Aquatic Plant Management - Biological Control

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. The milfoil weevil photograph was used with permission from Dr. Ray Newman.

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. The United States only clears extensively researched, host-specific organisms 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. Work is underway on a biological control for common reed (non-native genotype).

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.

Recently, 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) conducted research to to determine the suitability of this insect as a biocontrol agent. The University of Washington conducted surveys in Washington to determine the native range of the weevil here. Ecology staff conducted research into the suitability of the milfoil weevil for the biological control of milfoil in Washington lakes and rivers. Click here for a summary of Ecology's pilot project in Mattoon Lake. Scroll down for a list of special projects including weevil research.

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 continue through 2009.

To read about the milfoil weevil in detail please see these two links: and


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 sells milfoil weevils. However, to import these out-of-state weevils into Washington requires a permit from the Washington Department of Agriculture. As of December 2009 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.


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).

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