Purple loosestrife
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| Lythrum salicaria L. |
Purple loosestrife (Lythrum salicaria) is a semi-aquatic herbaceous plant belonging to the loosestrife family, Lythraceae, native to the wetlands of Eurasia.
It is a herbaceous perennial plant, growing 1-2 m tall, forming clonal colonies 1.5 m or more in width with numerous erect stems growing from a single woody root mass. The stems are reddish-purple and square in cross-section. The leaves are lanceolate, 3-10 cm long and 1-2 cm broad, downy and sessile, and arranged opposite or in whorls of three. The flowers are reddish purple, 10-15 mm diameter, with six petals (occasionally five), and are clustered tightly in the axils of bracts or leaves. The fruit is a small 3-4 mm capsule containing numerous minute seeds. Flowering lasts throughout the summer. When the seeds are mature, the leaves often turn bright red through dehydration in early autumn; the red colour may last for almost two weeks. The dead stalks from previous growing seasons are brown.
Other names include spiked loosestrife, purple lythrum, or salicaire. It should not be confused with other plants sharing the name loosestrife that are members of the family Primulaceae.
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[edit] Cultivation and uses
Purple loosestrife has become an invasive species since its introduction into temperate New Zealand and North America where it is now considered a noxious weed. The seeds probably first arrived in the plant's non-native areas in muddy ballast water emptied from ships. It has also been used as a medicinal herb and cultivated as a garden plant. The flowers are quite showy and bright, and monotypic fields of purple loosestrife are deceptively attractive. The truth is that purple loosestrife has had a very destructive impact on North American wetland ecology since the early 19th century.
The plants grow vigorously and spread very fast when removed from their natural controlling agents. Infestations result in dramatic disruption in water flow in rivers and canals, and a sharp decline in biological diversity. Native food and cover plant species, notably cattails, are completely crowded out, and the life cycles of organisms from waterfowl to amphibians to algae are affected. There is, however, a confounding view to the latter claims, namely that they are not supported by conclusive scientific evidence, but rather an overwhelming number of subjective observations (Hager and McCoy 1998).
Purple loosestrife is still sold in some areas as an ornamental. Plants marketed under the name European wand loosestrife (L. virgatum) are in fact the same species as purple loosestrife despite the different name. In some cases the plants sold are assumed to be sterile; in fact, this is rarely the case. The sale of purple loosestrife is illegal in many areas. Its detrimental effects are simply too costly to risk.
In North America, purple loosestrife may be distinguished from similar native plants (e.g. fireweed Epilobium angustifolium, blue vervain Verbena hastata, Liatris Liatris spp., and spiraea Spiraea douglasii) by its angular stalks which are square in outline, as well by it leaves, which are in pairs that alternate at right angle and are not serated.
[edit] Biological control
A single plant may produce up to three million tiny seeds annually. Easily carried by wind and water, the seeds germinate in moist soils after overwintering. The plant can also sprout anew from pieces of root left in the soil or water. Once established, infestations are extremely difficult and costly to remove by mechanical and chemical means.
Purple loosestrife provides a model of successful biological pest control. Research began in 1985 and today the plant is managed well with a number of insects that feed on it. Four species of beetle use purple loosestrife as their natural food source and they can do significant damage to the plant. The beetles used as biological control agents include two species of leaf beetle and two species of weevil.
- The black-margined loosestrife beetle Galerucella calmariensis is a brown beetle with a black line on its thorax. The adult feeds on the leaves of the plant, producing characteristic round holes. Its larvae destroy tender leaf buds and strip the tissue from the leaves.
- The golden loosestrife beetle Galerucella pusilla is nearly identical to G. calmariensis, but usually lacks the black thoracic line. Its feeding habits are also quite similar to the other leaf beetle. An infestation of either of these insects is extremely effective in wiping out a stand of purple loosestrife, defoliating up to 100% of the plants in an area.
- The loosestrife root weevil Hylobius transversovittatus is a large red nocturnal weevil, which spends its nights feeding on leaves and leaf buds. The larvae emerge from their eggs and immediately burrow into the root of the plant, which they feed on continuously for over a year. This root damage stunts the plant's growth and ability to create seeds. If several larvae inhabit the same root, the plant can be killed.
- The loosestrife flower weevil Nanophyes marmoratus is a tiny weevil which lays a single egg in each flower. When the larvae emerge they eat the flowers' ovaries, and the plant is unable to create seed. The larvae usually proceed to hollow out the flower buds and use them as safe places to pupate.
[edit] External links
[edit] Further reading
Wilson, L.M., Schwarzlaender, M., Blossey, B., & Randall, C.B. (2004). Biology and Biological Control of Purple Loosestrife. Morgantown, WV: Forest Health Technology Enterprise Team, United States Department of Agriculture.
Hager, H. and McCoy, K. 1998. The implacations of accepting untested hypothesis:a review of the effects of purple loosestrife (Lythrum salicaria) in North America. Biodiversity and Conservation 7: 1069-1079.
