Biocontrol: Protection on a Large Scale, for the Long Run
No stranger to Hawaii, biological control complements other techniques and offers a safe and effective means for managing invasive species.
For years, land managers have applied herbicides and have manually cut strawberry guava stems to control the plant's growth and spread. But only relatively small patches of forest can be managed in this way--chemical and physical management becomes extremely expensive and difficult over vast landscapes and over the long term.
Because chemical and physical control has been impossible to apply on a large scale, and strawberry guava continues its spread into new native landscapes, land managers have turned to biological control.
What Biocontrol Is--and Isn't
Biological control, or "biocontrol," is a management approach that involves the introduction of a plant-feeding insect or disease to control the growth or spread of an invasive plant. It is based on the principle that specialized insects or diseases that keep a plant species in check in its native range can be used to help restore balance in places where the plant has become invasive.
Biocontrol agents are natural predators of the plant in its home range and feed on or damage a part of the plant, making it easier to manage. In some cases biocontrol can reduce the abundance of invasive species gradually, but it never eliminates them completely.
Biocontrol is commonly used around the world by land managers, regional and national governments, and others charged with controlling highly invasive plants. It is an accepted and valued management practice in over 100 countries, including the United States. In Hawaii, biocontrol has been used for more than 100 years, resulting in the successful control of invasive plants such as prickly pear cactus, lantana, pamakani, emex, Klamath weed, and ivy gourd.
Advantages of Biocontrol
Biocontrol is used when other management techniques prove ineffective or prohibitively expensive. It has the ability to:
- Control invasive species across vast, remote, or inaccessible landscapes
- Manage invasive species over the long term without repeated applications
- Reduce the need for herbicides and other chemicals.
Biocontrol also has economic benefits. After an initial cost for study of an agent and its release, biocontrol agents operate continuously with little need for additional labor or other inputs.
Safety of Biocontrol
There is a natural tendency to confuse disastrous introductions of the past with contemporary biocontrol efforts. The introduction of mongooses to Hawaii in the 1800s by a private landowner, for example, is not at all representative of the modern practice of biocontrol--it occurred without the extensive scientific and regulatory reviews that are central to today's biocontrol programs.
For an organism to even be considered a potential biocontrol agent today, it must first be proven to be highly host-specific, meaning it affects only a single host species or a very narrow range of species. For this reason, an animal like the mongoose, which preys on a wide variety of species, would never be used as a biocontrol agent.
Before a proposed agent is considered for release, it is subjected to rigorous laboratory and field studies in both its native range and the area in which it is invasive. These studies establish the agent's suitability for biocontrol by demonstrating that it is effective against the target invasive and poses no threat to other plants in the area.
Strawberry Guava Biocontrol Plan
In 2005--following 15 years of research in Hawaii and Brazil--researchers with the Institute of Pacific Islands Forestry submitted a proposal to release a scale insect in Hawaii to control the growth and spread of strawberry guava.
The insect, a Brazilian scale (Tectococcus ovatus), was selected as a candidate after many years of research showed that it could safely and effectively control the invasive tree in Hawaii without posing a threat to other plant species. Studies demonstrated that the scale is adapted to feed only on strawberry guava, ultimately causing reduced growth and seed production.
The Brazilian scale is a member of a group of insects known as "gall formers." Their young settle and feed on newly sprouted strawberry guava leaves and cause them to form growths, known as "galls." Because the plant expends energy on forming the galls, fewer resources are available for producing new stems and fruit. As a natural predator of strawberry guava, the Brazilian scale can slow the tree's growth and spread, making it less competitive against native plants. The scale is not capable of killing strawberry guava trees, only acting as a check on their growth and spread.
The young scale insects can move about by crawling or by floating with wind currents and, if released, would gradually spread over a period of decades. At the points of introduction of the scale, reduced strawberry guava growth and seed production should be noticeable within a few years. Because scale insects are entirely dependent on their plant hosts, if strawberry guava populations were to gradually decline, so, too, would the scale population.
Environmental Assessments of the proposed strawberry guava biocontrol, [pdf, 19.4 MB], were completed by the USDA Animal and Plant Health Inspection Service in 2008 and the Hawaii Department of Agriculture in 2011. Following scientific reviews and public consultations statewide, release of the biocontrol was permitted. As the State of Hawaii and other partners work to establish the insect on strawberry guava in invaded forests over the next several years, Forest Service researchers will continue to provide technical assistance and monitor the impacts of biocontrol.
Frequently Asked Questions (FAQs)
Q. What is an "invasive species"?
A. "Invasive species" is a globally used term to describe plants, animals, and other organisms that have been introduced into a new environment and that are causing, or are likely to cause, economic or environmental harm or harm to human health. Not all introduced species are considered invasive.
