Purple Loosestrife Essay, Research Paper
Purple Loosestrife
The scene is breathtakingly beautiful, a thick brush of purple flowers blankets
Canada’s wetlands. This blanket silences the expected sounds of the wetland
environment, birds chirping, ducks splashing, insects buzzing and animals
thriving. This unnatural silence is disturbing, the favourite flowers that used
to litter this landscape are no longer visible, the water that used to ripple
continuously is perfectly still. The wetland is dead, except for this
overpowering, hardy purple flower that has choked out all other vegetation and
species. Purple loosestrife now controls this landscape.
Purple loosestrife is an exotic species that was introduced to North America
from Europe during the early 1800’s. Europeans sailing to North America would
fill their ships ballast with wet sand taken from shores of Europe, a habitat
where purple loosestrife thrived. Upon arrival in North America the ballast
would be dumped overboard on the shoreline. By 1830 the plant was well
established along the New England seaboard. Purple loosestrife seeds were also
found in sheep and livestock feed that was imported from Europe during this
period. This new organism was introduced to a new habitat free from traditional
parasites, predators and competitors, purple loosestrife thrived in the
environmental conditions and by 1880 was rapidly spreading north and west
through the canal and marine routes. Purple loosestrife stands also increased
due to the importation of seeds and root stalks by horticulturists. It was
introduced to many communities as an herb, an ornamental garden flower and as a
desirable honey plant.
One of the earliest reported studies of purple loosestrife being a problem in
Canada was documented by Mr. Louis – Marie, in 1944. He stated that purple
loosestrife was invading the St. Lawrence flood plain pastures between Montreal
and Quebec. At that time Louis – Marie conducted a study to find suitable
control methods for purple loosestrife. His results indicated that repeated
mowing, continuous grazing, deep discing and harrowing were effective in keeping
the spread of purple loosestrife controlled on agriculture land. Since the
1940’s purple loosestrife infestations have increased greatly and the plant is
now a major problem threatening many wetland ecosystems across North America.
Figure 1 – Purple loosestrife flowers.
(Parker 1993)
Lythrum Salicaria, commonly known as purple loosestrife belongs to the
Lythraceae family, which consists of 25 genera and 550 species worldwide. The
genus Lythrum consists of thirty – five species, two of which are located in
North America, Lythrum Purish which is native to the continent and the invasive
purple loosestrife. Through cross breeding, purple loosestrife is quickly
overtaking Lythrum Purish and causing a decrease in native species. “The
generic name comes from the Greek luthrum, blood, possibly in reference to the
colour of the flowers or to one of it’s herbal uses, as an astringent to stop
the flow of blood.” (Canadian Wildlife Federation 1993, 38) Purple loosestrife,
an aggressive, competitive, invasive weed often grows to the height of a human
and when it is mature can be 1.5 metres in width. The stalk of the plant is
square and woody and may grow to 50 centimeters in diameter. The perennial
rootstock can give rise to 50 stems annually which produce smooth edged leaves
on oppositesides of the stalk. Purple loosestrife flowers are long pink and
purple spikes which bloom from June to September (Figure 1). One purple
loosestrife plant alone is solid and hardy but when this plant invades an area
it creates a “dense, impermeable stands which
Figure 2 – Purple loosestrife growing in a typical habitat.
(Parker 1993)
are unsuitable as cover, food or resting sites for a wide range of native
wetland animals…” (Michigan Department of Natural Resources 1994). Due to the
lack of predators which feed upon purple loosestrife, this dominant plant has an
advantage when competing against most other native wetland species for food
sunlight and space. These advantages allow purple loosestrife to create dense,
monotypic stands which reduce the size and diversity of native plant populations.
Purple loosestrife can also grow on a range of substrates and under nutrient
deficit conditions. It has the ability to regenerate quickly after cutting or
damage and can withstand flooding once adult plants have been established. There
are no native species that are as hardy as purple loosestrife, therefore without
competition and predators the wetland ecosystem cannot control the spread of
purple loosestrife.
