Feature Article

Classical Biological Control of Purple Loosestrife in the Upper Midwest

Origin and status of exotic Lythrum spp. Purple loosestrife, Lythrum salicaria, is a perennial weed commonly found in wetlands throughout North America, north of the 35th parallel (Stuckey 1980, Anderson and Ascher 1993, Mal et al. 1992). The plant is native to Europe and was likely brought to North America as a contaminant in ballast of sailing ships in the late 17th century and also purposefully introduced as a garden plant (Stuckey 1980, Hight et al. 1995). In the absence of natural enemies, purple loosestrife reaches densities much greater than those seen in Europe (Thompson et al. 1987), it displaces native plants (Mal et al. 1992), degrades wetlands by reducing vegetational diversity, often producing dense monotypic stands of purple loosestrife. Purple loosestrife also clogs waterways and reduces open water in shallow wetlands which in turn deteriorates the quality of wetlands as wildlife habitat (Rawinski and Malecki 1984).

Because purple loosestrife has a showy flowering spike which often exceeds 2 feet in length, is hardy in northern climates, and tolerates saturated soils, purple loosestrife has been a popular garden plant. Beekeepers were even encouraged to plant purple loosestrife to provide honey bees with flowers to visit in wetlands since most wet meadows and swamps are considered unproductive as bee forage. Lythrum virgatum or 'wand loosestrife' is believed by some to be a separate species, but North American taxonomists don't recognize this as a valid species. Horticultural selections of Lythrum spp. such as 'Morden Pink' and 'Dropmore Purple' were thought to be sterile, but recent research at the University of Minnesota and elsewhere showed that cultivars are not sterile and they can serve as pollen sources for L. salicaria and will hybridize with winged loosestrife, Lythrum alatum, a native species (Anderson and Ascher 1994). Because these cultivars can hybridize with purple loosestrife or become weeds themselves, most states now consider all loosestrife (cultivars, L. virgatum and L. salicaria) as primary noxious weeds. Noxious weed laws vary from state to state but in general they prohibit the cultivation, propagation, transportation and sale of these plants.

Growth habits and impact on wetlands. Purple loosestrife forms a woody crown from which it sends up new shoots each year. The old stems often stay erect and in dense stands prevent light from penetrating to the soil surface effectively reducing competition by native species. In a loosestrife infested wetland, the erect dead stems restrict access to open water by resident and migratory waterfowl. Although there has been little research documenting reduction in waterfowl numbers in loosestrife infested wetlands, there is agreement among many research scientists and wildlife managers that purple loosestrife does reduce the quality of wildlife habitat. No wildlife species, with the exception of red winged black birds, uses purple loosestrife as a food or nesting site (Rawinski and Malecki 1984).

Seed is the primary means of dispersal. Seed is exceedingly small and dispersal is by moving water or by transport of contaminated soil as it clings to motor vehicles and animals as they move through a loosestrife infested wetland. Mature purple loosestrife plants are enormously prolific seed producers, averaging 2.7 million seed per mature plant (Thompson et al. 1987). A single plant that is left uncontrolled poses a significant threat to any wetland. Seedbanks of purple loosestrife remain viable for many years with more than 80% of the seed able to germinate after 3 years and up to 40% are still viable after 8 years (Becker, unpublished data). With the exception of the Colorado, Arkansas and the Rio Grande rivers, purple loosestrife has colonized all major watersheds of the arid west (Thompson et al. 1987).

Control. Conventional control practices range from herbicides to hand removal of plants and manipulation of water levels. If the infestation is relatively small (from 1-100 plants) hand removal and herbicide application has a greater chance of eliminating the problem, especially if disturbance of the site is minimal and if sufficient numbers of native plants are able to shade out purple loosestrife seedlings. In large infestations where purple loosestrife has formed dense monotypic stands, or where the extent of the infestation covers large areas with purple loosestrife at low to moderate density, conventional control is far less effective and suppression rarely lasts for more than a few growing seasons. In general, using nonselective herbicides in large expanses of purple loosestrife is not recommended. As an alternative to conventional control methods, Minnesota, in conjunction with several northern states, began a purple loosestrife biological control program in 1992. Building on the basic findings of Malecki et al. (1993) where natural enemies of purple loosestrife were studied in Europe, insects that are implicated in control of purple loosestrife in Europe have been imported to North America.

Biological Control. Using insects to control exotic weed species is not a new concept and there are many examples where weeds have been successfully controlled by importing insects from the weed's native range. However, there are strict guidelines to follow before an exotic insect is approved for release in the U.S. For purple loosestrife biological control, extensive feeding studies were conducted in Europe to assure that the candidate insects were sufficiently host specific. Currently, there are five species approved for release in the U.S. and Canada, two leaf feeding beetles, Galerucella calmariensis and G. pusilla, a root feeding weevil, Hylobius transversovittatus, and two seed feeding weevils, Nanophyes marmoratus and N. brevis. With the exception of N. brevis all species have been released in North America. The current focus of most purple loosestrife biological control programs in North America is the rearing and release of the leaf feeding beetles. These insects are easy to rear and handle. However, it may take all species or different insects yet to be released to control purple loosestrife in all locations.

