Managing Invasive Plants: Concepts, Principles, and Practices link

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MANAGING INVASIVE PLANTS: Concepts, Principles, and Practices

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Management Methods: Biological Control

Biocontrol In Practice

The practice of biocontrol is regulated and guided by federal and state laws, an International Code of Best Practices, and specific protocols that are all designed to ensure the safety and effectiveness of biocontrol programs. Although some argue that safety is almost entirely a matter of selecting host-specific agents (McClay 1999), the success or failure of a biocontrol project or program is a function of a number of biotic (living), abiotic (nonliving), and procedural factors. Successful biocontrol efforts require a commitment to thorough assessment of these factors on a case-by-case basis as well as compliance with the International Code of Best Practices for biocontrol.


United States Importation

Agents selected for biocontrol of invasive plants are imported into the United States after rigorous testing for host-specificity to ensure that the potential biocontrol organism attacks only the target invasive plant. Federal and state governments provide rigid guidelines for testing, importation, and quarantine of selected natural enemies, and biological data are required by state and federal agencies for agents to be released from quarantine.

Foreign exploration, quarantine, rearing, release, and host specificity testing all follow a specific set of guidelines and protocols, established and monitored by the Technical Advisory Group on the Introduction of Biological Control Agents of Weeds (TAG) of the USDA Animal and Plant Health Inspection Service (APHIS). TAG members review petitions for candidate biocontrol agents and provide information and advice to researchers and those in APHIS responsible for issuing permits for importation, testing, and field release of biocontrol agents.

Read more about TAG and the permitting process:

Interstate Movements

Movement of exotic biocontrol organisms across state lines must be authorized by a USDA-APHIS-PPQ permit. A supplemental Environmental Assessment (EA) and Endangered Species Act (ESA) Section 7 consultation must be conducted before releases into additional states can be approved.

Release in National Wildlife Refuges

Laws, regulations, and policy govern the release of USDA permitted biological control organisms on national wildlife refuges. The USFWS Biological Integrity, Diversity, and Environmental Health policy 601 FW 3 states that we do not introduce species on refuges outside their historic range or introduce species, unless such introduction is essential for the control of an invasive species and prescribed in an integrated pest management plan.

Refuge staff who propose to release a biological control agent to manage an invasive species on-refuge complete the following:

  • Endangered Species Act intra-Service section 7 analysis for federally listed, proposed, and candidate species or designated critical habitat that may occur in the action area or be impacted by the release of the biocontrol agent.
  • NEPA compliance documentation. If there is a USDA completed National Environmental Policy Act (NEPA) environmental analysis (EA) and the analysis of impacts is applicable and adequately evaluates any potential impacts to refuge natural resources, then that maybe adopted by reference with a FONSI statement. If a NEPA environmental analysis has not been conducted or is not applicable to the refuge natural resources, then a NEPA analysis and compliance documentation is completed.
  • NOTE: If the release is proposed to be completed by an agency or organization other than the USFWS, the action is reviewed in accordance with the USFWS's Appropriate Refuge Uses (603 FW 1) and Compatibility (603 FW 2) policies.

Refuge managers are responsible for documenting and reporting information about the release of exotic species to the regional office, and maintaining a current list of state and local regulations governing exotic species.

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During the past decades, more land management agencies and other interested parties have incorporated biocontrol as part of their integrated pest management programs for invasive plants. Training, experience, and tolerance for risk vary greatly among practitioners around the world that are now involved in various aspects of biocontrol.

Guidance provided by the TAG committee and federal and state statutory regulations provide some oversight and guidance to invasive plant biocontrol practitioners conducting initial biocontrol agent release activities. However, redistribution and other post-release aspects of biocontrol are less regulated.

The International Code of Best Practices (Balciunas 2000) was designed to provide practitioners with “internationally agreed-upon guidelines to help select the actions that reduce risk and enhance the positive impact of biocontrol” (Balciunas and Coombs 2004). In addition to addressing traditional concerns for host-specificity, these guidelines emphasize predicting the effectiveness of agents, monitoring the consequences of releases (including redistribution releases), and forming partnerships to ensure adequate monitoring. Adherence to the Code will help ensure that classical biocontrol remains a viable and desirable option for managing invasive plants.

Twelve Guidelines for the International Code of Best Practices for Classical Biocontrol
  1. Ensure the target weed’s potential impact justifies the risk of releasing nonendemic agents
  2. Obtain multi-agency approval for the target weed
  3. Select agents with the potential to control the target weed
  4. Release safe and approved agents
  5. Ensure only the intended agent is released
  6. Use appropriate protocols for release and documentation
  7. Monitor impact on the target plant species
  8. Stop releases of ineffective agents, or when control is achieved
  9. Monitor impacts on potential nontarget species
  10. Encourage assessment of changes in plant and animal communities
  11. Monitor interactions among agents
  12. Communicate the results to the public

For an expanded discussion of the Code, see Balciunas and Coombs 2004.

