Biological Control

Jennifer E. Andreas, Joseph Milan, Carol Randall, and Joel Price
March 2022

Classical biological weed control involves the introduction and management of selected host-specific natural enemies to reduce and suppress problematic non-native weeds. The majority of the Pacific Northwest’s weeds are exotic, originating from other continents. These newly introduced plants, freed from the natural enemies found in their native range, gain a competitive advantage over native plants which allows the introduced plants to become weedy. Once weed populations become well distributed, herbicides, cultural, and mechanical methods of weed control are not always economical or physically able to meet weed management objectives. Classical weed biological control (biocontrol) is a weed management tool that is inexpensive, self-sustaining, and host-specific and may help reduce the negative impact of weeds, particularly where other control methods are not feasible.

Biocontrol differs from other weed control methods because it does not result in immediate weed population reductions. Successful biological control reduces the competitive ability of target weeds, allowing other plants to successfully compete with and suppress target weed populations through time. Unlike other weed control options, biocontrol rarely causes substantial weed population reductions in the initial year of release; however, successful biocontrol can result in significant, sustained weed reductions over a period of years. In the Pacific Northwest, biocontrol has been successful in controlling Mediterranean sage, St. Johnswort (Klamath weed), tansy ragwort, Dalmatian and yellow toadflax, purple loosestrife, diffuse knapweed, and leafy spurge.

Biocontrol is a slow process, and its efficacy is highly variable. Since it can take several years for biological control agent (BCA) populations to build to levels capable of weed suppression, many BCAs have not been present or studied long enough in the Pacific Northwest to determine their ability to control their host weed. Only after monitoring target weed and BCA populations over time will the impact of newly introduced BCAs become evident.

BCAs impact their target weeds directly and indirectly. They directly impact weeds by destroying plant tissues and interfering with plant functions. BCAs cause indirect impact by increasing stress on the weed, which may reduce its ability to compete with desirable plants.

Biocontrol can be integrated with other management practices to reduce target weed populations. For example, once weeds are weakened by BCAs, competitive plantings can reduce the availability of light, water, and nutrients to already stressed weed populations. In addition, satellite weed populations can be controlled by herbicides or physical means to reduce weed spread while BCAs attack the primary infestation.

Similar to other weed control methods, biocontrol is not a silver bullet; it will not eradicate all of the target weed since a BCA’s survival depends on the presence of its host plant. Therefore, when using BCAs, expect weed populations to persist, but at much lower levels. After target weed populations decrease, populations of BCAs will decrease correspondingly. This is a natural cycle. A resurgence of weed populations may occur due to seed reserves in the soil, host weed plants missed by the BCA, and lagging populations of BCAs. In areas where the BCAs do not provide sufficient control, other weed control methods may need to be integrated to achieve desired results, or a search for additional BCAs may 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 host weed species) and they are regulated by the Animal and Plant Health Inspection Service (APHIS) to ensure that they will not become pests. Host-specificity testing is an expensive and time-consuming task that must be done before a BCA is allowed to be introduced into the United States. Some candidate BCAs, which could be effective in reducing target weed populations, may not be approved for introduction because they are not host-specific enough. A candidate BCA that is not sufficiently host-specific will be rejected for release in the United States to ensure that the BCA, in the absence of its host weed, will not move to crops, native flora, or endangered plant species.

Biocontrol of certain weeds may not work in your area, even if it does elsewhere. BCAs require specific conditions to survive and thrive. Climate variations (e.g. cold winters), plant biotype differences, and nearby insecticidal treatments (e.g. mosquito abatement or agricultural pest insect control) may account for some past biocontrol failures. To maximize success, trained personnel should supervise and monitor biocontrol efforts. To monitor BCAs, the Standardized Impact Monitoring Protocol (SIMP) is recommended. More information on SIMP can be found by accessing the ISDA/BLM biocontrol website (

Once release sites for BCAs have been selected, protecting those areas from disturbance so BCAs can get established is essential. Initial BCA releases often consist of a small number of individual BCAs. Establishing collectable populations of the BCA for redistribution to other weed infestations can take three to five years and is generally the first goal of a new biocontrol program. Care should be taken to ensure that BCAs being collected for redistribution are not diseased or parasitized, as this will reduce their efficacy in the new area. The current status and types of BCAs used in the Pacific Northwest are found in the tables in this section. It may be necessary to release BCAs over successive years to ensure that the BCAs become established and reach the population levels needed to result in target weed suppression. In addition, BCAs will likely need to be reestablished in areas where disturbances, like wildfire or flooding, may have killed previously established populations. Biocontrol has many advantages but also some disadvantages. Advantages include: reducing herbicide residues in the environment; specificity on target weeds; use in areas with limited accessibility; long-term, self-perpetuating control; low cost per acre; host-finding capabilities; synchronizing BCAs to hosts weeds’ life cycles; and the unlikelihood that host weeds will develop resistance to BCAs. Disadvantages of biocontrol include: the limited availability of BCAs 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 or climate matching; and host-specificity when host populations are low.

Biocontrol programs require consistent funding, expertise, and interdisciplinary cooperation. Biocontrol of weeds is not a perfect solution for all weed management programs. It can, however, be effective as an additional tool in combination with other weed management methods. Interdisciplinary vegetation management teams are working strategically to find the best single or combination of weed control tools to minimize adverse environmental and socioeconomic impacts.

If you are considering importing BCAs from another state, be aware that federal interstate transport permits may be required. For more information, contact your nearest USDA APHIS PPQ office, or visit or your state weed biocontrol specialist. Permits for approved BCAs are free but take time to review and issue so it is best to apply well before your anticipated ship date.

Anyone who implements classical biological weed control is encouraged to follow the International Code of Best Practices for Classical Biological Control of Weeds, adopted by the delegates at the 1999 International Symposium on Biological Control of Weeds. The twelve codes for safe and effective biocontrol practices can be found at Briefly, the key guidelines for biocontrol implementation includes:

  • Releasing only safe and approved BCAs.
  • Using the most effective BCAs.
  • Documenting releases. State agencies can provide release forms and appreciate receiving release information for their records.
  • Releasing only the intended target BCA by thoroughly removing all other insects and plants from collecting material.
  • Stopping releases of ineffective BCA, or when control is achieved.
  • Monitoring for impacts on the host weed, non-target species, and the environment.

As highlighted in the code of best practices, do not use non-APHIS approved natural enemies as BCAs because unapproved natural enemies have the potential to cause negative nontarget impacts. Examples include accidentally introduced species such as Larinus carlinae (formerly L. planus, Canada thistle weevil), Chaetorellia succinea (yellow starthistle seedhead fly), Coniatus splendidulus (splendid tamarisk weevil), and Cassida rubiginosa (thistle tortoise beetle). In addition, BCAs against weedy thistle species (Rhinocyllus conicus and Trichosirocalus horridus) are no longer approved for interstate shipment because of concerns over potential impacts to native thistles. As of 2009, USDA APHIS suspended all permits for the interstate movement of the saltcedar beetle, Diorhabda carinulata (formerly D. elongata). The accidentally introduced houndstongue root weevil, Mogulones crucifer (formerly M. cruciger), is listed as a pest by USDA APHIS PPQ. Movement of M. crucifer may result in severe penalties, as stated in the PPQ Pest Alert: “Unauthorized collection, transportation, and release of Mogulones crucifer as a BCA 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.”

The following tables contain lists of target weeds, and the current status of selected BCAs by state. For more information about accessing, using, and monitoring BCAs, contact your state department of agriculture or land-grant university.