CABI News, Issue 1 / May 2007
Biocontrol fights weeds in North America
Many plants that don’t cause particular problems in their natural environment become aggressive, fast-spreading weeds if transferred to a new area. Controlling these invasive species isn’t easy, and people are often forced to rely on environmentally damaging chemical sprays. However, these aren’t the only options.
by CABI Europe – Switzerland
Natural enemies like insects and fungi can be used instead. CABI’s Centre in Switzerland has been researching and identifying biological controls for a range of pests since it opened in 1947, and work has just begun on three new projects.
CABI's Centre in Switzerland focuses on weeds from Eurasia that cause economic and environmental damage in North America.Three new projects have recently begun to find ways of combating swallow-worts (Vincetoxicum spp.), common tansy (Tanacetum vulgare) and Eurasian watermilfoil (Myriophyllum spicatum). Because biological control agents have to be carefully tested, these projects may take several years to give results.
However, many past projects run by CABI provide good examples of just how worthwhile this work is.
Leafy spurge
(Euphorbia esula) |
CABI’s work to combat leafy spurge (Euphorbia esula), is one such example of just how effective carefully conducted biological control work can be.
This weed is a deep-rooted herbaceous perennial that infests more than two million hectares of rangeland and grassland in 35 US states and nearly all Canadian provinces. Again, this plant replaces local grasses and is poisonous to cattle and horses – causing significant financial losses.
Extensive research between 1960 and 1998 resulted in 11 leafy-spurge-attacking insects being released in North America. Of these, the root-feeding flea beetles (Aphthona czwalinae and A. lacertosa) have had a major impact in areas with moist soils. In fact, in some cases they have reduced plant cover from 100% to less than 10% – a low enough level to allow native vegetation to recover. Another type of flea beetle (A. nigriscutis) is providing similar levels of control in areas with warm, dry, open, coarse soils, which don’t suit A. czwalinae or A. lacertosa. |
|
CABI has also been working since the 1960s to assess 20 insect species that could be used to control spotted and diffuse knapweeds (Centaurea stoebe and C. diffusa). These weeds infest rangelands in south-western Canada and the north-western US. Since they produce large amounts of seed, early efforts focused on finding insects that would attack the seeds.
In the 1990s, however, it became clear that seed feeders alone would not be effective, so four additional root-feeding insects were studied and subsequently released in North America. The root-mining weevil Cyphocleonus achates is now providing excellent control of spotted knapweed in western Montana and British Columbia, while the seed-eating Larinus minutus is providing a successful and permanent control for diffuse knapweed in eastern Washington and Oregon. |
|
The story is similar for CABI’s work with Dalmatian toadflax (Linaria dalmatica) in the US and Canada, where these weeds crowd out native species like grasses and reduce the amount of quality fodder available to livestock. Since work began in 1987, CABI has identified five moths and weevils that specifically attack these plants – and these were released between 1991 and 2001.
As a result, the stem-boring weevil Mecinus janthinus (released in 1994) is now successfully controlling Dalmatian toadflax in the north-western US and in British Columbia in Canada, emerging in large numbers in spring and eating the toadflax’s shoot tips. This severely stunts shoot growth and can completely suppress flowering – which prevents the plant producing seeds and spreading. Work continues to find effective biological control agents for the closely related yellow toadflax (Linaria vulgaris). |
Yellow toadflax
(Linara vulgaris) |
Purple loosestrife
(Lythrum salicaria)
|
Rangelands are not the only ecologically and economically important areas attacked by invasive weeds. Purple loosestrife (Lythrum salicaria) is a perennial weed that takes over large areas of wetlands, causing major damage to these important ecosystems. Again, the costs associated with purple loosestrife are large – in the late 1980s around $45.9 million was spent each year to control this weed across 19 US states.
Work began in 1987, and CABI’s scientists evaluated more than 100 insect species associated with purple loosestrife in Europe. Six proved to be safe for release in North America, and two leaf-eating beetles, Galerucella calmariensis and G. pusilla, are particularly effective. As a result of this work, this aggressive invader is now being successfully controlled in the western and midwestern USA, with 95% of purple loosestrife biomass being destroyed – which allows native wetland plants to permanently replace stands of the weed.
|
|
CABI’s work to control the houndstongue (Cynoglossum officinale) is another example of just how effective carefully conducted biological control work can be. This weed, which can poison livestock, is found on rangeland in western Canada and the western USA.
CABI’s search to find an effective natural parasite for houndstongue began in 1988. And as a result, and after extensive research, the root-mining weevil Mogulones cruciger was released in Canada in 1997. Spreading at a rate of 500m per year, the weevil is now well established and is having a major impact on this difficult weed. In British Columbia, for example, the release of as few as 100 weevils into quarter-acre patches of dense houndstongue brought the weed under control within two or three years. |
Houndstongue
(Cynoglossum officinale) |
We thank all our North American partners for their collaboration on these projects and USDA Forest Service, AAFC Lethbridge, University of Idaho, Minnesota Department of Agriculture and USDA-APHIS for letting us use some of the pictures shown here.