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Successful establishment of biocontrol agents is a function of a number of abiotic, biotic, and procedural factors. Identifying and avoiding common mistakes will help to improve the effectiveness of biocontrol.

Table on factors affecting biocontrol success
Diagram content adapted from Coombs et al. 1999

Understanding these factors can help practitioners

  • select appropriate release sites
  • select safe and effective biocontrol agents
  • obtain and properly handle biocontrol agents
  • determine the optimal size and number of releases
  • determine the best time of year or time of day to release agents
  • consider and address potential conflicts with land use or other management activities

Specific procedures involved in developing, implementing, and managing biocontrol projects can vary greatly between target host plants, biocontrol agents, and managed sites. For instance, biocontrol programs for invasive plants require different release procedures in an aquatic habitat versus a terrestrial habitat.

Detailed procedural manuals have been developed for biocontrol of some target invasive plant species (i.e., VanDreische et al. 2002, Wilson et al. 2005, Wilson and Randall 2001, Bourchier et al. 2005, Wilson et al. 2004, Wilson et al. 1998). These guides provide information about the biology and ecology of biocontrol agents and the target host plant, as well as guidelines and considerations for planning, implementing, and monitoring a biocontrol program.

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Photo of man walking through a field of knapweed (left) and a man wearing floats on his feet to walk across mats of alligator weed (right).
Biocontrol practitioners use the same sweep net method to collect and monitor insects released to control knapweed on rangeland (left) and alligator weed in an aquatic habitat. Photo credits: K Loeffelman/Univ. Idaho (left); USDA ARS,

The International Code of Best Practices emphasizes the importance of documenting and monitoring biocontrol projects. Standardized protocols provide a consistent framework by which the effectiveness and impacts of biocontrol programs can be monitored over the long term and across different regions. Maintaining good records during pre-release, release, and post-release activities can provide important insight into why some biocontrol efforts succeed while others fail, and can help improve the scientific basis of biocontrol (Blossey and Skinner 2000).

Post-release monitoring should be designed to detect

  • establishment of the biocontrol agent
  • intensity of biocontrol agent attack on the target plants
  • effect of attack on target plants at the individual and population levels
  • effect of biocontrol releases on nontarget flora and fauna
  • interactions between the biocontrol agent and the new environment

Biocontrol monitoring activities involve sampling insect populations as well as target plant populations and the plant community. Various biocontrol agents and target plants will require different monitoring protocols. Monitoring protocols have been published for a few invasive plant species (i.e., VanDreische et al. 2002, Wilson et al. 2005, Wilson and Randall 2001, Bourchier et al. 2005, Wilson et al. 2004, Wilson et al. 1998). Blossey (2004) provides a number of general guidelines that are applicable to any biocontrol program.

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Invasive Plant Management Options

Biological control can be used to implement a number of invasive plant options.


Since the target host plant must be present in order for biocontrol agents to establish and survive, biocontrol is not a logical choice for prevention strategies.


Biocontrol is truly an invasive plant suppression tool. The slow and steady actions of biocontrol agents gradually weaken target plants and impair their ability to compete within the plant community. Biocontrol can be an excellent option for well-established, widespread infestations that occur in areas where rapid removal of invasive species is not necessary and some level of infestation is acceptable. Biocontrol may be best used in dense or large core infestations where large-scale or repeated herbicide applications are not feasible.


Some biocontrol agents can be effective in gradually suppressing seed production and vegetative reproduction in some target plant species. By reducing or eliminating the target population’s capacity to reproduce and spread, biocontrol may help support other more fast-acting methods when containment is the desired outcome. However, because the effects of biocontrol are very slow to appear, containment may not be realistic when using biocontrol methods alone.


Newly invading plant species are often candidates for early detection/rapid response (EDRR) programs that require eliminating populations in a short period of time. Newly invading species often do not occur at adequate densities to support biocontrol agents. Because biocontrol agents will not eliminate their host, biocontrol is not an appropriate control method for areas where all target invasive plants must be eradicated.

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Integrated Methods

When used alone, biocontrol can effectively suppress its target host plant population. However, the results may not become apparent for a number of years, and other nontarget invasive plant species will not be controlled. For this reason, biocontrol is often applied as part of an integrated strategy that incorporates a number of invasive plant control methods.

Every effort should be made to conserve biological control agents once they are established in an area. Biocontrol agents require living target plants on which to develop and reproduce. Management methods that hinder normal plant growth and development may not be compatible with biocontrol. Disruptive practices such as improperly timed herbicide application, grazing, mowing, or burning should be avoided. Timing application of management methods relative to biocontrol agent life cycles is a fundamental consideration and should not impede biocontrol agent development or reproduction.

