Invasive Species in the Chesapeake Watershed
What is Lythrum salicaria?
Lythrum salicaria is an erect herbaceous perennial with a strong taproot. Height ranges from 0.5 to 2.5 m. Stems vary from glabrous (smooth, hairless) to pubescent (covered with fine hairs). Each root crown supports 30 to 50 stems. Leaves, arranged opposite or in whorls of 3, are lanceolate to oblong with cordate bases. The spike-like inflorescence is 10 to 40 cm in length; each flower has 5 to 7 petals. Floral masses are typically magenta, but white or pink flowers may also be observed. Flowering occurs from late June to early September in most areas (Thompson et al. 1987, Balogh 1985, Rawinski 1982, Gleason 1957, Fernald 1950).
Native to Europe, purple loosestrife is distributed in the temperate northern hemisphere (below 65°N), some subtropical climates and Australia. Populations of purple loosestrife have not been confirmed in South Africa or South America (Thompson et al. 1987).
At maturity, seed production is estimated to be 2.7 million seeds per plant (Thompson et al. 1987). Weighing 0.05 to 0.06 g each, seeds are viable within 3 weeks of anthesis (flowering) (McCaughey and Stephenson 2000, Shamsi and Whitehead 1974). Wind, mud, wildlife and human-associated transport are vectors of seed dispersal (Thompson et al. 1987, Shamsi and Whitehead 1974). However, wind probably plays a limited role in seed transport. Thompson et al. (1987) observed declines in seedling density within 10 m of the parent plant, and a tendency towards down slope versus downwind transport.
Optimal germination occurs between pH 4.0 to 9.1 at temperatures between 15 to 20°C. Moisture is considered to be the most important determinant of growth and reproduction, but germination occurs across a variety of substrate conditions (Thompson et al. 1987, Balogh 1985, Shamsi and Whitehead 1974). In a study of seedling recruitment of wetland plant species, water level gradients between -5 to 10 cm yielded no significant differences among the percent of seedling germination (Keddy and Ellis 1985). Established seedlings are also capable of surviving shallow (30 to 45 cm depth) flooding (Thompson and Stuckey 1980). Seedling densities may approach 10,000 to 20,000 plants/m2; growth rates may exceed 1cm/day (Thompson et al. 1987, Rawinski 1982). The duration between germination and flowering is 8 to 10 weeks (Rawinski 1982). Growth and development are nitrogen-limited; decreases in nitrogen increase shoot to root ratios (Shamsi and Whitehead 1977).
Purple loosestrife colonizes both brackish and freshwater habitats, spreading reproductively and vegetatively from lateral shoot meristems (Stevens et al. 1997, Thompson et al. 1987). It commonly occurs with Typha sp., reed canary grass, sedges and rushes (Thompson et al. 1987). Although purple loosestrife primarily invades disturbed wetlands, it also becomes established among natural wetlands, wet meadows, swamps, riverbanks and edges of ponds and reservoirs (Rawinski 1982). While mammalian herbivores may prevent production of terminal inflorescence, formation of monospecific stands often occurs, due to a lack of native herbivores and host-specific pathogens (Rachich and Reader 1999, Hight 1990). Monospecific stands in the Northeast are capable of self-replacement for at least 20 years (Thompson et al. 1987).
A contaminant of European ship ballast, purple loosestrife was well established along New England coasts by the 1830s. Lythrum salicaria was also imported as a medicinal herb for the treatment of diarrhea, dysentery, bleeding, wounds, ulcers and sores (Malecki et al. 1993). Conversion of wetlands to agricultural lands and construction of canals for waterborne commerce facilitated the inland spread of L. salicaria (Thompson et al. 1987). Intentional introductions have also enabled purple loosestrife to achieve a broad distribution across the U.S. It was commonly planted in Virginia's English style gardens, and naturalization by beekeepers may have contributed to its westward spread (Thompson et al. 1987). The rate of distribution has increased exponentially since the 1880s, with a marked acceleration around 1940. In a survey of four northeastern and Midwest states, Thompson et al. (1987) estimated the rate of expansion in natural habitats to be 1,157 km2/yr between 1940 and 1980.
Seed mixes and commercial cultivars are another source of North American introductions. In a survey of commercial wildflower and native prairie seed mixes, 10% of the 25% of seed mixes containing non-native seed species also contained L. salicaria (Wade, as cited in Thompson et al. 1987). Pollen and seed from sterile cultivars may also contribute to the spread of purple loosestrife. When L. virgatum, commercially sold as "Morden Pink," was transplanted into wild stands of L. salicaria, Lindgren and Clay (1993) found evidence of cross-pollination. Using a tetrazolium test, 83% of the seeds collected from Morden Pink transplants were viable. Anderson and Ascher (1993) found similar evidence of cross-pollination among male and female loosestrife cultivars crossed with L. salicaria . Seed germination rates ranged from 30 to 100%.
