Ministry of Agriculture
Biological Weed Control in British Columbia
Weeds are costly, unwanted plants.
In British Columbia, crop losses due to weeds, and costs associated with their control, are estimated to exceed $50 million annually.
But the use of natural enemies in an integrated control program can reduce the costs of chemical and other non-chemical control methods. This is especially true in the fight against noxious weeds - alien plants which invade and reduce the productivity of our agricultural lands and natural grasslands.
British Columbia began using natural enemies to control noxious grassland and pasture weeds in 1951. The introduction of a beetle has successfully controlled St. Johnswort - a poisonous pasture weed. More recently, nodding thistle has been controlled by two weevil insects.
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Monitoring St. Johnswort biocontrol agents |
By 1993, fifty-one biological control organisms had been released in British Columbia to attack nineteen noxious weeds.
What is Biological Weed Control?
Biological weed control, known as biocontrol, is a method of controlling undesirable, introduced plants by exposing them to their natural enemies.
The majority of these noxious weeds were brought into our province from Europe and Asia.
Unfortunately, when the weeds arrived in British Columbia, the natural enemies that control them in their native countries did not come with them. In the absence of those enemies, the new weeds quickly spread and overran our desirable native plants.
Biological control attempts to establish a natural balance between the weed and its environment by introducing weed-specific insects or diseases to attack the noxious plants.
The goal is not to eliminate the weed but to reduce its population to levels that no longer cause environmental or economic concerns.
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Cyphocleonus root-crown weevil on knapweed |
Steps in Selecting a Biological Control
Significant progress has been achieved in biocontrol development through the cooperative efforts of the International Institute of Biological Control (Switzerland), Agriculture and Agri-Food Canada, the ministries of Agriculture, and Forests, and through the support of private sector groups such as the B.C. Cattlemen's Association.
- Candidate natural enemies that feed on targeted weed and show promise for control are studied in their native habitat.
- Exhaustive studies are carried out to ensure the insect will attack only the targeted weed and not other vegetation.
- Long-term results are reviewed by North American Biocontrol agencies. If the natural agent is proven to damage the weed without attacking other vegetation, it is approved for release.
- The B.C. Plant Protection Advisory Council approves or rejects the release of federally approved natural weed control agents to British Columbia.
- Initial releases in British Columbia are made under controlled conditions to enable the control agents to become established and to increase populations for redistribution in the province.
There are a number of action steps before any natural enemy is released in British Columbia:
Usually, several insect species are released to improve the chances of long-term control of the weed. Insect breeding cages, maintained by the Ministry of Forests in the southern interior, are used to initially establish most new enemies. When populations warrant, the insects are then redistributed throughout the province.
The Future
Biological weed control is a long, gradual and expensive process, but researchers suggest that overall return over costs is generally at least 50:1 and often up to 100:1. Biocontrol does not aim to eradicate the weed. Successful biocontrol will, at best, reduce the vigour, abundance and economic losses caused by the weed.
The use of natural enemies will continue to play an integral and expanding role in integrated vegetation management systems in British Columbia, reducing the costs associated with chemical and other non-chemical control programs.
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| Range grass severely suppressed by knapweed (light strip). Herbicides may be used in integrated weed control programs (dark strip and foreground). | Chrysolina beetle feeding on St. Johnswort |
British Columbia Examples
St. Johnswort
The first biocontrol agent released in British Columbia was a leaf-feeding beetle used to attack the poisonous pasture weed St. Johnswort. Less than 10 years after its 1951 release, the weed was under control throughout most of its range in British Columbia.
Tansy Ragwort
This toxic plant causes liver damage to grazing animals. It infests pastures in the Lower Fraser Valley, Vancouver Island and the Gulf Islands. A recent infestation was found in the Okanagan as well. Four agents, including a leaf-eating moth, a seed-eating fly, a root-feeding flea beetle and a root-feeding moth are established in coastal climates.
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| Tansy ragwort flea beetle (Longitarsus jacobaeae) and "shothole" damage on rosette leaf. | Cinnabar moth (Tyria jacobaeae) |
The conspicuous black and red Cinnabar moth and its yellow and black striped caterpillar larva cause local defoliation of ragwort. The very inconspicuous ragwort flea beetle that feeds in the root crowns causes the most damage. These have been responsible for reducing ragwort populations at many sites.
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Cinnabar moth larvae defoliating tansy ragwort |
Knapweed
Diffuse and spotted knapweeds infest tens of thousands of acres of British Columbia rangelands and seriously reduce the amount of food available for grazing livestock and wildlife. Since the first insect release in 1971, a total of 12 natural agents have been approved and are in various stages of establishment, propagation and redistribution in British Columbia.
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Knapweed seed-reducing fly (Urophora sp.) is established throughout B.C. |
Two small flies (Urophora sp.) have reduced knapweed seed production by up to 90 percent. Further seed reduction and seedling mortality result from attack by the knapweed beetle (Sphenoptera) whose larvae mine the roots. These agents are well established throughout the province.
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Sphenoptera adults. Larvae of this beetle feed in knapweed roots. |
A number of other insects are also being distributed throughout the province. Knapweed populations are declining where combined attack by natural agents is at a high level.
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The knapweed root-feeding Agapeta moth prefers moist sites |
Leafy Spurge
Leafy spurge is a creeping perennial weed that displaces useful forage plants on rangeland and pasture. It is also toxic to grazing animals. This weed is exceptionally difficult and costly to control with chemicals.
Spurge flea beetles, released to numerous sites since 1986, are beginning to result in population reductions in the Kamloops, Chilcotin and East Kootenay regions. Where the agents are established, circular areas where the insects have reduced the number of flowering stems, are evident.
Other Weeds
A number of other introduced noxious weeds are also targeted for biological control. These include yellow and dalmation toadflax, bull thistle, Canada thistle, nodding thistle, hound's-tongue and scentless chamomile.
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Specialized fly larvae reduce seed production in bull thistle. |
The rapidly-spreading wetland invader, purple loosestrife, that chokes out native wildlife vegetation, is also a target for biocontrol. A root-feeding weevil and two defoliating beetles that stress the plants will be established throughout the province in an effort to protect wetland habitats.
Production of the printed brochure was made possible by contributions from
the following organizations:
B.C. Ministry of Agriculture
B.C. Ministry of Environment
Canada's Green Plan
British Columbia Cattlemen's Association
Agriculture and Agri-Food Canada
