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Biological Control

Mecinus janthinus the "Toadflax bug" 

 

Classical biological control involves the introduction and management of selected natural enemies of a weed. Many of our worst weeds originated in foreign countries. These newly introduced plants, free from the natural enemies found in their homelands, gained a competitive advantage over native plants. Once they were out of control, other methods of weed management were not always economical or physically possible. The need for a method of weed reduction that was economical, self-sustaining, and environmentally safe provided opportunities for biological control.

After noxious weeds infestations are out of control, it is often assumed that biological control will solve the problem. But at this time, each new introduction of a biological weed control agent (BCA) in the United States has only about a one in three chance of success. There are several well-documented successes of biological control in our area: Mediterranean sage, St. Johnswort (Klamath weed), and tansy ragwort. Emerging successes on the following weeds (and the most effective agents) include Dalmatian toadflax (Mecinus janthinus), diffuse knapweed (Larinus minutus), leafy spurge (Aphthona spp.), purple loosestrife (Galerucella spp.), and yellow starthistle (Eustenopus villosus).

Biological control is a slow process, and its efficacy is highly variable. Many BCAs are so new that their ability to control their host weed has not yet been determined. Only after several years of monitoring weed populations will the impact of newly introduced agents become evident.

Biological control agents impact weeds directly and indirectly. Direct impact destroys vital plant tissues and functions. Indirect impact increases stress on the weeds, which may reduce their ability to compete with desirable plants.

Biological control can be integrated with other weed control practices to reduce weed populations. For example, once weeds are weakened by BCAs, competitive plantings may be used to out-compete the weeds.

Biological control is not a panacea; it will not eradicate noxious weeds. When using BCAs, residual levels of the weed populations must be expected; the agents’ survival depends on the density of their host weeds. After host weed populations decrease, populations of BCAs will decrease correspondingly. This is a natural cycle. Therefore, a resurgence of weed populations may occur due to seed reserves in the soil, missed plants, and lagging populations of BCAs. In areas where the BCAs do not provide sufficient control, other methods may need to be integrated or the search for additional BCAs must be pursued.

The BCAs released in our area have been tested to ensure they are host specific. This is an expensive and time-consuming task that must be done before the agents are allowed to be introduced into the United States. Some BCAs cleared for use may have only a minor impact on host plant density. Other BCAs that may be effective in reducing weed populations may not be cleared for introduction. One reason for this may be that they are not host specific; that is, the agents in the absence of their host weeds may switch to crops, native flora, and endangered plant species.

Biological control of certain weeds may not work in your area, even if it does in others. Climate variations such as cold winters and plant biotype differences may account for some failures in the past. To ensure maximum success, trained personnel should supervise biological control programs. Biological control agents require specific conditions to survive.

Once release sites for BCAs have been selected, it is essential that those areas be protected. BCA releases are often made with small numbers. It can take 3 to 5 years to establish collectable populations. After BCA populations have sufficiently increased at nursery sites, they may then be introduced into new areas. The types and status of BCAs used in the Pacific Northwest are in the tables in this section. In areas where control was achieved but the host plants make a resurgence—for example, after severe weather—reintroducing the agents may be required.

Biological control has many benefits and some disadvantages. Benefits include: reducing herbicide residues in the environment, host specificity on target weeds, long-term self-perpetuating control, low cost per acre, searching ability to locate hosts, synchronizing agents to hosts’ life cycles, and the unlikelihood that hosts will develop resistance to agents. Disadvantages of biological control include the limited availability of agents from their native lands, the dependence of control on plant density, the slow rate at which control occurs, biotype matching, the difficulty and expense of conflicts over control of the weed, and host specificity when host populations are low.

Biological control programs require consistent funding, expertise, and interdisciplinary cooperation. Biological control of weeds is not a perfect solution for all noxious weed control programs. It can, however, be integrated as an additional tool with other weed management methods. Interdisciplinary vegetation management teams are working on noxious weed problems to minimize adverse environmental and socioeconomic impacts.

If you are considering importing biocontrol agents from another state, be aware that federal permits are required. To obtain USDA-APHIS-PPQ Form 526, write: USDA-APHIS-PPQ, Biological Assessment and Taxonomic Support, 4700 River Road, Unit 113, Riverdale, MD 20737; online at http://www.aphis.usda.gov/ppq/permits/biological/weedbio.html

We encourage all who implement classical biological weed control to follow the International Code of Best Practices, presented at the 1999 International Symposium on Biological Control of Weeds. Briefly, this includes:

   • Releasing only approved agents

   • Using the most effective agents

   • Documenting releases

   • Monitoring for impacts on the target weed, non-target species, and the environment

 

http://pnwpest.org/pnw/weeds?02W_BIOC02.dat

  

http://pnwpest.org/pnw/weeds?02W_BIOC03.dat

 ERIC M. COOMBS, GARY L. PIPER, MARK SCHWARZLAENDER, JENNIFER ANDREAS, and JOSEPH MILAN