (in areas where water levels can be controlled), and installing bottom barriers in areas of small infestation. Contact your local resource agency to determine if permits are required in your area.

Biological control is an attempt to reunite a plant with its natural enemies. Complete eradication is unlikely, but the goal of biocontrol is to reduce numbers of the target plant to lessen its ability to displace native vegetation. Research and development is being undertaken to discover the efficiency of a native weevil to help control milfoil. The North American weevil normally eats Northern watermilfoil, however, it will also eat milfoil. The weevil lays its eggs on the tips of the plant where they hatch, and young burrow into the stem and eat their way through the plant, slowing plant growth, and weakening the plant. This may prove to be a cost effective, safe, non-toxic, long-term solution to the invasion of milfoil.