Local nursery associations such as the Virginia Landscape and Nursery Association and the Maryland Nurserymens Association do not advocate loosestrife varieties in their buyer's guides; however hybrids, cultivars and seeds continue to be commercially available on the Internet. Although European companies primarily advertise seeds, seedlings are available from nurseries outside the Chesapeake Bay region. (Surprisingly, one Virginia nursery recommended Morden Pink as a choice perennial for Washington gardens.)
Wetland disturbance increases susceptibility to purple loosestrife invasions. In a comparison of L. salicaria seedling germination among disturbed and undisturbed plots of Phalaris arundinaceae, Rachich et al. (1999) observed >50% establishment in disturbed plots. Seedlings did not become established in undisturbed plots of P. arundinaceae. Mixing of genotypes may also be a factor in the invasiveness of loosestrife across North America. Repeated ballast introductions originating from multiple European ports combined with the cross-pollination of cultivars and wild species may have increased the adaptability of L. salicaria to differing climate and hydrologic regimes (Thompson et al. 1987).
In contrast to environmental disturbance and gene flow resulting from cross-pollination, flood tolerance is not a significant predictor of invasion. In a comparison of six Lythraceae species, increased plant height and development of an aerenchymatous phellem were consistent across all six species (Lempe et al. 2001). Morphological adaptations to flooding were not species-specific, which suggest that invasiveness is not attributed solely to flood tolerance in L. salicaria .
Where is Lythrum salicaria Found in the Chesapeake Bay Watershed?
L. salicaria has been reported from 15 Maryland counties; 19 individual sites have been confirmed by the Department of Agriculture. In counties where purple loosestrife has been detected but sites not identified, reports were received from reliable sources, though they have not been verified with GPS or mapping (Dick Bean pers. comm.).
Purple loosestrife occurs throughout Pennsylvania, particularly in the Susquehanna River Basin and south central counties. Total acreage in Pennsylvania is unknown; however biological control has been implemented at 35 sites in 21 counties since 1991 (Will Mountain pers. comm.).
Based on state herbarium records and field surveys, purple loosestrife has been identified at 25 sites statewide. Although present along Virginia's coastal plain, purple loosestrife is most abundant in Northern Virginia. While existing infestations appear to be expanding, few new introductions are occurring (Steve Capel pers. comm.). Purple loosestrife has not been recorded from National Wildlife Refuges in Virginia and Maryland (Jan Taylor pers. comm.).
Purple loosestrife is present in freshwater tidal marshes along the Anacostia River watershed. While percent coverage has remained minimal (~5%) in Kenilworth Marsh, purple loosestrife is a dominant plant in a newly reconstructed wetland at Kingman Lake. Dominance at Kingman Lake may be due to the fact that resident geese foraging on marsh plantings do not target L. salicaria (Dick Hammerschlag pers. comm.).
Are There Existing Management Efforts for L. salicaria in the Chesapeake Bay Watershed?
Purple loosestrife, hybrids and cultivars are regulated as noxious weeds in Virginia (§3.1-296.11 et seq.) and Pennsylvania (3 P.S. 255.1 et seq.), but are not listed in Maryland (Dick Bean pers. comm.). The Virginia law declares it illegal to move, transport, deliver, ship or offer for shipment into the state. The Pennsylvania law prohibits sale, transport, planting and propagation. Although it is legal to sell L. salicaria in Maryland, individual nurseries have voluntarily discontinued its sale as a potted plant (Dick Bean pers. comm.).
Since 1999, the Maryland Department of Agriculture has been releasing and monitoring species at several locations in Howard, Prince Georges and Caroline counties. The Department's nursery inspectors continue to educate industry members about potential impacts of the non-native plant (Dick Bean pers. comm.).
Since 1995, the Pennsylvania Dept of Agriculture and USDA/APHIS have released Galerucella, Hylobius and Nanophyes for biological control of purple loosestrife (see map for locations). Success is monitored each fall via a leaf damage survey. Between 1995-2001, USDA's biological control program in Pennsylvania cost approximately $50,000 (Gary Clement pers. comm.). In a cooperative effort, the Pennsylvania Game Commission and Department of Agriculture are managing purple loosestrife infestations at Middle Creek Wildlife Management Area (Lancaster County). While Galerucella have been released, biological control has not been successful in eradication. Manual control was effective short-term, but too labor intensive. In contrast, herbicide application was successful on a small scale, and may be used more extensively in the future (Ian Gregg, pers. comm..) The Nature Conservancy is also managing for purple loosestrife at Valley Creek, Chester County (Betsy Lyman pers. comm.).
