The
milfoil weevil photograph used with permission - Dr. Ray Newman
Description of Method
Many problematic aquatic plants in the western United States are nonindigenous species.
Plants like Eurasian watermilfoil, Brazilian elodea, and purple loosestrife have been
introduced to North America from other continents. Here they grow extremely aggressively
forming monocultures that exclude native aquatic plants and degrade fish and wildlife
habitat. Yet, often these same species are not aggressive or invasive in their native
range. This may be in part, because their populations are kept under control by insects,
diseases, or other factors not found in their introduced range.
The biological control of aquatic plants focuses on the selection and introduction of
organisms that have an impact on the growth or reproduction of a target plant.
Theoretically, by stocking an infested waterbody or wetland with these organisms, the
target plant can be reduced and native plants can recover.
Classic biological control uses control agents that are host specific.
These organisms attack only the species targeted for control. Generally these biocontrol
agents are found in the native range of the nuisance aquatic plants and, like the targeted
plant, these biocontrol agents are also nonindigenous species. With classic biological
control an exotic species is introduced to control another exotic species. However,
extensive research is conducted before release to ensure that biological control agents
are host specific and will not harm the environment in other ways. The
authors of Biological Control of Weeds -- A World Catalogue of Agents
and Their Target Weeds state that after 100 years of using biocontrol
agents, there are only eight examples, world-wide, of damage to non-target
plants, "none of which has caused serious economic or environmental
damage ..."
Search for a classical biological control agent typically starts in the region of the
world that is home to the nuisance aquatic plant. Researchers collect and rear insects
and/or pathogens that appear to have an impact on the growth or reproduction of the target
species. Those insects/pathogens that appear to be generalists (feeding or impacting other
aquatic plant species) are rejected as biological control agents. Insects that impact the
target species (or very closely related species) exclusively are considered for release.
Once collected, these insects are reared and tested for host specificity and other
parameters. Only extensively researched, host-specific organisms are cleared by the United
States for release. It generally takes a number of years of study and specific testing
before a biological control agent is approved.
Even with an approved host-specific biocontrol agent, control can be difficult to
achieve. Some biological control organisms are very successful in controlling exotic
species and others are of little value. A number of factors come into play. It is
sometimes difficult to establish reproducing populations of a biocontrol agent. Climate
or other factors may prevent its establishment. Sometimes the biocontrol insects become
prey for native predator species, and sometimes the impact of the insect on the target
plant just isn't enough to control the growth and reproduction of the species.
People who work in this field say that the more biological control species that you can
put to work on a problem plant, the better success you will have in controlling the
targeted species. There are some good examples where numerous biological control agents
have had little effect on a targeted species, and other examples where one biocontrol
agent was responsible for the complete control of a problem species.
However, even when biological control works, a classic biological control agent
generally does not totally eliminate all target plants. A predator-prey cycle establishes
where increasing predator populations will reduce the targeted species. In response to
decreased food supply (the target plant is the sole food source for the predator), the
predator species will decline. The target plant species rebounds due to the decline of the
predator species. The cycle continues with the predator populations building in response
to an increased food supply.
Although a successful biological control agent rarely eradicates a problem species, it
can reduce populations substantially, allowing native species to return. Used in an
integrated approach with other control techniques, biological agents can stress target
plants making them more susceptible to other control methods.
A number of exotic aquatic species have approved classic biological control agents
available for release in the US. These species include: Hydrilla, water hyacinth,
alligator weed, and purple loosestrife.
In 1992 three beetles were released in Washington for purple loosestrife control. Their
damaging impact on purple loosestrife populations was evident in the Winchester Wasteway
area of Grant County in 1996. In 1998, 1999, and 2000, the Washington State Noxious Weed Control
Board organized insect collection for state, local, and federal staff. Thousands of
insects were collected and distributed to purple loosestrife sites throughout the
state and even the United States.
The Cornell
University web site provides a good overview of the insects imported for
purple loosestrife control.
Another type of biological control uses general agents such as grass carp to manage problem plants.
Unlike classical biocontrol agents, these fish are not host specific and will not target
specific species. Although grass carp do have food preferences, under some circumstances,
they can eliminate all submersed vegetation in a waterbody. Like classic biological
control agents, grass carp are exotic species and originate from Asia. In Washington,
all grass carp must be certified sterile before they can be imported into the state. There
are many waterbodies in Washington (mostly smaller sites) where grass carp are being used
to control the growth of aquatic plants.
During the past decade a third type of control agent has emerged. In this case, a
native insect that feeds and reproduces on northern milfoil (milfoil native to North
America - Myriophyllum sibericum) was found to also utilize Eurasian watermilfoil
- Myriophyllum spicatum (introduced milfoil). Vermont government scientists first
noticed that Eurasian watermilfoil had declined in some lakes and brought this to the
attention of researchers. It was discovered that weevils feeding on Eurasian watermilfoil
caused the stems to collapse. Because native milfoil has thicker stems than Eurasian
watermilfoil, the mining activity of the larvae does not cause it the same kind of damage.
A number of declines of Eurasian watermilfoil have been documented around the United
States and researchers believe that weevils may be implicated in many of these declines.
Several researchers around the United States (Vermont, Minnesota, Wisconsin, Ohio,
& Washington) have been working to determine the suitability of this insect as a biocontrol agent. The University of Washington is conducting research into the suitability of
the milfoil weevil for the biological control of milfoil in Washington lakes and rivers.
Surveys have shown that in Washington the weevil is found more often in eastern Washington
lakes and it seems to prefer more alkaline waters. However, it is also present in cooler,
wetter western Washington too. Washington research will be continuing through 2001.
To read about the milfoil weevil in detail please see: http://www.fw.umn.edu/research/milfoil/milfoilbc.html
for an excellent overview. Follow
this link to see what University of Washington researchers have discovered
about the milfoil weevil in Washington.
Permits
To stock grass carp you need a fish stocking permit from the Washington Department of
Fish and Wildlife. Check with your regional office for permit applications.
The milfoil weevil is native to Washington and is present in a number of lakes and
rivers. It is found associated with both native northern milfoil and Eurasian
watermilfoil. A company is selling milfoil weevils commercially. However, to import these
out-of-state weevils into Washington requires a permit from the Washington Department of
Agriculture. As of December 28, 2000 no permits have been issued for Washington.
Check with your local
Noxious Weed Control
Board to learn more about the requirements for stocking biocontrol agents for
other plants such as purple loosestrife.
Costs
The costs for researchers to locate, culture, and test biocontrol agents is high. Once
approved for use, insects can sell for $1.00 or more per insect. Sometimes it is possible
to establish nurseries where weed specialists can collect insects for reestablishment
elsewhere. Grass carp can cost from $5.00 to $20 per fish (depending on quantity and
shipping method).
Home
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| Harvesting | Rotovation | Diver Dredging | Drawdown | Weed
Rolling | Aquatic Herbicides
| Biological Control
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Lake Page
Questions about
this page?
Contact Kathy Hamel by e-mail at kham461@ecy.wa.gov
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