Biological Weed Control
Biological weed control involves the release of organisms that attack plants to control weeds. The aim of biological control is to shift the balance of competition between the weed and the crop in favour of the crop and against the weed. The biological control agent, normally a fungus or insect, may not necessarily kill the target weed but should, at the least, reduce its vigour and competitive ability. From a practical point of view the organism or agent should prevent the weed setting seed or producing other reproductive parts. There is considerable potential for encouraging the use of native biological control agents against weeds and substantial research effort has been put into biological control in general. However, the application of biological weed control in agricultural systems in Europe has proved difficult and their are no well documented successes.
In practice, there are three basic types of biological control:
Classical (or innoculative) biological control involves the release of exotic natural enemies to control exotic weeds and has been successful against weeds like thistles in the US and Australia where weevils (native to Europe) have been introduced onto the thistles. It has been suggested that some introduced weeds like hogweed (Heracleum mantegazzium), Himalayan balsam (Impatiens glandulifera) and the Japanese knotweeds (Reynoutria spp.) would be ideal candidates for classical biological control but so far it has only been attempted with bracken (Pteridium aquilinum) where attempts to use two South African moths as potential biological control agents were not successful.
The introduction of a classical biological control agent may not be deliberate. A rust, Puccinia lagenophorae, of Australian origin, which attacks a range of Senecio species, was unknown in Europe before 1960 but has since become established in France and the UK on groundsel where it reduces the viability of groundsel plants on which it can be regularly found. It is unknown how this pathogen reached Europe or how it established.
Inundative control involves the mass production and release of native natural enemies against native weeds, for example rust fungus is often used against weeds. Work in this area has concentrated on fungal pathogens of plants as they can potentially be applied as sprays in the same way as conventional herbicides (hence their name myco- or bio-herbicides). Studies on bioherbicides have concentrated mainly on foliar treatments using fungi. Commercial products have been developed (mainly in the US) but success has been limited Soil micro-organisms are often overlooked but are also important as plant pathogens. Several are being investigated as potential biological control agents particularly for control of grass weeds such as downy brome, wild oat and green foxtail.
Although much of the work on biological control agents has concentrated on the growing weed plant, there is considerable potential for using micro-organisms to manipulate or deplete the soil weed seedbank. The persistence of weed seeds in the soil is the key to their success in continuing to emerge despite repeated control measures over many years. Greater predation or an increase in natural decay would reduce the soil seedbank and hence future weed populations. However, there are as of yet no practical or commercial applications available.
Conservation control is an indirect method, which manipulates the habitat around the weeds with the aim of encourging those organisms that attack the weed. This is a long term strategy that requires a detailed knowledge of the ecology of the crop weed habitat, the target weeds and the control agents. It has received little attention to date. One recent example is the upsurge of interest in looking at encouraging the dock beetle on dock plants by creating conditions that favour the beetle.
Livestock can also be considered as biological control agents which can give a broad spectrum control of weeds in various situations. These are discussed in the section on livestock.
Allelopathy refers to the direct or indirect chemical effects of one plant on the germination, growth, or development of neighbouring plants and can be legitimately regarded as a component of biological control whatever the type. The effect is exerted through the release of allelochemicals, either by growing plants or their residues. More extensive information on this topic can be found on our allelopathy page.
The assessment of the potential risks involved in introducing biological control agents remains a difficult and (sometimes) contentious issue as any predictions of how biological control may affect the interaction between species, and influence the life cycle of non-target species is extremely complicated. Even if there were no risk to non-target species, there could still be a conflict of interests because some may perceive a particular plant as a weed while others see it as a desirable wild flower, or even a potential crop. For this reason it seems difficult to imagine that of the shelf biological control of weeds is a realistic prospect in the short to medium term.
- For more details consult our fully referenced review of biological weed control (141 Kb pdf updated in October 2007).
- The Organic Agricultural Centre of Canada factsheet on biological control of weeds
- EWRS Education and Training Working Group Web Site on Biological Weed Control
- General information on biological control of weeds including pictures of bugs.
- Frequently asked questions about biological control of weeds