Biocontrol and Physical Methods iconBiological Control and Physical Methods

Few studies have examined the effects of mechanical control on insects and fungi. Methods such as tilling, pulling, and digging may disturb the soil and remove or destroy host plants. Soil-disturbing methods are likely to have a major impact on organisms that live in the soil (Hatcher and Melander 2003). Methods that completely remove host plants may not be desirable because they remove food and habitat for biocontrol agents, and may remove the biocontrol agents themselves.

Physical methods such as mowing and cutting should be properly timed to avoid disturbing or destroying biocontrol agents in the aboveground portions of the plant. Properly timed, these methods may enhance the effects of biocontrol agents by providing new growth for biocontrol agents to feed upon, and by further stressing plants that expend energy to regrow. In studies by Wiggers et al. (2004), biocontrol agents significantly suppressed regrowth from pruned stumps of the invasive tree melaleuca (Melaleuca quinquenervia). The biocontrol agents were so effective in this case that the researchers suggest biocontrol agents may provide a satisfactory alternative to herbicides for suppression of regrowth from cut melaleuca stumps.

Biocontrol and Chemical Methods iconBiological Control and Chemical Methods

The effects of combining herbicides with biocontrol methods have been examined by a number of authors (Lym et al. 1996, Nelson et al. 1998, Enloe and DiTomaso 1999, Lee 1986). Herbicides should only be used when and where they will not impede development of the biocontrol agent (Messersmith and Adkins 1995). Studies by Center et al. (1999) show that the integration of herbicides and Neochetina spp. to control water hyacinth (Eichhornia spp.) could work if sufficient non-sprayed areas are left for weevils to over-winter and maintain sustainable populations.

In some instances, low rates of herbicides can be used to stress target plants, making them more susceptible to the effects of biocontrol (i.e., Brown et al. 1999). Herbicide and biocontrol can be applied separately to achieve different management objectives in different areas of the same management site. For example, the leafy spurge (Euphorbia esula) gall midge (Spurgia esulae) was released to reduce seed production in wooded areas while herbicides were applied to prevent further spread beyond the treeline (Lym and Carlson 1994).

Standard herbicide application can also be used directly with some biocontrol agents. For example, incorporation of Aphthona spp. with herbicides has resulted in more rapid and complete leafy spurge control than either method used alone (Lym and Nelson 2002). In some cases, herbicides may elevate the nutritional value of host plants for the feeding biocontrol agents (Center et al. 1999).

Biocontrol and Grazing Methods iconBiological Control and Prescribed Grazing

Positive and negative interactions between grazing and biological control agents have been documented. In Montana, three years of sheep grazing and flea beetles (Aphthona nigriscutis) reduced densities of leafy spurge more than either sheep grazing or flea beetles alone (Hansen 1993). However, in Colorado, goat grazing resulted in fewer Dalmatian toadflax (Linaria dalmatica) biological control insects (Brachypterolus pulicarius) per flowering plant compared to ungrazed plots (Norton and Hufbauer, unpublished data).

Biocontrol and Prescribed Fire iconBiological Control and Prescribed Burning

Prescribed burning, if improperly timed, can destroy biocontrol agents overwintering within stems or seed heads but might have little effect on root-infesting natural enemies.

Prescribed fires can aid establishment of biocontrol agents in marginally suitable environments, but timing of the fire must be coordinated to the insect's life cycle to ensure survival (Lym 2005). If applied when insects are not active, burning treatments may enhance establishment of biocontrol populations. Fellows and Newton (1999) attributed the increase in leafy spurge (Euphorbia esula) flea beetle populations (Aphthona nigriscutis) to increased insect colonization in bareground of burned plots.

Burning may cause initial decline in biocontrol populations. Briese (1996) observed a decline in Chrysolina quadrigemina populations following a burn. Biocontrol populations rapidly recolonized the burned area from adjacent St. Johnswort infestations and became well established, perhaps benefiting from increased nitrogen uptake by recovering St. Johnswort plants.

Biocontrol and Restoration/Revegetation iconBiological Control and Restoration/Revegetation

It is important to consider the availability of desirable plant species at the site where biocontrol is applied. Lack of desirable plants available in the plant community can result in the "biocontrol treadmill" or "ecological replacement" phenomena discussed previously.

If revegetation is necessary, care should be taken to avoid disturbing or damaging established biocontrol populations. In some cases, integration of biocontrol agents with revegetation programs may require the agent to be the last method introduced because the cultivation and herbicide treatments necessary to establish desirable grasses and forbs may be destructive to insects (Lym 2005, Enloe and DiTomaso 1999).

October 7, 2019