Frequently Asked Questions

We've assembled some of the most common questions related to biocontrol. If after reading them, you still have questions, we encourage you to talk to your local county weed superintendants. They will have the most up-to-date information on biological control for your specific state and county.

Cite source: J.E. Andreas- Washington State University Extension; R.L. Winston- MIA Consulting

Most invasive plants (weeds) in the USA are not native to North America; they arrived with immigrants, through commerce, or by accident from different parts of the world. These non-native plants are generally introduced without their natural enemies—the insects, mites, nematodes, and pathogens that keep their populations in check in their native ranges. Classical weed biological control (often referred to as biocontrol) is the intentional use of an invasive plant’s natural enemies to reduce its vigor and reproductive potential. 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.
The first intentional use of an insect to control an invasive weed dates back to the mid-1800s in India. Since then, over 500 biological control agents have been introduced against approximately 200 weed species in over 90 countries worldwide.
In the USA, over 200 biocontrol agents have been introduced against 76 weed species since 1902.

In addition to classical weed biological control described above, augmentative and conservation biological control are sometimes applied to pest plants, insects, mites, and pathogens. Augmentative biological control typically involves periodic releases of natural enemies when too few are present to control a pest effectively. This type of biological control is most often used against pest insects and mites.

Biological control agents may attack a weed’s flowers, seeds, roots, foliage, and/or stems. Regardless of the plant part attacked, the aim is always to reduce the growth and population size of the target weed. Effective biocontrol agents seldom kill weeds outright. By reducing the reproductive capability and vigor of their target weed, they compromise the weed’s ability to compete with other plant species.

Biological control is just one of many weed control methods practiced worldwide. More conventional control methods, including chemical applications and physical control (e.g., hand pulling and tillage), have long been used in agricultural settings. These control strategies, however, are often not feasible or economically sustainable for vast tracts of publicly owned lands, difficult-to-access terrain, or areas containing sensitive vegetation. Conventional control methods are also increasingly restricted or prohibited altogether in some areas, e.g., along or close to bodies of water. Finally, some weed species are developing resistance to chemical products currently on the market. In contrast, approved biological control agents attack only the target weed, are self-dispersing (even in rough or remote terrain), provide continuous action without weeds developing resistance, and integrate well with other control methods. Consequently, biological control is an important part of a cost-effective, long-term solution for large-scale weed management.

Like all weed management tools, biocontrol is not a silver bullet and has its limitations.

  • Not a quick fix
  • Predators may limit populations
  • Results often vary
  • Some biocontrol agents not readily available
  • New biocontrol agent approval is slow process

Potential biocontrol agents are vetted at a number of stages in the approval process. The United States Department of Agriculture’s Animal and Plant Health Inspection Service–Plant Protection and Quarantine (USDA-APHIS-PPQ) is the federal regulatory agency responsible for providing testing guidelines and authorizing the importation of biocontrol agents into the USA. The Technical Advisory Group (TAG) for Biological Control Agents of Weeds is an expert committee with representatives from USA federal regulatory, resource management, and environmental protection agencies, and regulatory counterparts from Canada and Mexico. TAG members review all petitions to import new biocontrol agents into the USA and, based on the extensive test results, make recommendations to USDA-APHIS-PPQ regarding the safety and potential impact of prospective biocontrol agents. If TAG approves a candidate, the petition undergoes further review by the US Fish and Wildlife Service (USFWS) and Native American Tribal governing bodies. If both groups accept the petition, the general public is invited to provide input. Only after approval by TAG, USFWS, and Tribal Councils does USDA-APHIS-PPQ officially approve a candidate weed biocontrol agent for release. In addition, some states have their own approval process to permit field release of weed biocontrol agents.

The effectiveness of weed biological control depends on the definition of success, and this varies from place to place, depending on the goals and objectives of the weed control project.
Some biocontrol agent introductions have resulted in spectacular reductions of their target weeds. Most biocontrol programs are considered successful when the target weeds are still present but reduced to the point where the damage they inflict is below an acceptable economic or ecological threshold. It is important to bear in mind that in the weed’s native range, the weed is a fixed component of the plant community. Because the goal of weed biological control is to restore the balance between a weed and its natural enemies, eradication should never be the goal for this form of weed control.

Opponents of weed biological control often cite the same two examples of “biological control gone wrong.” In the 1800s, plantation owners in the Caribbean and Hawaii introduced the Javan mongoose (Herpestes javanicus) to control black rat (Rattus rattus) populations destroying their sugar cane. In 1935, the cane toad Bufo marinus was introduced to Australia to combat damage caused by outbreaks of native sugar cane beetles Dermolepida albohirtum and Lepidiota frenchi. Both introductions had disastrous environmental impacts. Neither species had natural enemies in their introduced ranges that could keep their numbers in check, so their populations exploded. The toxic cane toad has also caused extensive non-target damage to predators ingesting its toxin. Both the mongoose and the cane toads attacked numerous non-target species, wreaking havoc on native bird, reptile, and insect populations. Neither introduction has had significant impacts on sugar cane pest populations because of asynchrony in the daily cycles of the mongoose and cane toad compared to the targeted sugar cane pests. In both cases, no formal host-specificity testing and approval took place, and both introduced species were known to attack multiple additional species prior to their release. These introductions should not be considered examples of classical biological control and would never be approved today.
While the field of classical weed biocontrol has itself made mistakes since its inception, the discipline has changed markedly with time. In the early days of weed biocontrol in North America, the main focus was reducing invasive plants to the benefit of commercial crops and rangeland species. Native species were not recognized as an important priority. Consequently, native species were not always included during host-specificity testing, or sometimes attack to native species was deemed acceptable. For example, during the testing for thistle biological control agents in the 1960s and 70s, many considered native thistles to be a hindrance to the grazing sector, along with exotic thistles. Feeding damage to native thistles was therefore not of concern, and three biological control agents that attacked both native and exotic thistles were approved and released. These agents, Larinus carlinae (formerly L. planus), Rhinocyllus conicus and Trichosirocalus horridus, are no longer approved for redistribution in the USA.
In modern times, perceptions have changed, and native species are tested extensively during feeding and development trials. Any biological control agents approved for release in North America in the modern era must be proven to be extremely host-specific. These agents have co-evolved with their target weed host, feeding and developing only on their target host; they are not adapted to survive on other plant species. During extensive host-specificity testing, if potential biocontrol agents are found to attack any crop or protected plant species, they are rejected and not released.

Most weed biocontrol agents go unnoticed by the majority of people in North America. They feed on their target plants for all or just a part of the growing season and often overwinter unseen in the soil or soil litter. Because of their proven host specificity, they are unlikely to be observed away from their host plant, and do not seek out people, pets, or homes.


Schwarzländer, M., H.L. Hinz, R.L. Winston RL, and M.D. Day. 2018. Biological control of weeds: an analysis of introductions, rates of establishment and estimates of success, worldwide. BioControl 63: 319–331.

Whaley, D.K., and J.E. Andreas. 2015. Collecting and Releasing Biological Weed Control Agents in Washington State. Washington State University Extension Publication FS177E.

Winston R.L., M. Schwarzländer, H.L. Hinz, M.D. Day, M.J.W. Cock, and M.H. Julien (Eds). 2019. Biological Control of Weeds: A World Catalogue of Agents and Their Target Weeds. Based on FHTET-2014-04, USDA Forest Service, Forest Health Technology Enterprise Team. Available online at (accessed 31 October 2019).