Miscellaneous Facts about Purple Loosestrife

Our intent here is to throw out some miscellaneous facts, some of which might be useful in the design of teaching units about the spread or control of an invasive species.

Pollination Requirements of Purple Loosestrife
Several people have asked about the "complex pollination requirements" of purple loosestrife. The genus Lythrum is peculiar and unique, as there are three distinct types of flower, each with a different arrangement of the stamen and pistils. Each purple loosestrife plant has only one type of flower, and must be cross-pollinated by one of the other types, to produce viable seeds. This explains why it sometimes takes many years for one or two plants to establish a major infestation. It also explains why the so-called "sterile" horticultural varieties are not truly sterile. Each variety may, in fact, be self-sterile (that is, it cannot cross-pollinate with one of its own), but if another variety or wild plant with a differing flower configuration is in the area, it could become pollinated and produce seeds.  It also explains why it is so important to remove the plant early, when only a one plant is present, since viable seeds are less likely.    Darwin loved this plant with its peculiar pollination properties, and is quoted in a letter to his friend Asa Gray "I am almost stark, staring mad over Lythrum.  For the love of Heaven, have a look at some of your species, and if you can get me some seed, do"

How much of a threat to wetlands?
Purple loosestrife is partly credited with decline of a threatened bulrush (Scirpus longii) in MA, the rare dwarf spike-rush (Eleocharis parvula) in NY, the endangered bog turtle (Clemmys muhlenbergii) in the northern part of its range, and makes habitat unsuitable for breeding by the black tern (Chilonius niger). Loss of biodiversity and habitat can be observed. (see  Assessment and Management of Plant Invasions, Luken, JO and Thieret, JW editors,1997, Springer -- this book is in Dana Library, Dartmouth campus) 

Life of the Purple Loosestrife Plant (Lythrum salicaria
A mature plant can occupy a large space, both above and below the ground: a single mature plant can produce 30-50 herbaceous stems that rise to above 2 meters, and the entire plant can be 1.5 meters across. The rootstocks can grow to more than .5 meters in diameter (acting as storage organs for carbohydrates, allowing for rapid resprouting after defoliation or cutting). A mature plant can produce as many as 2.7 million seeds, which are light in weight, easily dispersed by wind or water or mud clinging to wildlife (or vehicles). There is an average of 120 seeds per seed capsule, and up to 900 capsules per plant. Seeds have a high germination rate, up to 84% in diffuse light conditions, which can result in as many as 20,000 seedlings per square meter . Seeds can remain dormant for three years with no effect on germination, and researchers found 40% of Purple Loosestrife seeds still viable even after eight years in the seed bank. Seeds can germinate in alkaline or acidic soils, and soils which are nutrient rich or nutrient poor. Light requirements for germination are minimal. Temperature at soil surface is critical: seeds will germinate in a range from 15 to 20 degrees C. Seedlings that germinate in the spring grow rapidly and can produce a floral shoot up to 30 cm in length the first year. Summer-germinated seedlings develop only five or six pairs of leaves before the end of the growing season. Not surprisingly, spring germinated seedlings have a higher survival rate than summer-germinated seedlings. Open grown shoots have a greater reproductive output than shoots growing in dense stands. [Data compiled from literature survey in Element Stewardship Abstract for Lythrum Salicaria, prepared by the Nature Conservancy's Stewardship staff. 1988]

Life of the Beetles -- Galerucella pusilla and G. calmariensis
These two beetles have been introduced to control Purple Loosestrife, and are very similar but distinct species, and co-exist within the same niche on the plant, and are difficult to distinguish as they have very similar color (light brown), size (4 - 6 mm) and shape. Adults emerge in the spring when plants are at least 6-8 inches high, start feeding immediately and lay eggs on leaves and stems within 7-10 days of emergence. Adults live up to 40 days, and in that time can lay as many as 10 eggs/day. The eggs are usually in clutches of 6-7, laid over with frass. Eggs then hatch 2-3 weeks after they are laid. Newly hatched larvae feed at the tips of the plants, and destroy growing tissue. As the larvae mature through five instar stages, they begin a "window-pane feeding" on the leaves and stem, leaving a skeletonized portion of the leaf. Complete larvae development takes 2-3 weeks. The mature larvae then crawl down the stem and into the soil to pupate, and after 2-3 weeks adults emerge. The adults occasionally sexually reproduce prior to diapause, but most wait until the next year. These new adults feed for the remainder of the growing season, then burrow into the leaf layer to overwinter. Shorter days in the fall, as well as a decrease in food, may induce the beetles to diapause, and activity level of Galerucella spp are positively correlated to sunlight. Because beetles only reproduce once a year, sometimes we don't see the effect of bio-control until 3 - 8 years after the release when beetle populations are large enough to have a significant impact on PL. Beetles reared under controlled conditions, protected from predators by net sleeve bags, are capable of reaching 500-1000 new beetles in a 6-8 week period, starting from an initial supply of just 25 beetles. 

[source:Blossy, B.; Schroeder, D.; Hight, SD.; and Malecki, R.A. 1994. Host Specificity and Environmental Impact of Two Leaf Beetles (Galerucella calmariensis and G. Pusilla) for the Biological Control of Purple Loosestrife (Lythrum salicaria). Weed Science 42:134-140     We have a copy of these papers]