Purple loosestrife is now found world wide in wet, marshy places, coastal areas,
ditches and stream banks. (See Figure 2) It is prevalent in most of Europe and
Asia, the former USSR, the Middle East, North Africa, Tasmania, Australia and
North America. It has not been found in cold Arctic regions. In North America
purple loosestrife is located between the Canadian territories and north of the
35th parallel with the exception of Montana. The most serious infestations are
found in the wetlands of Southern Quebec and Ontario and along the Red and
Assiniboine Rivers in Manitoba. Second to Manitoba, British Columbia has the
next largest purple loosestrife infestation, weed populations are reported from
Vancouver Island to the lower Fraser and Okanogan Rivers south of Penticton. In
Saskatchewan and Alberta. small, isolated stands of purple loosestrife are
reported and the Atlantic Provinces are quickly being invaded. Currently areas
that are sensitive to the new invasions are the salt and freshwater marshes in
the Maritimes. (See Figure 3)
Figure 4 – Purple loosestrife seedling.
(Parker 1993)
Regardless of the purple loosestrife location, one of the main reasons for the
rapid infestations is due to the plants prolific seed production and
reproduction cycle. “It has been estimated that a mature plant can produce 2.7
million seeds per growing season” (DeClerck-Float 1992,15). Purple loosestrife
seeds are small and easily transported by water or by mud that attaches to the
feet of birds or off road vehicles. The seeds remain dormant over the winter and
germinate in late spring or early summer. They are capable of germinating in
either the mud or when submerged under water providing the water temperature is
between 15 – 20 oC and there are adequate light levels (See Figure 4). Through
experiments performed by S.R.A. Shamsi and F.H. Whitehead, it has been
determined that “prolonged seed dormancy may be possible, since seeds stored for
three years in a refrigerator at 3 – 4 oC were still useable and had a
germination rate of 80% ” (DeClerck-Float 1992,15). The production of purple
loosestrife seeds and their exclusive characteristics allow the plants to
develop large seed banks at a site which is a factor that makes purple
loosestrife so difficult to control. The plant has the ability to reproduce from
the seed bank. Purple loosestrife can also spread vegetatively by adventitious
shoots and roots from clipped or tramped plants. Any part of the plant that
falls to the ground, even from a wheelbarrow, can develop into a plant. This
shows the plant’s desire to live no matter what obstacles it faces, again making
it very difficult to control. Purple loosestrife plants have three style lengths
(short, mid, long) and three strengths (short, mid, long). Pollination occurs
between plants with the same style and stamen length. Purple loosestrife flowers
have of one style length and the two sets of stamens are different lengths,
therefore a plant is technically self – incompatible. “However, Ottenbriet
(1991), found that the self – incompatibility system is not strict, as mid -
styled plants showed a highdegree of self fertility with themselves and other
mid – styled plants.” (DeClerck – Float 1992, 16) This proves, it is not safe to
plant self – incompatible purple loosestrife, there is a risk of pollination
which will lead to further distribution of the plant. This misconception is a
problem because nurseries are selling self – incompatible plants as garden
flowers which reproduce with themselves or with other species from the
loosestrife family creating more invasive stands.
Purple loosestrife’s hardy, competitive and reproductive characteristics
classifies it as a large environmental concern. The plant is threatening wet
lands, decreasing water foul population, clogging irrigation systems and
becoming a threat to the fisheries. “Mosquin and Whiting (1992) regard purple
loosestrife to be one of the five invasive alien plants that have had a major
impact on natural ecosystems in Canada.” (Canadian Wildlife Federation 1993,41)
Canadian wetlands are rapidly being over taken by purple loosestrife, large
stands of the plant displace native species that can’t compete against this
exotic species. The loss of native flora and fauna means the loss of habitat and
food for wetland animals, this destroys the well balanced, wetland ecosystem.
Across the Maritimes, prarie sloughs are becoming increasingly infested with
purple loosestrife thus destroying the breeding ground of many North American
waterfowl. This additional stress compiled with urbanization and pollution could
cause theextinction of North America’s waterfowl population. The invasion of
purple loosestrife across the Maritimes is causing extra labor for farmers as
well as an increased cost because the plants are clogging the irrigation systems.
