Dr. Michael J. Pitcairn, Dept. of Food and Agriculture, Biological Control Program, 3288 Meadowview Road, Sacramento, CA 95832. Email: mike.pitcairn@cdfa.ca.gov, phone: 916-262-2049.

Dr. Lincoln Smith, Research Entomologist, U.S. Department of Agriculture, Agricultural Research Service, Invasive Species and Pollinator Health Research Unit, 800 Buchanan St., Albany, CA 94710. Email: Link.smith@usda.gov, phone: 510-559-5674.

Classical biological control was first used in California in 1945 to control St. Johnswort, or Klamathweed (Hypericum perforatum) (Winston et al. 2012). Five species of insects were introduced, and within a decade the weed population was reduced by at least 97%. Today there are occasional 'outbreaks' of the weed, but the insects eventually colonize the patches and bring them back under control. A total of 77 agents have been released against 39 weed species in California, and 82% of the insects have established populations (Pitcairn 2018). This section addresses 22 target weeds that are most likely to be affected by existing biological control agents in California. These and additional species have been released in other states (Nechols et al. 1995, Van Driesche 2002, Coombs et al. 2004, Winston et al. 2014).

How it works. "Classical biological control" involves the use of naturally-occurring, host-specific insects, mites or pathogens to help control invasive alien weeds (Smith 2007, Van Driesche et al. 2008, 2010, Smith et al. 2014). Although some insects or mites feed on many types of plants, most herbivorous species specialize on a limited number of plants to which they have adapted over evolutionary time. When a plant arrives in a new location that has a suitable environment and that lacks the herbivores or pathogens that normally attack the plant, it can multiply and become an invasive weed. The strategy of biological control is to find host-specific natural enemies (herbivores or pathogens) of the target weed in its region of origin and to release them in the invaded region to help reduce the weed's impact. Effective agents multiply and disperse, providing a self-sustaining population that helps to reduce the target weed. Often more than one biological control agent is needed before attaining satisfactory control of the target weed. Biological control does not aim to completely eliminate the target weed, and the level of control is likely to vary from year-to-year and among different types of habitats. However, the self-perpetuating reduction of the weed population can be highly cost-effective. For example, a comprehensive review of all programs in Australia, including those that failed to affect the weed target, reported a benefit to cost ratio of 23:1 (Page and Lacey 2006). Benefit ratios of projects around the world range from 8:1 to 7405:1 (van Wilgen et al. 2020). A recent review found that 61% of weeds targeted in North America have been effectively controlled (Schwarzländer et al. 2018).

Regulation. In order to not harm other plants, it is important to thoroughly test the host specificity of an agent before it is released. The USA has a well-established formal process for evaluating and approving new agents for release (Hinz et al. 2019). The Animal and Plant Health Inspection Service (APHIS) is a unit within the United States Department of Agriculture (USDA) that regulates the introduction and movement of organisms into and within the USA. APHIS formed the Technical Advisory Group for Biological Control Agents of Weeds (TAG), a committee that has representatives from all of the federal agencies that have land management responsibilities, as well as from the National Plant Board, the Weed Science Society of America, and the departments of agriculture of Canada and Mexico. TAG reviews the scientific merit of petitions, in which everything that is known about the biology of the target weed and the prospective biological control agent is summarized. Petitions include a list of the species of plants tested, the results of host specificity experiments, and assessments of potential risks to humans and the environment. TAG provides a recommendation to APHIS regarding the merits of the petition. Next, APHIS consults the US Fish and Wildlife Service (USFWS), regarding threatened and endangered species, as required by the Endangered Species Act. APHIS writes a Biological Assessment (BA) that documents possible risks to any threatened or endangered species. The USFWSreviews the BA and issues a Letter of Concurrence if they agree that the proposed action does not threaten any listed species. APHIS also consults with Native American tribes. APHIS then prepares an Environmental Assessment (EA) that documents possible risks to the humans and environment, in compliance with the National Environmental Policy Act, which is published in the Federal Register to allow for a 30-day period to receive public comments. APHIS must respond to any comments received. If APHIS determines that there are no significant risks, it then signs a Finding of No Significant Impact (FONSI) before it can issue any release permits. This process can take several years or more to complete.

