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Purple Loosestrife
Taxonomy:
- Lythrum salicaria L. (Lythraceae)
Origin:
- L. salicaria, a plant of
European origin, has spread and degraded temperate North American
wetlands since the early nineteenth century. The plant was
introduced both as a contaminant of European ship ballast and as
medicinal herb for treatment of diarrhea, dysentery, bleeding,
wounds, ulcers and sores.
Ecology:
- L. salicaria is a herbaceous,
wetland perennial that grows in a wide range of habitats.
Established plants can reach heights of 2m with 30-50 stems forming
wide-topped crowns that dominate the herbaceous canopy. One mature
plant can produce more than 2 million seeds annually. Seeds are
easily dispersed by water and in mud adhered to aquatic wildlife,
livestock and people. High temperatures (>20°C) and open, moist
soils are required for successful germination and seedling densities
can approach 10,000-20,000 plants/m2. A woody rootstock
serves as a storage organ, providing resources for growth in spring
and regrowth if the above-ground shoots are cut or damaged.
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Distribution and Spread:
- By the 1830's, L. salicaria was
well established along the New England seaboard. The construction of inland
canals and waterways in the 1880's favored the expansion of the plant into
interior New York and the St. Lawrence River Valley. The continued expansion
of L. salicaria coincided with increased development and use of
road systems, commercial distribution of the plant for horticultural
purposes, and regional propagation of seed for bee forage. As of 1996,
L. salicaria is found in all contiguous states (except Florida) and all
Canadian provinces.
Problem:
- Invasion of L. salicaria into a
wetland can result in the suppression of the resident plant
community and the eventual alteration of the wetland's structure and
function. Large monotypic stands of L. salicaria jeopardize various
threatened and endangered native wetland plants and wildlife by eliminating
natural foods and cover. Dense plant establishments in irrigation systems
has impeded the flow of water.
Previous Control Methods:
- No effective method is available to control
L. salicaria, except where it occurs in small localized stands and can
be intensively managed. In such isolated areas, uprooting the plant by hand
and ensuring the removal of all vegetative parts can eliminate L.
salicaria. Other control techniques include water-level manipulation,
mowing or cutting, burning, and herbicide application. These control methods
are costly, require continued long-term maintenance and, in the case of
herbicides, are non-selective and environmentally degrading.
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Biological Control:
- Ideally, natural enemies, as well as
competition with other plants, prevent many plants from expanding their
distributions. In turn, the abundance of the plant (acting as a host)
influences the abundance of its natural enemies. L. salicaria was
introduced to North America without its natural enemies and the objective of
our program is to restore the self-regulatory potential of this plant-insect
interaction by using biological weed control. Four host specific insect
species approved by USDA-APHIS have been released in the US. These species
are
Hylobius transversovittatus, a
root-mining weevil,
Galerucella calmariensis and
Galerucella pusilla, two
leaf-eating beetles, and
Nanophyes marmoratus a
flower-feeding weevil. Nanophyes brevis, a seed feeding weevil, has been
approved for introduction, however, European specimens are infested with a
nematode, and this infection has prevented its introduction. Although
infested adults of N. brevis do not show reduced life-spans or
increased mortality and females lay fertile eggs, the potential for harmful
effects of the nematode to indigenous North American insects exists. At
present we have stopped any attempts to introduce N. brevis. In the
biocontrol program targeting L. salicaria, our strategy is to
achieve long-term control, not complete eradication, through provision of a
simple, yet diverse, collection of natural enemies. Historically,
introduction strategies for biological control agents have ranged from
single-species to multiple-species releases. Through evaluation of single-
and multiple-species releases, basic investigations of the biology and
ecology of target plant and control agents, and follow-up
monitoring, we hope to refine current
methodologies for selecting natural enemies for the biological control of
invasive, non-indigenous plants. These studies should allow to improve the
scientific basis of biological weed control and help to establish this
technique as a viable and environmentally friendly alternative to
conventional control measures for the next century.
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©2002
Bernd Blossey |
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