In B.C. purple loosestrife is invading the salt water shores and is becoming a
threat to the fisheries. The overpowering stands of purple loosestrife are
increasing costs and frustrations for many industries across Canada.
On the contrary, bee keepers and hortic
purple loosestrife. Bee keepers favour purple loosestrife because the plant
forms dense stands and large quantities of pollen in July and August. Purple
loosestrife is one of the few plants producing large amounts of nectar during
the late summer. The downfall to purple loosestrife honey is that it is ill
tasting and greenish, although this can be diluted by the good nectar from other
flowers. Canadian bee keepers do not want purple loosestrife to spread for fear
of losing the nectar from the good flowers but they also don’t want to lose the
large quantities of nectar obtained from purple loosestrife. Horticulturists
favour purple loosestrife as a garden perennial in the prarie provinces. It is
favoured because it’s both showy and hardy and able to withstand the fluctuating
climate. Horticulturists are finally realizing that the pros of purple
loosestrife as a garden perennial are far outweighed by the cons of purple
loosestrife as an exotic invader.
The most pressing question with regards to purple loosestrife right now is, how
can we control it? Studies have been conducted since 1941 with the aim of
finding effective control processes – one has still not been found. To gain
control over purple loosestrife and to reduce it’s impact on the environment
three goals that must be attained: 1) Eliminate the species from highly
significant sites where a low infestation is present. 2) Eliminate the species
in geographical areas where it is just beginning to establish itself. 3) Contain
the plant in large sites in order to slow down it’s spread. By achieving these
goals the impact of purple loosestrife across Canada will be stabilized until an
effective biological control agent is found. (Canadian Wildlife Federation 1993,
41)
There are three forms of control used on plant species, cultural control,
chemical control and biological control. Cultural control involves manual labor
such as mowing, cultivating, inundation, hand pulling, shearing, fire and
flooding. Each method is moderately successful depending on the specific
situation. Mowing, cultivating and inundation are not suitable control
mechanisms for purple loosestrife in many natural areas because by destroying
the exotic plant you also kill the struggling native species. In private areas
which are overrun with purple loosestrife these methods will reduce the spread
of seeds but will not kill the plants and therefore they will return the
following year. Hand pulling and shearing are only suitable for very limited
infestations due to their labor intensive nature. For these methods to be
effective all roots, stems, leaves and flowers must be removed and destroyed.
Fire has proven to be an ineffective method of control because the purple
loosestrife root crown iswell protected below the surface, the hot fire that is
necessary to kill the crown cannot be created. Flooding as a method of control
has proven redundant against mature plants. Adult purple loosestrife plants can
survive in water levels of 90cm. Flooding does however affect immature plants
but the water levels must be extremely high and it appears to take several years
to have an appreciable affect in the reduction. Unfortunately flooding will
also have a serious effect on native flora and fauna. Cultural control is both
labor intensive and not very productive.
Chemical controls for purple loosestrife have been tested in both Canada and the
USA but no herbicides have been accepted for use in Canada. In the USA, Rodeo,
See 2 and 4-D have been registered for use but there is limited benefit compared
to the high cost and temporary effectiveness. Canada has been testing Triclopyr
amine, which is a broad leaf herbicide, that can be used for control of purple
loosestrife. Researchers feel that it is an effective and safe product that can
be used to keep purple loosestrife in check. The largest problem when using
chemical controls is insuring that the effects of the herbicide will not
negatively effect the native species as well as purple loosestrife.
The final method of purple loosestrife control and the most promising for the
future, is biological control. This involves the introduction and management of
selected natural enemies of purple loosestrife. It is a slow process and is not
always efficient depending on the circumstances. The results are often long term
and the infested sites must be monitored for several years. Biological control
agents affect weed population indirectly by increasing the stress on the weeds
which may reduce their ability to complete with the native plants. Biological
control of purple loosestrife was initially investigated by the International
Institute of Biological Control (IIBC) in Europe. The USA contracted the
institute to conduct a study of possible biological control agents that could be
used to control purple loosestrife. (Canadian Wildlife Federation 1993, 42) As a
result of this study three insects were approved for release in the USA in June
of 1992 and at this time the insects were also released into field trials in
Canada. These three insects are Hylobius Transversouittatus, Galerucella
Calmariensis and Galerucella Pusilla.