For a biological control organism that will be released in California for the first time, the California Department of Food and Agriculture (CDFA) requires a separate state permit after the federal permit is issued. CDFA will perform an independent analysis of the risk of the introduction and issue a state permit. California has many endemic plant species that do not occur in other states, so if any of them are at risk of injury by a prospective agent, the state would probably not issue a permit. Some weed biological control agents that APHIS has permitted for use in the USA are not permitted for release in California. Recently, APHIS has issued a “330 list of exempt organisms” which exempts many biological control organisms from the need for a federal permit to move them from one state to another. This rule is particularly helpful for commercial suppliers of biological control organisms who ship to several states. However, a State of California permit is required before moving one of these organisms into California, unless it has previously been approved by CDFA.

Safety. Because regulatory agencies have a low tolerance of risk to nontarget species, authorized biological control agents are highly specific and in general do not attack any nontarget plants. However, the oldest agents were evaluated under less stringent criteria and some of them feed on a few nontarget species. Such nontarget feeding is most likely to occur when the biological control agent populations are high (which may occur soon before the weed population shrinks) and the nontarget plants are near heavily infested target weeds. Such nontarget damage is usually transitory and ends after the target weed and biological control agent populations decrease. The species of plants most at risk of attack are often those most closely related to the target weed. A review of biological control in the USA reported that 15 (13%) of 112 introduced species of insect biological control agents attacked nontarget plants to some extent (Pemberton 2000). 40 of 41 plant species attacked were in the same or a very closely related genus of the target weed. The one exception was an insect introduced to Hawaii in 1912 that had never been tested. Most of the damage to nontarget species is transitory or inconsequential to their populations (Hinz et al. 2019). A worldwide review concluded that 99% of 512 species of biological control agents caused no significant nontarget attack (Suckling and Sforza 2014).

In the USA, the agent causing the most damage to nontargets is the thistle seed head weevil (Rhinocyllus conicus), which was introduced in 1969, at a time when there was little concern about protecting native plant species. This weevil has been reported to attack at least 22 species of native thistles in the USA, including causing significant damage to a federally threatened plant species (Pemberton 2000, Louda and O’Brien 2002, Hinz et al. 2019). Similarly, the thistle rosette weevil (Trichosirocalus horridus) was introduced in 1974 to control various exotic thistles (Cardueae), but it also attacks some native thistle species (Takahashi et al. 2009, Wiggins et al. 2009). The cactus moth (Cactoblastis cactorum), which very successfully controlled invasive Opuntia cactus in Australia, was released on some Caribbean islands in 1957 (Zimmermann et al. 2000). It appeared on its own in southern Florida in 1989 attacking native Opuntia species and is spreading westward (Hight et al. 2002). Although this insect was never permitted for use in North America, this example shows how a natural enemy can be a safe and effective agent in one region, but a harmful invasive species in another.

It is important to understand the distinction between permitted agents and other adventive or accidentally introduced species that attack an invasive weed. The latter have not been evaluated for host plant specificity and may pose a threat to nontarget plant species. For example, the thistle seed head weevil (Larinuscarlinae[often called L.planus]) has never been permitted in the USA but appeared in Maryland in the 1960s. However, it has a strong affinity with Canada thistle (Cirsium arvense) and was subsequently redistributed in some western states to control Canada thistle (Louda and O’Brien 2002). This weevil is known to attack at least 4 species of native thistles (Cirsium spp.), including a federally-listed threatened species (Havens et al. 2012). Redistribution of this weevil has harmed some native species while having relatively little effect on controlling Canada thistle.

