Invasive Species Council of Manitoba

Eurasian Watermilfoil(Myriophyllum spicatum)

Eurasian Watermilfoil is an invasive, submersed aquatic perennial with smooth stems that branch near the surface. Eurasian Watermilfoil is native to Europe, Asia, and Northern Africa, and was introduced into North America in the 1940s, causing ecological damage to rivers, lakes and wetlands.

Eurasian Watermilfoil has spread to 45 U.S. states and three Canadian provinces (Ontario, Quebec and British Columbia). It has been reported but not confirmed in Manitoba. It was reported in 1998 and again in 2001 in a channel portion of the Souris River in southwestern Manitoba. Of concern to Manitoba is the general northward spread of the plant and established populations in Minnesota and Wisconsin. The species has been confirmed in the Red River Watershed in North Dakota southwest of Fargo.

Description

General: A submersed aquatic rooted plant, growing 2-3 metres in length with vine-like stems and featherlike leaves.

Leaves: Feather-like, whorled about the stem in fours. Leaves on the primary stem may be rigid, linear, and kidney shaped.

Stem: Long, vine-like, branching

Flowers: The terminal spike is about 2-10 cm long and is often standing above the water.

Habitat

Grows in depths of up to 7 metres of water, in any type of aquatic substrate including silt, sand, or rocks.

Dispersal

Eurasian Watermilfoil produces seeds, but spreads primarily by plant fragments that grow roots, stems and leaves as they float. In winter, upper portions of the stems break off and are capable of starting new populations in new habitats. Lower portions of the plant remain alive and green throughout the winter and send up new shoots in the spring and summer growing up to 57 cm per day.

Recreational activities are thought to be the major source of introduction. Plant materials caught on boat motors, trailers, nets, boat propellers, and fishing gear can cause introduction to new waterbodies. Dispersal is also linked to the aquarium and aquatic nursery trade. Eurasian Watermilfoil is a popular aquarium plant and unwanted aquarium contents discarded into various water sources including wetlands, lakes, streams and rivers can cause new introductions. Storms, and flood events can further contribute to dispersal.

Impacts

Eurasian watermilfoil threatens human health, as infestations create ideal habitat for mosquitoes including Culex spp. which has been implicated in the spread of West Nile Virus in Manitoba. Eurasian Watermilfoil will out-compete or eliminate native aquatic and wetland plants reducing overall biological diversity and reducing water quality. Waterfowl are reported to avoid aquatic areas infested by the plant. Eurasian watermilfoil grows in dense bands along riparian areas making fishing, boating and swimming impossible. Economic impacts have been estimated in the millions of dollars every year.

The plant has replaced the native northern watermilfoil (M. sibiricum) over much of the North American range. Of major concern is the ability of these two plants to hybridize, creating even stronger hybrid plants that may be resistant to biological control agents.

Control Methods

Once established there is no management tool currently available to eradicate Eurasian Watermilfoil. Mechanical control options such as bottom barriers, suction harvesting, hydro rakes, pulling, mechanical harvesters and rotavating as mechanical control option have been attempted but are very costly, labour intensive and provide only short term control, while negatively affecting non-target vegetation.

Chemicals have been used throughout the United States to control and contain Eurasian Watermilfoil but would not be a management option in Manitoba or Canada. There is also the concern that chemical controls may harm non-target organisms. Management tools may also include boat inspections, public education and surveys at boat launches or highway access point. Biological control may be viable option for Eurasian Watermilfoil. Research suggests that the native herbivorous weevil may provide some control, and the other biological control agents are being examined.

Eurasian Watermilfoil

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