Collecting and Releasing Biological Weed Control Agents in Washington State

Collecting and Releasing Biological Weed Control Agents in Washington State

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Dale Whaley, Extension Educator, WSU Douglas County Extension, Jennifer Andreas, Integrated Weed Control Project Director, WSU Extension Puyallup Research Center
The Integrated Weed Control Project (IWCP) from Washington State University is a way for landowners and land managers to learn about biological weed control and to learn about biological agents available to suppress non-native noxious weeds on their properties. This publication defines biological weed control and explains safety, where it’s appropriate for use, what biological agents may be useful, and where and how to deploy them.
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Integrated Weed Control Project

Washington State University’s Integrated Weed Control Project (IWCP) was developed to help educate landowners and land managers about biological weed control, and expand the availability of biological agents for the suppression of non-native noxious weeds across the state.

As a statewide program, the IWCP provides on-site recommendations and biological control agents free of charge to those with appropriate release sites. This publication will help you decide if biological control is appropriate for your land, and assist you in combining it with other methods in your integrated weed management (IWM) plan.

What is Biological Weed Control?

Classical biological weed control (often referred to as biocontrol) is the intentional use of an invasive plant’s natural enemies to reduce its vigor and reproductive potential. Many of Washington’s noxious weeds are native to Europe and Asia. When introduced into North America, they often arrive without the insects, mites, and pathogens that keep their populations in check in their native ranges. By releasing biocontrol agents, we aim to restore the balance found in the weeds’ native ranges, thereby shifting the competitive edge back to native or more desirable vegetation, such as forbs and grasses.

Safety of Biological Control Agents

In order to find biocontrol agents, researchers travel to the plant’s native range and survey for biocontrol species that may eventually be approved for release in North America. Potential agents are tested for their effectiveness against the weed and their host-specificity is assessed to ensure they will not attack commercial crops or native species.

Biocontrol agents are not available for use until they have undergone rigorous testing and have been approved for release by the United States Department of Agriculture—Animal and Plant Health Inspection Service—Plant Protection and Quarantine (USDA-APHIS-PPQ).

This process can take ten years or more to complete. The IWCP has adopted the International Code of Best Practices for Classical Biological Control of Weeds (Balciunas and Coombs 2004), meaning that only safe, host-specific, effective, and USDA-APHIS-PPQ-approved agents will be used for biological control. One example of an approved insect is Larinus minutus G., also known as lesser knapweed flower weevil, which is widely used on diffuse knapweed.


Not all noxious weeds have approved biological control agents, but if they do, biocontrol can be an effective and important part of an IWM strategy. Biocontrol agents are most appropriate in situations where weed infestations are large and well established, and on sites where other control methods are not feasible. They are not appropriate for small weed infestations or on sites where weed eradication is the goal. Weed infestations can be brought under control and managed effectively with biocontrol agents, but this tool alone will not eradicate them.

Biocontrol agents often take a minimum of four years, and sometimes more than ten, to make an impact, so this method should be regarded as a long-term management strategy, not a short-term fix.

Choosing an Appropriate Release Site

There are several things to consider before proceeding with a biocontrol agent release. It is important to have the weed properly identified to ensure that the correct biocontrol agent is used. Representatives of the IWCP, your county WSU Extension office, and county noxious weed board control board (CNWCB) office can assist you with noxious weed identification.

In addition, it is critical to determine whether your other management techniques are compatible with biocontrol use. As a general rule, biocontrol cannot be used directly with herbicides or such physical control methods as mowing, but can be used directly with cultural control methods, such as revegetation. It is best to discuss your IWM plan with IWCP personnel to maximize your biocontrol results.



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