Land manager's role. Classical biological control is generally a "passive" technique of weed management in the sense that it works in the background, but a land manager can use their knowledge of how it works to apply other management strategies that are complementary. State and county personnel have released biological control agents (insects and pathogens) that have been officially authorized by state and federal regulatory authorities. If these agents are already present at your site, then there is probably no benefit to trying to obtain more to release because the number that you would release (dozens) is small compared to what is already in the field (thousands). However, it is useful to know if there are agents at your site and to observe how abundant they are to help you select other management strategies that complement their effect. Steps taken to protect the insects will help increase their impact in successive years. It is important to understand that agents are living organisms, and that their populations will increase from year to year unless something is causing them to die or fail to reproduce. Biological control is generally a slow process, but it can be effective over very wide regions and perpetuates year after year.

Tips for using biological control agents. Obtain insects from CDFA, the local county agricultural commissioner or from field sites that have established agents. Some agents may be available from commercial vendors.

In general, biological control agents will multiply if they have a suitable habitat and are protected from disruptions (e.g. fire, herbicides, mowing). Undisturbed sites are more likely to allow populations to increase year after year.

Species of biological control agents differ in their life cycles and their habitat and environmental preferences. Knowing these details will help land managers to choose which species are most likely to be useful at each site, and to understand where they are likely to establish and have the greatest effect.

After agents become established at a site and their populations increase, they can be collected for redistribution to other sites. This is best done before the weed population decreases because after it decreases, the populations of biological control agents will also decrease.

Knowing that biological control agents are present may help you to integrate other management strategies. For example, it would be better to use a treatment such as herbicides or other non-chemical methods to control the periphery of a weed infestation and at 'high value' sites and let the biological control agents multiply in the center of the weed infestation. Over time, the agents should reduce the weed density to acceptable levels, and/or enable more efficacious use of other treatments such as herbicide, mowing or fire.

Treatments that kill the target weed during the time of year that the biological control agent is on the plant are likely to reduce the agent's population and efficacy. Thus, it is worth considering treating plants before the agent is active, in which case the agent will seek out plants in areas that were not treated (e.g., too difficult to access, or in sensitive areas). Alternatively, treating after the agent is active might be an option, although this time of year is usually less efficacious for herbicides, mowing or fire.

Any treatment regime, whether biological, chemical, or otherwise, should include revegetation with native or other desirable plants to promote competition by desirable plants. Competition stresses weeds and increases the impact of other treatments, including biological control agents.

Caveats. Application of insecticides at or near the site (e.g. to control mosquitoes or crop pests) will likely kill the biological control agents if adults or externally-feeding larvae are active at the time of application.

Classical biological control can help reduce densities of the target weed, but it is not expected to completely eradicate it. Furthermore, the weed population may fluctuate from year to year, and location to location, but persistence of the biological control agents will help to prevent it from increasing to injurious levels.

Permits. A federal (USDA-APHIS) permit is required to introduce an agent from a foreign country or to move it from one state to another. Apply for a federal permit (PPQ form 526) at: www.aphis.usda.gov. A state (CDFA) permit is required for the first time an agent is introduced into California. Agents brought from other states would need a statepermit only if they have not previously been approved by CDFA. The list of approved agents can be found in Pitcairn et al. (2014). Apply for a state permit (Form 66-026) at www.cdfa.ca.gov and email permits@cdfa.ca.gov regarding questions. Once a permitted weed biocontrol agent is established, CDFA generally does not require a permit to collect and move it within California. However, contact CDFA first to verify this for the agent in question. For example, the Dalmatian toadflax stem weevil (Mecinus janthiniformis) can only be released in southern California. Permission of the landowner should be obtained before collecting or releasing agents on their property.

Field methods. A variety of methods can be used to collect insects or pathogens. Choose the methods that are most appropriate for the species of interest. For more details, see the US Forest Service FHTET publication that is listed in the references of the section on the weed target of interest. Specialized equipment can be obtained from entomological, forestry or scientific vendors such as Bioquip.com.

Aspirators are used to collect insects into a vial or tube by suction. They are most effective for insects that are small and not actively trying to fly. The suction can be applied orally or by battery power depending on the model. Oral models should include a filter to protect the user from inhaling dust or insect scales.

Beating involves hitting vegetation with a stick to knock off insects that fall into a sweep net or horizontally held sheet. The desired insects are then collected by aspirator or small container.

Sweep nets are used to collect adults and some external leaf-feeding insect larvae. A sweep net consists of a conical cloth or mesh bag attached by a hoop to a long handle. Sweep the net from side to side hitting the vegetation (target weed) to dislodge insects so that they fall into the net. Roll the handle 180° to momentarily close the bag. Insects tend to move upward and are attracted to light, which can help to separate them from the debris. Collect the desired species using a small container or aspirator. Immediately release other insect species that you do not want because they may include beneficial native species. Note that this method may injure delicate species (moths and flies).

Night collecting sheets consist of a white cloth that is suspended vertically and illuminated with a white or black light to attract flying insects at night. The desired species are collected from the sheet by aspirator or small container.

Transferring infested plants or plant parts. Insects that spend the winter inside seedheads or galls can be transferred by collecting mature seedheads or galls late in the season and putting them at release sites. It may be necessary to protect them from rodents and birds by placing them inside a metal cage. For agents that are known to have parasitoids, it is better to hold the seedheads or galls in a rearing chamber to rear out the adults and release only the correct species and not move the parasitoids to the release site.

Storing, transporting and releasing agents. Most insects tolerate cold (refrigerator) temperatures well, whereas they often die quickly when confined in warm containers. Most insects are inactive at temperatures lower than 50°F, which is helpful for storing and handling them. Avoid overcrowding insects and put cut strips of paper towel or host plant vegetation in the container to provide them places to hide. Allow ventilation and avoid creating condensation droplets inside containers because insects can get trapped and drown in the liquid water droplets. Cardboard 'ice cream' containers (unwaxed) are often used. Hold the insect containers in a refrigerator or a cooler with blue ice until ready for release. Avoid releasing insects at midday during hot weather and place them directly on the target weed to encourage them to settle rather than immediately fly away.

New targets. The development of a new biological control agent is expensive and involves many years of research to discover, identify, test, obtain permits, multiply, and release. Investment to develop biological control differs significantly from that to develop chemical or mechanical methods of control because an individual investor cannot 'monopolize' or capture the profit from their investment. When an agent is released on someone's property, there is nothing to prevent it from eventually dispersing to their neighbors, who would benefit from the initial release. Thus, there is little motivation for an individual or private corporation to invest in classical biological control; however, the high benefit-to-cost ratio of classical biological control makes it a very attractive investment for society as a whole. Projects are usually supported with public funds at the federal, state or county levels. Perhaps the most effective role that weed managers can play is to inform their government representatives of the importance of their weed targets and the need to support research and development of solutions.

Although classical biological control can be highly effective to achieve long-term control of an invasive weed over a wide region, it is not a suitable strategy for all weed targets. Projects involve a large investment and require many years to achieve results, so the target weed should be important enough to justify this. For recently established invasive weeds, it may be more realistic to target them for eradication and containment. Typically, weeds that have become widespread and that cannot be cost-effectively controlled by other methods are targeted for biological control. Thus, biological control is often a tool of 'last resort'. If the target weed has close relatives that are native or cultivated species, then it is less likely that an agent that is sufficiently host-specific can be found. However, modern molecular genetic tools are greatly increasing our ability to understand the genetic variation of both the target plants and the prospective biological control agents, which is improving our ability to discover new prospective agents.

