Biological Control

Jennifer E. Andreas, Eric M. Coombs, Joseph Milan, and Mark Schwarzländer
Revised March 2015

Classical biological weed control involves the introduction and management of selected natural enemies to reduce and suppress problematic noxious weeds. The majority of the Pacific Northwest’s weeds originated from other continents. These newly introduced plants, free from the natural enemies found in their native range, gained a competitive advantage over native plants. Once these populations become unmanageable, other methods of weed control are not always economical or physically possible. The need for a method of weed reduction that is inexpensive, self-sustaining, and environmentally safe provides opportunities for biological control.

Once noxious weed infestations become widespread, 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 a success rate of only about one in three. In the Pacific Northwest, BCAs have shown well-documented success in the control of Mediterranean sage, St. Johnswort (Klamath weed), tansy ragwort, Dalmatian toadflax, and purple loosestrife. Preliminary information indicates emerging successes for diffuse knapweed using Larinus minutus and leafy spurge using Aphthona spp.

Biological control is a slow process, and its efficacy is highly variable. Many BCAs have not been present long enough to determine their ability to control their host weed, as it can take several years for their densities to increase and begin impacting weed populations. Only after monitoring weed populations over time will the impact of newly introduced agents become evident.

Biological control agents impact weeds directly and indirectly. They directly impact weeds by destroying vital plant tissues and functions. They cause indirect impact by increasing stress, which may reduce their ability to compete with desirable plants.

Biological control can be integrated with other management practices to reduce weed populations. For example, once weeds are weakened by BCAs, competitive plantings may be used to out-compete the weeds. In addition, satellite weed populations can be controlled by chemical or physical means to reduce weed spread while BCAs attack the primary infestation.

Biological control is not a panacea; it will not eradicate noxious weeds. When using BCAs, expect residual levels of the weed populations, since an agent’s survival depends on the density of its host weed. 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 a search for additional BCAs must be pursued.

The BCAs released in the Pacific Northwest have been tested to ensure they are host specific (i.e., they will only feed on the intended weed species). Testing 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 potential BCAs that could be effective in reducing weed populations may not be approved 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 move to crops, native flora, or endangered plant species.

Biological control of certain weeds may not work in your area, even if it does in others. Biological control agents require specific conditions to survive. Climate variations, such as cold winters and plant biotype differences, may account for some past failures. To ensure maximum success, trained personnel should supervise and monitor biological control programs. To monitor biological control agents, using the Standardized Impact Monitoring Protocol (SIMP) is recommended. More information on SIMP can be found by accessing the BLM biological control website ( ), or by searching online for “BLM biological control.”

Once release sites for BCAs have been selected, protecting those areas is essential. BCA releases are often made with a small number of organisms. Establishing collectable populations can take three to five years. After BCA populations have sufficiently increased at nursery sites, they may then be introduced into new areas. Care should be taken to avoid moving diseased or parasitized immature stages of BCAs to new areas; moving them will reduce their efficacy. The current status and types of BCAs used in the Pacific Northwest are found in the tables in this section. Reintroducing the agents may be required in areas where control was achieved but the host plants have made a resurgence, after severe weather or following wildfires, for example.

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; host-finding capabilities; 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 and uncertainty of the level of control; biotype matching; 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 used as an additional tool in combination with other weed management methods. Interdisciplinary vegetation management teams are working on noxious weed problems to minimize adverse environmental and socioeconomic impacts.

The following tables contain lists of target weeds, and the status of selected biological control agents by state.

If you are considering importing biocontrol agents from another state, be aware that federal permits are required. To obtain a USDA-APHIS-PPQ Form 526, write: USDA-APHIS-PPQ, Biological Assessment and Taxonomic Support, 4700 River Road, Unit 113, Riverdale, MD 20737; or visit

Anyone who implements classical biological weed control is encouraged 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, nontarget species, and the environment

For more information about monitoring biocontrol agents, contact your state department of agriculture or land-grant university.

The use of unapproved natural enemies [that is, accidentally introduced species such as Larinus carlinae (formerly L. planus), thistle seedhead weevil, Chaetorellia succinea, yellow starthistle seedhead fly, Coniatus splendidulus, saltcedar weevil, and Cassida rubiginosa, thistle tortoise beetle] as biological control agents is not recommended because of their potential negative nontarget impacts. In addition, the thistle seedhead weevil, Rhinocyllus conicus, and the root/crown weevil, Trichosirocalus horridus, are no longer approved for interstate shipment because of concerns over potential impacts to native thistle species. As of 2009, the USDA-APHIS suspended all permits for the interstate movement of the saltcedar beetle Diorhabda carinulata (formerly D. elongata). Recently, the accidentally introduced houndstongue root weevil, Mogulones crucifer (formerly M. cruciger), was listed as a pest by USDA-APHIS-PPQ. Movement of this insect may result in severe penalties, as stated in the PPQ Pest Alert; “Unauthorized collection, transportation, and release of Mogulones crucifer as a biological control agent against houndstongue in the United States is a criminal violation of the Endangered Species Act, with penalties for punishable offenses ranging from a maximum fine of up to $50,000 or imprisonment for 1 year, or both, and civil penalties of up to $25,000 per violation. These actions are also a violation of the Plant Protection Act, with fines of up to $250,000 per count for noncompliance, or up to $1 million per adjudication if the violation is willful.”