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General
Information about Eurasian Watermilfoil
Eurasian watermilfoil (Myriophyllum
spicatum) is an attractive plant with feathery underwater
foliage. It was once commonly sold as an aquarium plant. Eurasian
watermilfoil, hereafter called milfoil, originates from Europe and
Asia, but was introduced to North America many years ago and is
now found over much of the United States. This plant was
introduced to the eastern United States at least as long ago as
the 1940s, but it may have arrived as early as the late 1800s. The
first known herbarium specimen of milfoil in Washington was
collected from Lake Meridian near Seattle in 1965. By the mid
1970s it was also found in Lake Washington. During this same time
period milfoil became established in central British Columbia and
traveled downstream to Lake Osoyoos and the Okanogan River in
central Washington. Now milfoil is found in the Columbia,
Okanogan, Snake, and Pend Oreille Rivers and in many nearby lakes.
In western Washington, the distribution of milfoil closely follows
the Interstate 5 corridor. It is very apparent that milfoil has
been spread from lake to lake on boat trailers.
Growth Habit
Because it is widely
distributed and difficult to control, milfoil is considered to be
the most problematic plant in Washington. The introduction of
milfoil can drastically alter a waterbody's ecology. Milfoil forms
very dense mats of vegetation on the surface of the water. These
mats interfere with recreational activities such as swimming,
fishing, water skiing, and boating.
In eastern Washington milfoil interferes with power
generation and irrigation by clogging water intakes.
The sheer mass of plants
can cause flooding and the stagnant mats can create good habitat
for mosquitoes. Milfoil mats can rob oxygen from the water by
preventing the wind from mixing the oxygenated surface waters to
deeper water. The dense mats
of vegetation can also increase the sedimentation rate by trapping
sediments. Milfoil also starts spring growth sooner than native
aquatic plants and can shade out these beneficial plants. When
milfoil invades new territory, typically the species diversity of
aquatic plants declines. While some species of waterfowl will eat
milfoil, it is not considered to be a good food source.
Milfoil reproduces
extremely rapidly and can infest an entire lake within two years
of introduction to the system.
Although milfoil produces many seeds, we do not
believe that these seeds are important for milfoil reproduction in
Washington waters. However, milfoil is able to reproduce very
successfully and rapidly through the formation of plant fragments.
In the late summer and fall the plants become brittle and
naturally break apart. These fragments will float to other areas,
sink, and start new plants. Milfoil will also grow from fragments
created by boaters or other disturbances during any time of year.
A new plant can start from a tiny piece of a milfoil plant. This
is why milfoil can so easily be transported from lake to lake on
boat trailers or fishing gear. Once established in its new home,
water currents may carry milfoil fragments and start new colonies
within the same waterbody.
Management
Once milfoil becomes
well-established within a waterbody, it is difficult or impossible
to remove. In smaller
waterbodies (350 acres or less), we have had some limited success
using an aquatic herbicide called Sonar®
to remove milfoil. Recently the state legislature gave limited
approval for the use of 2,4-D to control pioneering milfoil
infestations. Other control methods include: Harvesting,
rotovation (underwater rototilling), installation of bottom
barriers, diver hand pulling, diver dredging, and in some very
limited situations the use of triploid (sterile) grass carp. We
are investigating other biological controls such as the milfoil
weevil. The management of milfoil costs the state and private
individuals up-to-one million dollars per year.
Identification
As a group, milfoil species are
easy to identify because all milfoils have feather-like leaves
arranged in whorls around the stem. Identifying individual
species is much more difficult and even the plant experts rely on
DNA or pigment analysis to tell some species from each
other. There are several native milfoil species in Washington and
some species like northern milfoil (Myriophyllum sibericum)
are commonly found in Washington lakes. In fact, for many years
northern milfoil and Eurasian watermilfoil were classified as the
same species. Currently botanists seem to be in agreement that the
two are separate species, though often they look very similar. The
milfoil leaflet to on the right is from a northern milfoil plant.
The threadlike segments are very widely separated and there are
few pairs per leaflet. The Eurasian watermilfoil leaflet (left
side of the photo) has many pairs of closely-spaced segments. Note
also that the shape of each leaflet is different. Unfortunately
not all northern and Eurasian milfoil leaflets look so differently
from each other. Sometimes these two species look very similar and
when that happens it is best to rely on DNA or pigment analysis
for positive identification.
Here are some tips to
identify Eurasian watermilfoil from the native milfoils.
- Count the pairs of
leaflets. Eurasian watermilfoil usually has twelve or more
pairs on each leaf.
- Eurasian watermilfoil
leaves tend to collapse around the stem when removed from the
water. Other milfoil
species have thicker stems and are usually more robust.
- The mature leaves are
typically arranged in whorls of four around the stem.
Source: Washington State
Department of Ecology |