Q. Why do some plants become invasive?
A. Ecosystems in Hawaii and elsewhere developed over millions of years in relative isolation. Early human settlers introduced plants and animals into new areas to meet their needs, but these introductions were very few compared with today. Modern transportation methods and increased global commerce have facilitated movement of vast numbers of species into new places, either intentionally or accidentally. In the new range, an introduced plant may be free from the insects and diseases in its home range, and sometimes can grow out of control. This can give the invasive plant an advantage over native plants, which still must contend with their own predators and diseases.
Q. How do invasive plants affect humans?
A. Humans depend on the functioning of ecosystems for many of our basic survival needs. Invasive plants can strongly affect how those ecosystems function. For instance, invasive plants can replace native vegetation in some areas, causing more frequent fires that are a threat to lives and property. In other places, invasives can change the processing of water through a watershed or the amount and type of nutrients in soil. Invasive species are extremely costly to agriculture around the world and are a key threat to healthy ecosystems that support rare and endangered species.
Q. What is biological control of invasive plants?
A. Biological control, or biocontrol, is the introduction of a plant-feeding insect or disease to control the growth or spread of an invasive plant. These biocontrol agents are natural enemies of the plant in its home range and feed on or damage a part of the plant, making it easier to manage. Biocontrol can reduce the abundance of an invasive species gradually but cannot eliminate it completely.
Q. Is biological control commonly used to control invasive plants?
A. Yes. Biological control is an approach extensively employed around the world by land managers, regional and national governments, and others charged with controlling highly invasive plants. It is an accepted management practice in over 100 countries, including the United States. In Hawaii, biocontrol has been used for over 100 years, resulting in successful control of invasive plants such as prickly pear cactus, lantana, pamakani, emex, Klamath weed, and ivy gourd.
Q. Why use biocontrol when other methods to control invasive plants are available?
A. Biocontrol offers important advantages as a management tool. It is relatively inexpensive compared with conventional methods such as hand cutting, especially for invasions that extend over large areas. It offers an ecological advantage because it can reduce the need to use herbicide. Most importantly, biocontrol is sustainable over the long term. After an initial cost for study and release, biocontrol agents operate continuously with little need for additional labor or other inputs.
Q. So biocontrol offers advantages. But is it safe?
A. Biocontrol of invasive plants has been practiced worldwide for over a century. History shows that when proper procedures and careful research are in place, negative consequences are rare and risks are reduced to very low levels. The key to safety in biocontrol is to use only host-specific agents--those that are highly specialized to survive on a narrow range of host plants. Host specificity is determined through careful observations in a biocontrol agent's native country and through rigorous testing on nontarget plant species. Before it can be released, an agent must pass extensive scrutiny and scientific review at the state and federal levels. In addition, most programs now require postrelease monitoring to track the progress of the agent. In Hawaii, over 50 biocontrol agents have been released against insect and weed targets since strict regulatory processes were established in the 1970s. None has switched hosts to nontarget species or become invasive themselves.
Q. What about the mongoose and other disastrous introductions in Hawaii?
A. Introduction of mongooses by a private landowner in the 1800s, with no scientific or regulatory review, is not at all representative of the modern practice of biocontrol. Today, biocontrol introductions are based on very careful research and testing. Animals like mongoose and cane toad, which feed on a broad range of prey, would never be considered for biological control. The only acceptable organisms are highly host-specific--those that have a single host or very narrow range of hosts and that are unable to survive on other species.
Q. Won't a biocontrol move on to other plants after killing off its target host plant?
A. Although some biocontrol agents are capable of killing their host plants, biocontrol never results in complete eradication of the target plant. Instead, as the host plant population declines, so does the population of the agent. Highly host-specific biocontrol agents are so closely dependent on their host plant that they cannot use other plant species, even when faced with starvation.
Q. What about the possibility of a biocontrol agent adapting to attack new plants?
A. Rapid evolution of host switching by a host-specific plant feeder is extremely unlikely because of the many genetic changes that would have to occur. Organisms that depend on plants for their food adapt and change over time, as do their host plants. This process is gradual over thousands to millions of years. Such newly evolved host-switching has never been observed in over 1,000 cases of invasive plant biocontrol worldwide over the past 100 years.
Q. What good is biocontrol given the constant onslaught of new invasive species?
A. Biological control is not a cure-all. Solving invasive species problems in Hawaii requires integration of a variety of approaches. Preimportation risk assessment, quarantines at ports of entry, and rapid response to new invasions are all necessary to prevent "the next Miconia." Control using herbicides or mechanical removal are effective against some already established invasive plants. Unfortunately, we have many invasive species for which mechanical and herbicidal techniques are too risky or prohibitively expensive to apply, except in small areas. Carefully chosen biological agents are our only hope for providing long-term control of our worst invasive plants at an island-wide scale, including in remote areas of severely threatened Hawaiian forest.
Click photos to enlarge.