The Hylobius Transversouittatus is a root feeding weevil that is a parasite of
purple loosestrife. The climate in Europe, which is native to this insect is
very similar to the Canadian climate thus making it easy for the weevil to adapt.
The H. Transversouittatus larvae mine the roots and change the vascular system
which reduces seed production and germination. The adult weevils emerge in May
or June and begin laying their eggs in the roots. The females continue laying
their eggs until September thus covering 2/3 of the growing season. Over a
period of time the effect of the weevil will drastically reduce the purple
loosestrife stand. “The damage caused by the feeding of seven larvae per plant
was found to reduce seed germination by 50%.” (DeClerk – Float 1992, 10)
Similar to purple loosestrife, the H. Transversouittatus is easily adapted and
can withstand prolonged periods of flooding. The larvae do not feed off the
roots when the water levels are high, they go into diapause until the roots dry
out then they resume feeding. This weevil has only one natural enemy, the
Mymarid egg but this enemy is not parasitic and has little impact on the
population. H. Transversouittatus has been tested and results show that the
insect will not have an impact on native species growing in Canada but will have
a large impact on purple loosestrife. Feeding by the insects in high densities
causes defoliation in mature plants, kills seedlings and destroying or
preventing the formation of flower spikes. H. Transversouittatus appears to be a
very likely candidate as a biological control agent for purple loosestrife but
several years of trials will be necessary to determine it’s effectiveness. It
could take up to ten years to show it’s full potential.
Galerucella Calmariensis and Galerucella Pusilla can be classified together
because they are both leaf feeding beetles that have similar life histories,
occupy the same habitat and affect purple loosestrife in the same manner. These
two species are often found together in Northern Europe with one of the species
dominating destruction of the stand. G. Calmariensis extends farther north than
G. Pusilla and will be better suited for Canada’s northern sites of purple
loosestrife. Both species are parasites which have good host finding
capabilities. Females will move from one host to the next, once a certain level
of feeding damage has been reached, this guarantees the spread of the attack in
large purple loosestrife stands. After being put through the same tests as H.
Transversouitatus, Galerucella Calmariensis and Galerucella Pusilla were found
to be extremely host specific and do not pose a threat to native species in
Canada. In Europe these beetles are more commonly found than H.
Transversouitatus.All three of these insects appear to be very promising in
their control over purple loosestrife stands but, as mentioned earlier, it could
take a few years to notice any progress. The idea of introducing another species
to Canada’s wetland ecosystem is not approved by all due to the purple
loosestrife infestation incident. Many believe that tampering with nature is
what has caused the problems in the first place and hopefully by letting nature
run it’s course all will turn out for the best . Unfortunately this viewpoint
can not be supported for long. Canada is at a point right now that without the
biological control agents, purple loosestrfie will destroy a lot of wetland and
farmland. With biological control we can only hope that the ecosystems can be
brought back under control.
Purple loosestrife is a very serious problem. It’s rapid invasion is threatening
wetlands, waterfowl and fisheries as well as the diversity of Canada’s flora and
fauna. If this plant is not brought under control quickly then the result of
this exotic species being brought to Canada could be disastrous. The use of
cultural and chemical control has not been effective so we now rely on the
success of biological control to stop the spread of this hardy invasive plant
and to replenish the diversity of Canada’s wetland ecosystem. As a country we
must do everything we can to reduce the spread and growth of purple loosestrife.
As a concerned Canadian you can report any local purple loosestrife stands,
spread your knowledge about the problem, strongly discourage the plantings of
any new plants or the selling of the weed in nurseries and join the Ontario
Federation of Anglers and Hunters. By doing this you are donating money and
support the tests that are being conducted. We must work together to remove the
purple blanket that silences our wetlands.
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