A question that often arises when people first read about a purple loosestrife biological control program is, 'Once loosestrife is under control, what will these exotic insects eat?'. The answer is the beetle population will decline along with that of purple loosestrife. The insects went through years of testing in Europe to determine what plants they accept as food sources. None of the insects feed on any crop species; if they did, permission to introduce these insects would be denied. The two leaf feeding beetles, G. calmariensis and G. pusilla will feed on two native species closely related to purple loosestrife, swamp loosestrife, Decodon verticillatus, and winged loosestrife, L. alatum, but their preference is to feed and reproduce on purple loosestrife. In recent research conducted at the University of Minnesota, we have shown that some purple loosestrife populations are less preferred than others (Katovich et al., unpublished data) so in fact these insects may not feed equally on all variants of L. salicaria. The leaf beetles were approved for release even though they will feed, to a limited degree, on close relatives of purple loosestrife. The rationale for their release is based on perceived risk to these native plants from purple loosestrife itself. Because these native plants cannot compete with purple loosestrife the risk to these plants is greater if we do nothing than the risk of the leaf beetles using these native plants as a food source.

Release strategies and states involved. The purple loosestrife biological control program began in the U.S. and Canada in 1992 led by a group of scientists associated with Cornell University in New York state. Since the initial release in 1992, the Minnesota program has focused on rearing insects for release in Minnesota. In 1997 the Minnesota program trained 37 groups of people (nature center naturalists, personnel from the Departments of Natural Resources and Agriculture, K-12 educators, and other land managers) how to rear the leaf beetles and release them in wetlands. These cooperators reared an estimated 1.2 million insects and we now have over 200 sites in Minnesota where insects have been released. By the year 2000 we anticipate that all major watersheds in Minnesota that are infested with purple loosestrife will have active beetle colonies present. In the North Central region there are six states that have active rearing and releasing programs and in 1997 over 2 million leaf beetles were reared and released.

Results to Date. Currently there are scattered sites in most states where the leaf feeding beetles have increased in density and their feeding has caused a notable reduction in flowering and dominance of purple loosestrife. In Minnesota, we have several sites where the decline of purple loosestrife is substantial and the native vegetation is rebounding. In wetlands where insects have been established a few years, the insects have moved to nearby purple loosestrife infested areas on their own. In 1997, we collected several thousand insects from these sites to redistribute to other areas of the state. Clearly, the insects have established reproducing populations and we have reason to hope that purple loosestrife may be the next success story in weed biological control. However, leaf beetle populations have not increased at all sites. Do they simply need more time or are other insects needed to obtain control? Only time and continued research effort will answer these questions.

Unfortunately, the other insects approved for release are not as easy to rear as the leaf beetles. Mass rearing and release programs for these insects are still under development. In the future we plan to focus our efforts on training people to collect, rear, and release the leaf beetles as well as other insects as we discover how ourselves. For more information on purple loosestrife biological control programs in your state contact your Department of Natural Resources or similar agency. If you want more specific information on the purple loosestrife biological control program, visit the Purple Loosestrife InfoCentre web site developed and maintained by the Manitoba Purple Loosestrife Project.

- David W. Ragsdale, Dept. of Entomology, Elizabeth Stamm Katovich and Roger Becker, Dept. of Agronomy and Plant Genetics, University of Minnesota

Literature Cited:

Anderson, N. O., and Ascher, P. D. 1993. Male and female fertility of loosestrife (Lythrum) cultivars. J. Amer. Soc. Hort. Sci. 118(6): 851-858.

Hight, S. D., Blossey, B., Laing, J., and Declerck-Floate, R. 1995. Establishment of insect biological control agents from Europe against Lythrum salicaria in North America. Environ. Entomol. 24(4): 967-977.

Mal, T. K., Lovett-Doust, J., Lovett-Doust, L., and Mulligan, G. A. 1992. The biology of Canadian weeds. 100. Lythrum salicaria. Can. J. Plant Sci. 72: 1305-1330.

Malecki, R. A., Blossey, B., Hight, S. D., Schroeder, D., Kok, L. T., and Coulson, J. R. 1993. Biological Control of Purple Loosestrife: A case for using insects as control agents, after rigorous screening, and for integrating release strategies with research. Bioscience 43(10): 680-686.

Rawinski, T. J., and Malecki, R. A. 1984. Ecological relationships among purple loosestrife, cattail and wildlife at the Montezuma national wildlife refuge. NY Fish and Game Journal 31: 81-87.

Stuckey, R. L. 1980. Distributional history of Lythrum salicaria (purple loosestrife) in North America. Bartonia 47: 320.

Thompson, D. Q., Stuckey, R. L., and Thompson, E. B. 1987. Spread, impact, and control of purple loose-strife (Lythrum salicaria) in North American Wetlands. U.S. Dept. Interior Fish and Wildl. Service. 55pp.


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