Commercial vendors. We know of only two suppliers of weed biocontrol agents that ship to California:

Integrated Weed Control
4027 Bridger Canyon
Bozeman, MT 59715-8433
email: IWC@integratedweedcontrol.com
Web site: integratedweedcontrol.com
They sell agents for: Canada thistle, knapweeds, purple loosestrife, St. Johnswort

Weed Busters Biocontrol
5607 Hillview Way,
Missoula, MT 59803
(406) 251-4261
email: RobertKandace5@msn.com
Web site: weedbustersbiocontrol.com
They sell agents for: Canada thistle, leafy spurge, spotted and diffuse knapweed

Mention of company names, trades names or specific commercial products is solely for the purpose of providing specific information and does not constitute endorsement by the U.S. Department of Agriculture (USDA) or the California Department of Food and Agriculture. The USDA prohibits discrimination in all its programs and activities on the basis of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or family status. Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice and TDD).


accidental introduction -- species that appear in a new region (e.g. California) that were not authorized by the appropriate regulatory agencies. This applies to many alien weeds, but also to some of their natural enemies. Such species are not authorized for use to control invasive plants because they have not been evaluated for safety and potential risks.

adventive -- unpermitted alien species that arrived by unknown means.

APHIS -- Animal and Plant Health Inspection Service is the branch of the US Department of Agriculture that is responsible for regulating the movement of live organisms from other countries to the USA, and between states within the USA.

BA -- Biological Assessment, a document that assesses the risk of a proposed action on threatened and endangered species. This is in compliance with the Endangered Species Act of 1973.

biological control -- This term is sometimes broadly interpreted to include use of targeted grazing, botanical pesticides and genetic methods; however, in this document we intend for it to refer to use of living organisms (natural enemies) to help control populations of invasive weeds.

biological control agent -- a natural enemy that has been authorized by the appropriate regulatory agencies to be used within a specific political region.

classical biological control -- the use of exotic host-specific natural enemies to help control an invasive species (e.g., an alien weed). The natural enemies are discovered in the region of origin of the weed, tested for safety, permitted, and intentionally released to establish a self-sustaining population to reduce density of the target weed.

CDFA -- California Department of Food and Agriculture

diapause -- a state of arrested development usually during seasonal adverse environmental conditions, such as during summer (aestivation) or winter (hibernation). Diapause can occur in the egg, larval, pupal or adult stage, depending on the insect species.

EA -- Environmental Assessment, a document that assesses the risks of a proposed action on the environment or human beings. This document is prepared and made available to the public in compliance with the National Environmental Policy Act (NEPA) of 1970.

FONSI -- "Finding of No Significant Impact" a document required for a federal agency to comply with the National Environmental Policy Act (NEPA) that concludes that an agent is safe and can be authorized to be released.

natural enemies -- herbivores or pathogens that eat or infect an organism (e.g. a target weed).

notho -- prefix indicating that a taxon is the result of hybridization

parasitoid -- an insect that parasitizes another kind of insect, which usually kills it.

pupa (pl. pupae) -- the developmental stage in which insects transform from larvae to adults.

sp. -- species (singular); ssp (plural)

subsp. – subspecies

SWFL -- southwestern willow flycatcher (Empidonax traillii extimus), a federally endangered bird subspecies.

TAG -- The Technical Advisory Group for Biological Control Agents of Weeds is a committee to provide APHIS scientific review of petitions that evaluate the safety and potential risks of introducing a new biological control agent. It consists of representatives from federal agencies of the USA, Canada and Mexico that have land management responsibilities, the National Plant Board, and the Weed Science Society of America.

taxon (pl. taxa) -- a taxonomic group, such as species, genus or family.

tephritid -- this refers to the fruit fly family Tephritidae in the insect Order Diptera (flies).

USDA -- United States Department of Agriculture

USFWS -- United States Department of Interior’s Fish and Wildlife Service, the agency responsible for protecting federally-listed threatened or endangered species.


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