THE BIOLOGICAL POLLUTANT
Manitoba Purple Loosestrife Project Update - Spring 1993
Purple loosestrife (Lythrum salicaria) is a herbaceous wetland perennial that was introduced into North America from Europe in the early 1800's. It is believed that purple loosestrife was introduced into North America in the ballasts of cargo ships. Other possible modes of introduction included purposeful introduction as a herb, rootstalks brought over by horticulturalists, and seed transport via imported raw wool and sheep.
WHY IS PURPLE LOOSESTRIFE CONSIDERED A PROBLEM?
Purple loosestrife invades native wetland communities forming a monospecific stand that no Canadian bird, mammal or fish depends upon. Purple loosestrife germinates and grows faster than just about any native canadian wetland species. Loosestrife soon forms dense brushlike stands, usurping water while forcing out native plants. Researchers monitoring the spread of loosestrife at a waterfowl impoundment in New York found that within 13 years purple loosestrife went 90% of the total biomass. Loosestrife reduces the size and diversity of natural plant communities, threatening scarce species. Once purple loosestrife invades an area and out-competes the native flora, the wildlife that once depended upon the native flora, is forced to move into new areas. Waterfowl will not eat loosestrife seeds. Muskrats eat around loosestrife stands. Wildlife species are displaced and those that cannot move into new areas are lost. What remains is a biological desert devoid of native plant and wildlife species.
In its native range, purple loosestrife is not considered a problem plant. In Eurasia, there are over 120 species of insects that prey on purple loosestrife regulating population levels. The evolved predator/prey relationships was not transferred to North America with the introduction of purple loosestrife. Hence, there are no natural predators or diseases of loosestrife in North America to curtail its spread.
HOW DOES PURPLE LOOSESTRIFE SPREAD?
Purple loosestrife spreads primarily by seed, but will also spread by the resprouting of cut stems or roots. Purple loosestrife does not spread by rhizomes. it has been reported that a mature plant can produced as many as 2.5 million seeds annually. Seeds may be transported along waterways, through storm water systems, across snow and ice, in mud picked-up by tires or footwear, or in the cooling systems of outboard motors. A few individual plants can build-up an extensive seed bank capable of survival in the soil several years before germinating.
DISTRIBUTION
The first documented occurrence of loosestrife in Manitoba was a plant collected in Neepawa in 1896. Loosestrife began to impact upon Manitoba's wetlands and riparian areas in the 1950's. Purple Loosestrife can now be found across southern Manitoba and is spreading rapidly. Loosestrife has invaded every major river system in southern Manitoba.
Habitat surveys in 1992 revealed a much higher level of loosestrife in Manitoba than originally thought. Areas with high densities of loosestrife include the Assiniboine River through Winnipeg, the Red River from Winnipeg north to the Netley Libau Marsh and areas in the south basin of Lake Winnipeg. Dauphin Lake, the Cypress River area, and drainage ditches in southern Manitoba also contain high levels of loostrife.
This adaptable plant is migrating from wetland habitats onto agricultural lands in the Netley -Libau area. The presence of loosestrife can reduce land resale values as well as reducing crop yields. In 1992, loosestrife was found growing in grain fields in eastern Ontario. Purple loosestrife can stagnate water flow in irrigation and drainage ditches. Landowners and farmers will be faced with additional costs related to loosestrife control and removal.
NOXIOUS WEED STATUS
Purple loosestrife has been included on the Manitoba Noxious Weed List since the early 1980's. Confusion exists as to whether cultivars such as 'Modern Pink' are considered noxious. The Noxious Weed List is currently being revised and updated to include all purple loosestrife.
In response to purple loosestrife in parks and along riverbanks owned by the City of Winnipeg, a new policy regarding the control of loosestrife was adopted in 1993. This policy states that no form of purple loosestrife or its domestic cultivars are to be planted on City lands and all loosestrife currently planted be removed and destroyed.
CONTROL OF PURPLE LOOSESTRIFE
Attempts at controlling this invasive alien have not been successful. Burning does not create enough heat to kill the rootstalks under the soil. Mowing or cutting does not destroy the roostsalks and creates pieces that may resprout vegetatively. There are no herbicides registered for use over open water in Canada that will control loosestrife. Herbicide application would only affect the current standing plant growth and would not affect the developed seed bank.
Smaller populations (less than 100 plants) can be controlled by hand pulling or digging. A shovel may be used to remove the entire above ground portion of the plant and rootstalk. Care must be taken to remove all root or plant pieces from the wetland area. Removal should take place prior to seed set (early August). Plants should then be dried and burned. Populations can be contained by cutting off the flower portion of the plant before it produces seed, followed by drying and burning. Areas where control campaigns are initiated must be revisited in the following years to remove any regrowth from undetected roots and new seedlings.
