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Management Methods: Biological Control
Russian knapweed nematode, Melaleuca leaf weevil, rush skeletonweed rust, and Canada thistle mite. Photo credits: (left to right) T Caesar, S Ausmus, G Piper, E Erbe/USDA ARS, www.forestryimages.org
Introduction
Natural enemies, as well as a number of other factors, play an important role in regulating plant populations in their native environments. The absence of natural enemies may be an important contributing factor to the invasiveness of some nonnative species.
Biological control (or biocontrol) reunites invasive plants with their enemies to restore natural controls and reduce dominance of invasive plants within the plant community. Promoted as a self-sustaining, self-dispersing control method, biocontrol is often used to gradually suppress widespread infestations in low-value or remote areas where other methods are not economically feasible.
Despite a reputation as an environmentally benign, cost-effective approach to invasive plant management, the long-term efficacy and environmental impacts of releasing one organism to control another are not fully understood. Classical biocontrol is irreversible and therefore it is essential that all potential consequences are adequately considered beforehand.
In this module, you will become familiar with
- the role of natural enemies in regulating plant populations
- the theory behind biocontrol practices
- impacts of natural enemies on their host and the environment
- safety and regulation of biocontrol
- principles and tactics for implementing and monitoring biocontrol programs
- examples of how biocontrol can be integrated into invasive plant management
In general, plant populations are regulated by their environment (e.g., resources, climate, and competition) and the influence of natural enemies (Crawley 1997). "Natural enemy" is a collective term for parasites, parasitoids, pathogens, predators, and competitors that inflict mortality or injury on a population of a species. By feeding upon, infecting, or otherwise damaging individual plants, natural enemies help to mediate the host plant’s ability to compete in its environment.
Most invasive plants in the United States were introduced from other continents without the complex of natural enemies that coexisted with them in their native environments (Julien 1992). The absence of natural enemies may contribute to the invasiveness of some nonnative plants (Keane and Crawley 2002), whereas different mechanisms, such as community disturbance and varied inherent competitive ability among plants, may also be important (Shea and Chesson 2002). According to the "Enemy Release Hypothesis," when plants are introduced to areas outside of their native range, they may be less regulated by herbivores and other natural enemies, which in turn results in an increase in the plant’s distribution and abundance (Crawley 1997).
Biological control (biocontrol) is loosely defined as using one organism to control another and can be used to restore some of the regulating factors that limit the competitive ability of a pest. The concept of intentionally using one organism (a biocontrol agent) to control another is not a new one. The domestication of cats to control rodent pests over 9,500 years ago is evidence that early humans recognized that some organisms play an important role in regulating others (Pickrell 2004). Biocontrol methods have since been developed to manage a variety of insect, mite, pathogen, and plant pests, and occasionally vertebrate pest species.
From this point forward, the term "biocontrol" will refer to the classical biocontrol approach to managing invasive plants. Approaches to biocontrol are distinguished by how a biocontrol agent is used rather than the type of biocontrol agent (Eilenberg et al. 2001). Classical biocontrol by definition involves the "intentional introduction of an exotic (nonnative), usually coevolved, biological control agent for permanent establishment and long-term pest control" (Eilenberg et al. 2001). Classical biocontrol is by far the most common approach.
Approaches to Biocontrol |
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Classical |
The intentional introduction of an exotic, usually co-evolved, biological control agent for permanent establishment and long-term pest control. Exotic organisms are released with the aim of long-term control without additional releases. |
Inoculative |
The intentional release of a living organism as a biological control agent with the expectation that it will multiply and control the pest for an extended period, but not permanently. Exotic organisms are released with the aim of temporary control and additional releases are needed. |
Inundative/ |
The use of living organisms to control pests where control is achieved exclusively by the released organisms themselves, not their progeny. Any mass-release with the expectation of immediate effects by the individuals released is inundative biocontrol. The biocontrol agent is not expected to reproduce or persist in the environment. Agents used for inundative releases, especially micro-organisms are also commonly called biopesticides or bioherbicides. |
Conservation |
Modification of the environment or existing practices to protect and enhance specific natural enemies or other organisms to reduce the effect of pests. Conservation biocontrol is distinguished from other strategies in that natural enemies are not released. This approach is a combination of protecting biocontrol agents and providing resources so that they can be more effective. |
Broad Spectrum |
The use of polyphagous (eats many types of plants) natural enemies in small numbers and confined to limited spaces for short periods of time to control pests. Broad spectrum biocontrol may employ domestic livestock (prescribed grazing) or grass carp and use confinement technology (such as fences or barriers) to focus grazing efforts and prevent escape to other areas. |
Since its inception, the practice of biocontrol has enjoyed a number of successes as well as some hard lessons. Some of the early attempts at biocontrol did not undergo the rigorous testing now in place and caused unintended and sometimes severe consequences. Centuries of trial and error and the development of rigorous safety and regulatory protocols have helped the practice of biocontrol evolve from early haphazard introductions to the careful planning and evaluation involved today.