The Virginia Native Plants Society and Department of Conservation and Recreation has sponsored educational programs and workshops. The education of nursery wholesalers was largely ineffective because of turnover in management/personnel (Steve Capel pers. comm.). Virginia has also implemented a Galerucella biological control program through the Virginia Polytechnic Institute's Entomology Department and USDA/APHIS. However, their program has not been as extensive as other states due to concern over host-specificity of non-native beetles/weevils on Lythrum species. Informally, float fishermen have voluntarily removed purple loosestrife from fishable rivers during its flowering season (Steve Capel pers. comm.).
The National Park Service used a combination of biological and chemical control to treat two acres of purple loosestrife at Kenilworth Marsh, a freshwater tidal wetland in the Anacostia River watershed. Galerucella were released in 1996 and 1997, but did not overwinter successfully. Using the Blossey monitoring protocol, none of the beetles were observed the following spring. Hylobius inoculated plants were transplanted in Kenilworth Marsh in 1997, but again weevil populations did not become established. Rodeo applications were also ineffective. The estimated cost for treatment was $40,000, about $10,000 of which was supplies and materials. Despite the lack of success with biological and chemical control, percent cover has not increased significantly during the past eight years. The percent of purple loosestrife coverage remains about 5% (Dick Hammerschlag pers. comm.).
What Research and Control Efforts are Currently Underway for Lythrum salicaria?
Prior to the discovery of biological control agents, existing control technologies (manual removal, water-level manipulation, burning and herbicide application) were ineffective in eradicating large areas of purple loosestrife infestation (Malecki et al. 1993). The cost, long-term maintenance, and non-target effects of these methods prompted the investigation of plant-herbivore interactions for weed control (Blossey, webpage). Bernd Blossey coordinates the biological control program for purple loosestrife at Cornell University. The goal is to achieve long-term control of L. salicaria infestations through the use of natural enemies. Four species of host-specific herbivores have been approved by USDA/APHIS for release in the U.S. to control purple loosestrife.
Two native European beetle species, Galerucella calmariensis and Galerucella pusilla, were approved for release in 1992. Prior to their release, susceptibility of 50 native North American plants was examined. Only winged loosestrife (Lythrum alatum) and swamp loosestrife (Decodon verticillatus) were identified as potential hosts, and field experiments in Europe indicated preference for L. salicaria when available. Feeding on leaf, stem and bud tissues, Galerucella are host-specific herbivores for L. salicaria (Blossey et al. 1994). Leaf defoliation reduces aboveground biomass, and does not deplete carbohydrate reserves in root or crown tissue severely enough to yield plant mortality short-term (Katovich et al. 1999). However, leaf defoliation may decrease future seedbank replenishment. Plant defoliation as low as 10% reduces inflorescence length, number of flower buds and seed capsules (Katovich et al. 2001). Preferred release sites include areas where purple loosestrife is continuously distributed, and relatively free of standing water and shade. However, Landis and Klepinger (2000) observed slower rates of Galerucella colony establishment where water resources are scarce throughout the growing season.
Galerucella may provide a long-term solution upon establishment of beetle colonies. In a 5 to 10 acre site, colony establishment is expected to take 7 to 10 years (Weeden et al. webpage). Landis and Klepinger (2000) report 100% establishment at 23 sites within 2 to 6 years of Galerucella releases. However,
Recent research by Lindgren et al. (1999) indicates the potential for combining biological control with herbicide application to manage purple loosestrife infestations on short- and long-time scales. Galerucella exposure to 2 to 4% glyphosate ("Roundup") concentrations did not affect larval pupation, oviposition or adult survival.
The root-mining weevil Hylobius transversovittatus has also been approved as a biological control agent for purple loosestrife. Comparing the effects of root herbivory and plant competition during two growing seasons, Noetzold et al. (1998) found root herbivory to be more effective than plant competition at reducing height, biomass and inflorescence in established L. salicaria plants. As with Galerucella, Hylobius transversovittatus will not be effective in shaded areas or standing water (Weeden et al. webpage).
A flower-feeding weevil, Nanophyes marmoratus has also been approved for introduction. This species was released in Pennsylvania in 1999 (Will Mountain pers. comm.). Although a related seed-feeding weevil, N. brevis, has been approved for introduction, it has not been introduced because of nematode infestations in Europe (Blossey webpage).
In addition, Nyvall and Hu (1997) identified three species of North American fungi as potential biocontrol agents in laboratory experiments. Spores of Alternaria alternata, Botrytis cinerea and Phoma sorghina applied to L. salicaria foliage via a carrier matrix were pathogenic to 6-week old plants. Farr and Rossman (2001) have identified another potential pathogenic fungus, Harknessia lythri.
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Last modified October 10, 2012