HERBICIDE RESEARCH
Currently there is no herbicide that is registered for use over open water in Canada for controlling purple loosestrife. An experiment is being conducted in Manitoba by Ducks Unlimited Canada and Dow Elanco to determine the effects or triclopyr amine (Restore) on purple loosestrife and other non-target vegetation. Restore is a herbicide selective to dicot species and therefore should have minimal effects on native monocot plants. Preliminary results from similar research in Ontario indicate that Restore is effective for controlling purple loosestrife without having negative impacts on non-target vegetation. Research is now focus in on investigating the fate of the herbicide in soil and water and its toxicity to aquatic organisms.
BIOLOGICAL CONTROL
Efforts are underway to test the effectiveness of biological control agents. Insects that control loosestrife in Europe have been imported in hopes they can control loosestrife in Canada. They are a root weevil Hylobius transversovittatus, and two species of leaf eating beetles, Galerucella pusilla and Galerucella calmariensis. Testing has indicated that these species of insects feed only on L. salicaria and that the chance of a permanent host transfer is extremely low.
Approval for the release of the above biocontrol agents was received from the Canadian government in the summer of 1992.
In the fall of 1992 root-mining weevils were released in Manitoba to test for winter hardiness. Release site inspections in April of 1993 indicated that these weevils have survived their first winter. Adult weevils are brown, 6 mm wide by 13 mm long (about the size of 1/4 your finger nail), live for about 2 years., and are capable of destroying large root-stocks. In the June of 1993 releases of the leaf eating beetles are planned. These leaf-beetles can completely defoliate purple loosestrife plants and suppress flower formation. Adults are 2 mm wide and 4 mm long. if these control agents are able to establish colonies in Manitoba, it will take from 5 to 10 years before the insect populations are large enough, insects will then be redistributed to priority areas within Manitoba.
Biological control offers a long-term control method that will not eradicate loosestrife, but reduce population levels to ecologically tolerable levels. Biological control programs for other noxious weeds in Manitoba, including Leafy Spurge, look promising.
ENVIRONMENTAL IMPACTS OF PURPLE LOOSESTRIFE
DEGRADING WILDLIFE HABITAT
- displaces plants and animals
- replaces native wetland communities
- eliminates food and shelter for wildlife species
- reduces biodiversity
- impacts fish spawning habitats
- reduces available waterfowl habitats
- threatens prairie pothole habitat where a large portion of North America's waterfowl breed
- impairment of recreational uses of wetlands
DEGRADING AGRICULTURAL LANDS
- impedes water flow in drainage and irrigation ditches
- reduces crop yields and quality
- reduces agricultural land value
- threatens commercial wildrice production
CULTIVAR CONFUSION
Loosestrife cultivars have been developed for use as garden ornamentals and landscaping plants. Cultivars such as 'Morden Pink', 'Morden Rose', 'Morden Gleam' and 'Dropmore Purple' were thought to be safe for garden use, in that they would not produce viable seeds. Recent research has indicated that cultivars can cross amongst themselves or with wild loosestrife and produce viable seed, further contributing to the spread of loosestrife. Plants do not have to be near each other to cross pollinators. Loosestrife planted in gardens may eventually spread to drainage ditches, streams, creeks, wetlands, and rivers. Many wild populations can be traced back to garden or horticultural escapes.
WHAT CAN YOU DO?
- report any purple loosestrife you sight by calling (204) 467-3269 (Manitoba Purple Loosestrife Project) or 1-800-565-6305 (Canadian Wildlife Federation)
- Dig out small infestations, remove entire plant including roots
- Removed plant parts should be dried and preferably burned (where permitted)
- To prevent further seed spread, cut off the seed head/flower top in July before it goes to seed, then burn (where permitted)
- Organize local clean-out campaigns and monitor site over several years
- Use alternative perennials as landscape ornamentals
The Manitoba Purple Loosestrife Project is a non-profit organization
led by a steering committee composed of Agriculture Canada, Canadian
Wildlife Service, City of Winnipeg, Ducks Unlimited Canada, Manitoba
Agriculture, Manitoba Environment, Manitoba Habitat Heritage Corporation,
Manitoba Naturalists Society, Manitoba Nursery and Landscape Association,
and the Manitoba Weed Supervisors Association.
For more information contact Rodney Penner, Provincial Coordinator, at (204) 986-3166, or Email rodneypenner@winnipeg.ca