Biological Control of Weeds on the Prairies
Weeds are defined as "a plant out of place"; therefore, plants can be weeds when found where they compete with valuable crop and forage plants and threaten native plant species.
In the management of unwanted pests such as weeds, good integrated pest management techniques include using various methods of control. Mechanical control, with methods such as hand picking weeds, is ideal when the weed population first invades an area. Chemical controls work well for some weeds and in certain situations (cropland), however are often non-selective on non-target plants. When weeds are an issue in sensitive areas, chemical controls cannot be used. Biological controls offer another method of weed management that can be used alone or in conjunction with chemical and mechanical methods to manage invasive weed species.
Biological control is the deliberate use of a weed's "natural enemies" to suppress its population. A weed's natural enemies may be arthropods (insects, mites and their relatives), bacteria or fungi. These "control agents" feed upon or cause disease in the weed, thereby limiting its growth, reproduction and spread.
There are four approaches to biocontrol:
- Classical (inoculative) biocontrol – the release of a relatively small number of natural enemies. These feed on the weed, reproduce and gradually suppress the weed as their population grows. For this approach, arthropods are generally used as control agents.
- Inundative biocontrol – a large quantity of natural enemies are periodically applied to reduce weed populations.
- Biopesticides – the application of formulations containing whole natural enemies (generally a pathogen that causes disease in a weed) or toxic components of natural enemies to reduce the weed population.
- Conservation biocontrol – manipulation of habitats to encourage survival and increase of natural enemies to regulate pest populations.
How biological control agents identified and introduced
Classical biocontrol has generally been the release of arthropods (insects). To be considered a good candidate for classical biological control, a weed should:
- Be non-native;
- Be present in numbers and densities greater than in its native range and numerous enough to cause environmental or economic damage;
- Have taxonomic characteristics sufficiently distinct from those of economically important and native plant species; and
- Occur in relatively undisturbed areas to allow for the establishment of biological control agents. Cultivation, mowing and other disturbances can have a destructive effect on many arthropod biocontrol agents. Inundative and biopesticide biocontrol agents such as bacteria and fungi are less sensitive to these types of disturbances so may be used in cropland.
For any of the approaches to biocontrol, the first steps are to identify potential natural enemies and assess their level of specialization. For classical biocontrol, scientists observe weeds in their areas of origin and collect the insects and other organisms attacking the plants and affecting their survival. For inundative biocontrol or potential biopesticides, most of the pathogens being examined are native to North America .
Potential biocontrols must be screened for the specificity of their host and their effect on the weed to ensure safety of native plants and crops. The closer the evolutionary relationship between the natural enemy and the target weed is, the more likely it is the natural enemy will have a narrow host range, or will only be able to feed on, or complete its life cycle on, the target weed. Screening includes exposure to plants that are closely related to the target weed.
All biological control agents that are released into Canada must be approved under the Plant Protection Act or the Pest Control Products Act. Following federal approval, the classic biological control agents (arthropods) are released on their target weeds at selected experimental sites, which are closely monitored. Data from these sites help to assess both the natural enemies' potential for survival under field conditions in western Canada and their potential to cause damage to the target weed.
Not all classic biological control agents survive and establish throughout the target weed's range. It can also take many years for the population of the biocontrol to grow to a sufficient size to have an impact on the target weed. The longer the biological control has been established in Canada and the more sites it has established on, the more information there is available on the optimal release sites, methods and timing for introducing that agent. Once it's established at a site, monitoring for the control agent population and the effect on the weed population should be done periodically. Classical biocontrols (insects) can often be sampled with a sweep net, however only when the life stage of that biocontrol is likely to be on the foliage. Many successful biocontrol agents spend at least the larva stage of their life cycle in the roots or stems of the target weed.
During establishment of a classical control agent on a weed infestation and while it's increasing its numbers, the site should not be mowed or disturbed. Herbicide may be used along the boundaries of the weed infestation to help contain the weed while the biological agent is increasing in population and spread.
A large number of insects can suppress the weed in an area and may allow for harvesting and redistribution of the biocontrol. Insects collected from these sites can be distributed to other weed-infested areas. It can take years before a biocontrol agent’s population reaches this size, and insects should not be collected for redistribution before this time.
