Biological Control

What is Biocontrol?

Definition of Biocontrol

Biological Control or biocontrol is the use of an invasive plant’s natural enemies-agents (chiefly insects, parasites and pathogens)-to reduce its population below a desired level. It is the long-term, self-sustaining treatment method for managing invasive plants. It can be divided into the following two approaches, classical and inundative:

1. Classical biocontrol uses agent populations that would fluctuate in a natural predator/prey relationship. This method uses natural predators of the invasive plant to establish a long-term balance between the biological control organism and the plant. For example, St. John's wort is considered under successful biocontrol, its population is cycling in a classical biological control pattern.

2. Inundative biocontrol typically uses pathogens, such as rusts and nematodes, that are applied to the target weed at high rates in a manner similar to herbicide application. The intention is to use large quantities of the agent to ‘blitz’ the target, thereby wiping it out all at once. Like all such methods, it does not deal with the residual seed bank in the soil or prevent the invasive plant from establishing from another source at a later date.

The BC Ministry of Forests and Range’s Biocontrol Development and Invasive Plant Programs use the classical biocontrol approach to bring invasive plant species to an acceptable socio-economic and ecological level.

Since classical biological control employs a typical predator/prey cycling of the agent and plant populations, an invasive plant will never be eradicated with the use of a biocontrol agent as the agent’s population will decline after the invasive plant population has been reduced. As one’s population increases or decreases, the other’s will follow due to the increased or diminished supply of food, or conversely, feeding pressure. Biological control of an invasive plant is considered to have been achieved when the amount of the invasive plant in a landscape plant community is below a socio-economic and ecological threshold deemed tolerable by resource managers.

The agents of classical biocontrol may kill the invasive plants directly or indirectly by decreasing reproductive and competitive abilities, or plant vigour, which in turn encourages the re-establishment of native vegetation. Biological control is not a substitute for good land management, however, and must be part of an overall land management plan. Invasive plants will persist and re-establish, or new invasive plant species will invade, if soil and plant disturbances are excessive and the surrounding vegetation is not vigorous enough to take advantage of the invasive plant’s reduced competitive ability. The most successful areas for biological control in B.C. are those with healthy residual native plants. These native plants are able to re-claim the area when the invasive plant has been weakened or killed and they can resist the invasion of a successive invasive plant.

Why Use Biocontrol?

Invasive plants cost resource users and managers funds in economic losses and costs to control the invasive plants themselves in order to mitigate further economic losses. To date there are no compiled estimates of these costs in BC. However, several states in the US report such figures as:

  • In Montana they estimate the direct and in-direct impact from 3 knapweeds species alone to be $42 million annually which could support 518 jobs in the state (Sheley and Petroff 1999); and
  • In North Dakota from leafy spurge alone, they estimate $2.9 million annually lost from wildlife associated recreational expenditures (Sheley and Petroff 1999).

Biocontrol is the only long-term solution for management of invasive plants. Over time, when using biological control, other treatment methods become decreasingly necessary, and may eventually not be needed at all. It is desirable to decrease the amount of herbicide applied to the environment (or potentially applied in the future) and to decrease economic losses and costs for control. An economic study of tansy ragwort (Senecio jacobaea) in Oregon used three biological control agents to help mitigate the economic losses estimated in the millions of dollars due to (among others) decreased forage and cattle death from tansy ragwort poisonings and to decrease the costs necessary to control the invasive plant. The resulting report by Hans Radtke, An Economic Evaluation of Biological Control of Tansy Ragwort, discusses the success of this biological control program. In particular, data from “Table 6 Cost and Net Annual Benefits of Biological Control of Tansy Ragwort in 1974 Dollars” (Radtke 1993) has been plotted to depict the overall economic benefits realized from the biological control of tansy ragwort by the seed fly (Hylemya seneciella), the flea beetle (Longitarsus jacobaeae) and the cinnabar moth (Tyria jacobaeae).  The resulting chart can be viewed in the following link.

Financial Benefits of Bio-Control on Tansy Ragwort in Oregon.


Radtke, H. 1993. An Economic Evaluation of Biological Control of Tansy Ragwort. Oregon Dept. of Agric. State Weed Board, Oregon, U.S.A.

Sheley, R. L. and J. K. Petroff. 1999. Biology and Management of Noxious Rangeland Weeds. Oregon State Univ. Press, Oregon, U.S.A.

How does biocontrol work?

Each biocontrol agent attacks its host plant in a specific manner. The visible symptoms of attack may be damaged leaves, flowers, stems or roots and/or wilting, discoloration, dropping of leaves, reduced numbers and viability of seeds, and retarded growth or flowering periods. The factors used for recording biocontrol agent presence for data entry into the Invasive Alien Plant Program (IAPP) Application as described in the MFR Invasive Alien Plant Program (IAPP) Reference Guide Module 1.9 Biological Treatment and Monitoring are: foliar feeding damage; seed feeding damage; root feeding damage; adults present; larva(e) present; pupa(e) present; eggs present; oviposition marks; and exit holes/tunnels. Injured plants are also vulnerable to attack by other insects or pathogens particularly at wound positions caused by biocontrol agents. Multiple attacks at multiple attack locations on the plant usually reduce plant reproduction and even cause plant death.

Biological control agents can generally be described in the following categories:

Gall Producing Insects

Gall producing insects produce atypical growths on plants through either larval feeding or female oviposition. The resulting gall (enlargement) causes the plant to direct nutrients into the gall tissue, rather than into seeds or plant growth. For different species, galls can be located on different plant tissues, for example, on roots versus stems or seed heads.


Defoliators partially, or completely, consume or mine leaves and associated stem tissue. This loss of nutrition reduces the plant's ability to produce sugars for the root system, thus suppressing growth and survival.

Sap Suckers

Insects and mites with piercing-sucking mouthparts feed on nutrients in the plant’s circulatory system, weakening the plant. Viruses, bacteria and fungi that are pathogenic and specific to the plant can also be transmitted during these attacks.

Flower and Seed Feeders

Flower and seed feeders affect reproductive tissues such as flower tissue or some or all of the seeds by consuming tissue and/or nutrients intended for seed production. Seed viability is then greatly reduced.

Stem Miners

During larval development, insects mine within the plant’s tissues. Plant pathogens carried by the miners can cause secondary damage once the stem miner leaves its hosts. Feeding activity by insects in the plant’s stem reduces nutrient reserves and can impair the plant’s ability to translocate nutrients.

Crown Feeders

Feeding activity by insects in the plant’s crown reduces nutrient reserves and can impair the plant’s ability to translocate nutrients. Pathogens can cause secondary damage here as well.

Root Feeders

These insects bore into the roots or feed on root hairs and young roots, reducing nutrient reserves and the plant’s ability to acquire and translocate moisture and nutrients. Soil pathogens may also enter the roots through the wounds.


“An organism that lives on or in another organism, or at the expense of another organism” but does not kill its host.


“Any-disease-producing microorganism. Principal pathogens include bacteria, viruses, fungi and nematodes.”


Gordh, G. and D.H. Headrick. 2001. A Dictionary of Entomology. CABI Publishing, Oxon, UK

Biocontrol in British Columbia

The use of biocontrol in British Columbia involves a retrospective set of actions, separated into stages that can be thought of as the Biocontrol Cycle.


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