Classical weed biological control (referred to simply as biocontrol throughout this website) is the intentional importation and use of a weed’s natural enemies to reduce its vigor and reproductive potential. Weed biocontrol is regulated, and only biocontrol agents that have been tested and are host-specific to the target weed will be permitted for use in North America. Biocontrol agents may attack a weed’s flowers, seeds, roots, foliage, and/or stems, reducing the competitive advantage of many weed species and giving the competitive edge back to native or more desirable plants.
Read more about classical weed biocontrol.
In addition to classical (importation) weed biocontrol, cultural, augmentative, and conservation biocontrol are sometimes applied to pest plants, insects, mites, and pathogens. Cultural biocontrol agents are herbivores that feed on pest plants but in a non-selective manner, such as targeted grazing with goats or the use of grass carp to control aquatic weeds. Augmentative biocontrol involves periodic releases of natural enemies (often native species) when too few are present to control a pest effectively. For example, releasing the native milfoil weevil, Euhrychiopsis lecontei, to control the introduced Eurasian watermilfoil in North America. In an agricultural setting, many crop species are attacked by pest mites and insects, which are in turn attacked by native predatory mites and insects. Growers can buy large numbers of the native predatory mites, lacewings, or ladybugs to augment those already on their crops, thus achieving augmentative biocontrol of the pest species. Conservation biocontrol is typically used in a crop setting. In this form of biocontrol, growers manipulate the environment to favor natural enemies. This can include providing alternative hosts or prey, food or nesting sites, or modifying cropping practices.
Advantages of weed biocontrol include
Limitations of classical weed biocontrol include
The effectiveness of weed biocontrol depends on the land manager’s 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. For example, in the 1990s, Dalmatian toadflax was smothering large tracts of land in western North America and was spreading rapidly. Releases of the beetle Mecinus janthiniformis began controlling the majority of Dalmatian toadflax infestations within two decades. This scale of destruction is uncommon for biocontrol, however. In some instances, biocontrol agents have had little to no impact on their target weeds, for a variety of reasons. Most biocontrol programs are considered successful when the target weeds are still present but reduced to the point where the damage they cause is below an acceptable economic or ecological threshold.
You can read more about what to expect with weed biocontrol.
Biocontrol is just one of many weed control options available. Other options include chemical control with pesticides, physical control with tilling, mowing or digging, and cultural control with land management practices. The best weed control method will depend on your infestation, your management goals, and your available resources.
Determining whether or not biocontrol is an appropriate choice for your situation will depend on a number of factors, including the location and traits of the target infestation, the weed being targeted, the biocontrol agents available, your management goals, and what resources you have to devote to your weed management program. Below are questions you should ask before you begin a biocontrol program:
Yes. Biocontrol is just one of many approaches used in the management of weeds. Several other methods and approaches are also available; in many situations, a combination of these provides the best control of target weed infestations. Integrated Pest Management (or IPM for short) is a decision-making process that combines tools and strategies to identify and manage invasive pests in a way that minimizes economic, health, and environmental risks.
Read more about Integrated Pest Management strategies and methods.
Yes. True classical weed biocontrol requires rigorous testing to prove that biocontrol agents are host-specific, meaning they develop only on the target weed.
Read more about the testing and permitting process of biocontrol agents.
Opponents of weed biocontrol 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 Rhinella marina 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 nontarget damage to predators ingesting its toxin. Both the mongoose and the cane toads attacked numerous nontarget 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 weeds 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 biocontrol 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 biocontrol 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 biocontrol 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. Some limited amounts of feeding on species closely related to the target weed may be deemed acceptable, but only if that feeding is proven to have no lasting impact on the nontarget plant population.
It is rare for biocontrol agents to completely kill all their target host plants across a landscape. Sometimes outbreak populations of biocontrol agents occur, and the target plants within a localized area are destroyed. In these instances, the agents in that locality also die out, unless the adults can disperse and find other plants of the same species elsewhere. During the dispersal process, large numbers of the biocontrol agent may be found on unrelated plants surrounding their destroyed weed infestation, and small amounts of feeding may be observed on the unrelated plants. This type of feeding is well-studied and is termed “spillover” feeding. Because the unrelated plants are not suitable hosts for biocontrol agent development, the feeding is only temporary and has no lasting impact on the affected plants. For example, Galerucella beetles are very effective biocontrol agents of purple loosestrife. When large populations of these beetles completely defoliate an entire purple loosestrife infestation, adults have on occasion been observed feeding on surrounding vegetation before they successfully disperse to new purple loosestrife infestations. This feeding has not been shown to cause mortality in any of the temporarily attacked surrounding vegetation.
For an insect biocontrol agent to differentiate its host plant from the other plants in the environment, several things must happen. Typically, the insect will first smell its host plant. Then, as it gets closer, it will see the host plant. After landing on the plant, the insect will “taste” the plant using sensors on its feet. If it determines the plant is suitable, it will take a bite. For the insect to then successfully digest the host plant, it must have all the specialized enzymes necessary to break down the plant cells. For a biocontrol agent to attack and survive on a different plant species, each of these several stages in host selection would have had to undergo mutations, and those mutations would all have had to occur simultaneously.
Mutations occur during reproduction when new embryos and eggs are formed. Existing individuals do not mutate or evolve. Most mutations that occur in nature are harmful, and the organism dies without passing it on to the next generation. In order for mutations to become permanent in a population, an individual with mutations would also have to find and mate with individuals that have similar mutations, otherwise the mutations are lost. Therefore, the chances of a biocontrol agent mutating to attack and survive on another species, and those mutations becoming permanent in the biocontrol agent population, are extremely low.
A brief discussion on vocabulary is necessary in order to understand and answer this question. An exotic species is one that is not native to the area of discussion. Not all exotic species are bad; in fact, many exotic species are beneficial, including the honeybee and several crop species. Invasive species are a special kind of exotic species. Invasive species aggressively establish themselves at the expense of native or more desirable plants or animals or they cause economic difficulties in cultivated landscapes.
Numerous exotic species are introduced to North America every year, sometimes intentionally and sometimes as stowaways or contaminants in the global transport of people, food, and trade. Most exotic species die, but approximately 10% of all exotic introductions successfully establish. Of those which do, only about 10% become invasive.
Classical weed biocontrol agents are rigorously tested for their host-specificity over multiple years to ensure they feed and develop only on their target weed(s). Because they do not feed and develop on other plants, and because their populations will decrease when their target hosts decrease, they do not fit the definition of invasive species. Classical weed biocontrol agent populations will not increase at the expense of native or more desirable species, nor will they cause economic damage in a crop setting.
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.
Visit the Getting Started page for a step-by-step guide.