Biocontrol agents do not eradicate their target weed. Therefore, though the level of the control agent population at the site will decrease as the weed population decreases, a small population of the agent should stay on the remaining plants. If the insect population is above what can be supported by the weed population the insects will move to new weed stands or die. If the weed population increases and the biocontrol is well established its population will increase with that of the weed in a fluctuating equilibrium (see Figure 2).
Advantages and limitations
Biological control is advantageous because of its selectivity; there is little danger of damage to non-target plant species. Biological control agents are also very effective in inaccessible areas.
Another attractive feature of biological control is its negligible environmental impact. This weed control method does not bring any of the problems associated with herbicide residues, contaminated groundwater and weed resistance to herbicides.
Individual applications of classical biological control are also potentially much less expensive over time. A small number of biocontrol agents can, once established, grow to very high densities and provide continuous control of a weed over a large area.
When the cost of development is considered, classical biocontrol is generally less expensive than chemical control.
Classical biological control does have limitations as it lacks the immediacy of chemical control. Populations require time to become established, so signs of weed suppression are rarely evident in the first year. Screening work (determining the selectivity and effectiveness of a biocontrol agent) is also very time consuming and is subject to limited funding. More recently, countries have begun to adopt laws to restrict the ability of researchers to survey the range of natural enemies. This has important implications when those countries are areas where the weed originated and thus would have the largest diversity of potential biocontrol agents.
Approved biocontrol agents currently established and/or being evaluated
- Leafy spurge (Euphorbi esula) with five species of Aphthora beetle species.
- Scentless chamomile (Matricaria perforate) with the seed weevil Omphalapion hookerorum widespread, the stem mining weevil Microplontus edentulus poorly established (one site in Alberta) and the gall midge Rhopalomyia triplurospermi widespread.
- Purple loosestrife (Lythrum salicaria) with foliage feeding beetles Galerucella calmariensis and Galerucella pusilla established and the status of the root weevils Nanophyes marmoratus and Hylobius transversovittatus uncertain.
- Cleavers (Galium spurium) had the gall mite Cecidophyes rouhollahi approved for introduction, however the mite was not able to overwinter in Western Canada.
- Dalmatian toadflax (Linaria dalmatica) with the stem-mining weevil Mecinus janthiniformis (previously called M. janthinus), a seed feeding weevil, Rhinusa antirrhini and defoliating moth Calophasia lunula in British Columbia.
- Yellow toadflax (Linaria vulgaris) with the root-galling weevils Rhinusa linariae and R. neta and the flower-feeding beetle Brachypterolus pulicarius. The defoliating moth Calophasia lunula, though introduced to western Canada, has only established in British Columbia. The stem mining weevil Mecinus janthinus has established in British Columbia and potentially sites in Alberta.
- Diffuse Knapweed (Centaurea diffusa) and Spotted Knapweed (Centaurea stobebe subsp. micranthos) have 12 biological control agents released in Canada. The most successful at biological control are the seed head weevil Larinus minutus and the root weevil Cyphocleonus achates, which are only established in British Colombia and Alberta.
- Houndstongue (Cynoglossum officinale) with the root weevil Mogulones crucifer established where the weed occurs in British Colombia and Alberta.
Insect agents being tested
- Yellow toadflax (Linaria vulgaris) with testing of a stem mining weevil Mecinus heydeni and a stem galling weevil Rhinusa pilosa
- Dalmatian toadflax (Linaria dalmatica) with testing of a stem-galling weevil Rhinusa brondelii and a stem mining Mecinus sp.
- Common Tansy (Tanacetum vulgare) with testing on shoot boring weevil Micoplontus millefolii and flower-feeding moth Isophrictis striatella.
Trials focusing on inundative biological control are also underway. These often tend to work best as biopesticide formulations. There are several potential bioherbicides; however, difficulties in formulations and market preferences mean that they might not currently be manufactured.
- The first pathogen to be registered as a bioherbicide was the fungus Colletotrichum gloeosporioides f.sp malvae (Figure 11) for control of round-leaved mallow (Malva pusilla) in field crops (Figure 12).
- Control of dandelion (Taraxacum officinale) through Sclerotinia minor IMI 344141 or Phoma macrostoma isolate 94-44B.
Other target weeds for inundative biological control are wild oat, green foxtail, Canada thistle, cleavers and oxeye daisy.
Photos courtesy of Alberta Agriculture and Forestry; Agriculture and Agri-Food Canada; Alec McClay and Robert B. Hughes, Alberta Research Council.