Purple loosestrife:
Nice plant, shame about its habits

By: John Bueglas

Purple loosestrife (Lythrum salicaria) has been portrayed by self-interest groups as an “Alien Invader,” “Beautiful Killer,” and even distributed posters showing it as “Most Wanted.” This targeted weed sounds more like the victim of a poor press agent instead of a problem in our wetlands, uplands and roadside ditches. Purple loosestrife is a perennial plant that originated in Europe and was most likely introduced to North American shores during the 1800s. It grew and spread to many areas and was mistakenly included in a list of native plants a few decades later. However, it was not considered a problem weed until the early 1980s. Theories abound as to how it exploded into the weed consciousness but the most plausible is a combination of the increased use of fertilizers, land manipulation and the increased mobility of people into most areas. The plant thrives in areas of excess nutrients as well as disturbed soil. In areas of ever increasing populations, soil is dug, pushed and moved many miles away. This aids in rapid transportation of seeds and seedlings, which can establish quickly.

A proven aggressor
Although purple loosestrife is an excellent competitor, it is not much of a killer. Extracts from the plant have been investigated for use in the treatment of liver disease and glaucoma and was used as a medicinal herb for the treatment of diarrhea, dysentery, bleeding, wounds and ulcers. The plant gets its “killer” reputation because it has the ability to crowd out other plants and produce monospecific stands of itself. Most wildlife can’t eat it when it becomes woody and they can’t move through it, when it is too dense, so they just move on. Birds that weave nests in its stalks and insects are all that is left. When the root system is in deep water, it will collect passing soil around it until the water is unable to move through. Wetlands are effectively dried up and must find another low-lying location to establish. With the realization that wetlands are nature’s filters, comes the fear that we cannot afford to wait for what little wetlands we have to reappear in the landscape.

     The use of chemicals is too risky on a large scale due to the proximity of water around the plant. Roundup appeared to be most effective on the weed but beside the fact that it is a non-specific herbicide, the surfactant used in its formulation is unsafe for use near water. In the U.S., Rodeo is permitted near water but not so in Canada. Mechanical means are difficult on a large scale because of the biology of the plant. Purple loosestrife stalks are a lesson in survival. At each node there are two buds. One will produce a shoot and the other a root. This means that if a plant is cut the stalk will grow a new leader shoot from the remaining stalk. If the cutting drops into water and is in wet soil, it will produce roots and become another plant. Purple loosestrife is also a prolific seed producer and will drop seeds in the millions if cutting takes place after the plants have flowered. The seeds can remain viable for up to 10 years and can germinate with as little as 10 minutes of sunlight.

Hope on the horizon
It sounds like the fight against purple loosestrife is a losing battle, but it’s not. On a small scale, pulling and digging of the plant is reasonable as long as certain precautions are taken. When digging the plant up it is important to get the entire root crown. On older, mature plants the crown can be well over a foot or more in diameter. If the shovel hits something solid, move it further away. At one time it was believed that the plant may be spreading by rhizome but it is the outer stalks that have to grow horizontally before reaching vertically that led to that theory. It is the root buds on those stalks that come in contact with the ground that cause a new plant to form. The part of the stalk still attached to the mother plant was thought to be the rhizome. To reduce the spread of seed it is better to dig the plants before the plants flower or at least as soon as the flowers appear. If it is not possible to dig the plants by this time then the flower stalks should be cut off, removed from the area and burned. Once removed, all plant material should be burned. The area should be checked regularly for the appearance of seedlings, which should then be removed. Chemical treatment should be attempted using non-spray applications only. Painting or wick applications are the safest methods and no chemical treatment should be made on or near water if prohibited in your area.

     On a larger scale, biological control is the only option that has been approved and supported by governments and environmental groups. Biological control, simply put, is the manipulation of an organism’s natural enemies for the purposes of controlling, reducing or eradicating the target organism. This method has been around for hundreds of years but has only recently gained popularity, as society demands more non-chemical solutions. The Department of Agriculture and Agrifood Canada was the department traditionally involved in these programs but because the problem weed was in wetlands, it fell under the jurisdiction of Environment Canada. For researchers, there was a period of teaching a new dog old tricks and the department perceived biological control as a new control measure and were understandably nervous. After many meetings and information sessions, all concerned embraced the concept.

     For purple loosestrife, insects that feed on the plant were selected. The International Institute for Biological Control (IIBC) in Delmont, Switzerland was contracted to survey European purple loosestrife for natural enemies. They discovered five species of insects that feed exclusively on Lythrum salicaria. Although that seems like a simple discovery, it actually took years of research on both sides of the Atlantic to determine this fact. Once approved for importation, North American researchers looked at the insects, under strict laboratory confinement, to determine that the insects had to actually rely on purple loosestrife to survive and reproduce. The main thrust of this was the use of starvation tests. The insects were not allowed to feed for a period of time and then offered related plant species and economically important plants as the only food source. Researchers watched to see if the insects would eat, survive on, or reproduce on the plants. Once it was determined, beyond reasonable doubt, that the insects were obligated to feed on purple loosestrife, the insects were approved for mass rearing and release on this continent. It is important to note that, for each application for importation and release, both the U.S. and Canada have to agree to this before it can happen.

Two leaf feeding beetles, Gallerucella calmariensis and G. pusilla were selected as the prime organisms on which to focus. Their life cycle, habits and voraciousness were better suited to the task than the other three insect species. Very similar in appearance, both complete their life cycle on the same plant, at the same time. Adult beetles feed on the leaves of the weed by chewing holes right through the leaf. By ingesting the leaves of younger plants, the adults are stimulated to breed and lay eggs while they are feeding on the plant. Once the eggs hatch, the larvae begin feeding by chewing the underside of the leaves thereby giving it a skeleton-like look. At this stage in their lives, the larvae like to congregate together.

     Mature larvae stop feeding and drop the ground where they crawl underground, litter and pupate. After a period of one to two weeks, the adults emerge to continue feeding. The behaviour to congregate is lost and if there is over crowding, the adults will fly off to find other purple loosestrife plants. This is how the beetle disperses to new areas. To effectively reduce the vigor of an individual plant it was determined there needs to be a density of 200 egg masses per square metre. The difficulty is to have enough beetles to colonize an area and achieve this density. As of 1998, the University of Guelph had released 98,300 beetles in Southern Ontario but research was halted and a pay per beetle industry has appeared. Before research was sacrificed, it was determined that some sites like the large site at Dixie Rd. S. and Highway 401 have had population explosions of the beetles. The problem is that there is no one left to determine if the program is a success or if it will be in the future. For now it is, at best, a possible long-term solution to the spread of purple loosestrife. It will not work as an eradication method, as the stands of purple loosestrife still exist even though beetle populations are high. The stands of purple loosestrife don’t appear to be expanding. For now, it is better to remove purple loosestrife to eradicate it from a site and to continue removing it as it appears. Insects can be released to help prevent the plants from appearing in surrounding areas and spreading back into the site. The insects in Southern Ontario are plentiful enough to be harvested from an area and moved to another. This is what the companies that sell the beetles are doing. Just remember to ask permission from a landowner before doing so. It is also better to release the beetles at night, as they are less likely to fly off after being confined together.

     To determine which method(s) are best suited to your particular needs, visit www.seagrant.umn.edu/exotics/purple. The site contains a chart, which shows at what land size and which density of plants certain methods or combinations of methods are most appropriate.

About the Author
John Bueglas has a Master’s Degree in Entomology and the Biological Control of purple loosestrife. He presently resides in River John, Nova Scotia, where he is General Manager of Dubé Botanical Gardens, an aquatic plant nursery. Contact: