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Still
at Large
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Innocent
in appearance, purple loosestrife is one of America's
best-known alien invaders.
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Another
native of Europe, the infamous purple loosestrife is a familiar
site along highways all over North America. It arrived in
the eastern United States in the 1800's, either as an ornamental
plant or as an unwanted hitchhiker in the ballast of ships.
Covering nearly half a million acres of land in North America,
the perennial poses a serious threat to endangered wetland
habitats. The hardy purple weed edges out native plant species,
as well as the insects, fish, amphibians, birds and animals
adapted to feed and live among them.
Though
more than one hundred insects feed on purple loosestrife in
its native Europe, these natural enemies had to be screened
carefully before being released in North America. "Feeding
trials" began in 1987, exposing the candidate insects to native
wetland species and important agricultural species to ensure
that an introduced insect would not itself become a problem
invasive species.
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Though more than one hundred insects feed on purple
loosestrife in its native Europe, these natural enemies
had to be screened carefully before being released in
North America.
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After
rigorous testing, the United States and Canada approved the
release of five species of beetles in 1992. As of 1996, three
insect species have been released in 25 U.S. states and in
seven Canadian provinces. One species feeds on the foliage,
another lays its eggs in the stem and the third eats the plant's
roots. Though scientists expect the insects to reduce purple
loosestrife populations by up to 80%, they don't expect to
see this reduction for at least another ten to twenty years.
The verdict is still out, therefore, on the overall success
of this program. However, there have been local successes
at insect release sites in New York, Minnesota, Ontario and
Illinois- where the insects reduced the loosestrife populations
by 97% within three years of release.
Looking
Ahead
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Loosestrife-munching
beetles may be the only way to control this prevalent
weed.
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Seven thousand alien species currently live in the United
States, some of which can have severe negative economic and
ecological consequences if allowed to thrive unchecked.
"Invasive species aren't like a wild fire that blows up in
your face," says Noah Poritz of Biological Control of Weeds,
Inc. "It's a quiet fire burning with equally devastating effects."
Biological control has obvious advantages over other forms
of control. It's cheaper and less toxic than pesticide use,
and it's far less labor intensive than removing offending
foreigners by hand; however, biological control is not a magic
bullet.
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"Invasive species aren't like a wild fire that blows
up in your face," says Poritz. "It's a quiet fire burning
with equally devastating effects."
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For one thing, ecologist Daniel Simberloff of the University
of Tennessee at Knoxville estimates biological control works
just one time out of ten. And it can require years of careful
research and thoughtful planning in order to avoid creating
a bigger problem than the one it's meant to solve.
"Biological control should be the method of last resort,
rather than first," says Simberloff. "Its reputation as an
environmentally friendly means of control is largely undeserved."
In many cases, some combination of biological control, mechanical
removal and pesticides will work best to eradicate unwanted
species. But, for now, the most effective way to avoid the
ecological and economic devastation caused by invasive species
is to keep them out in the first place. 
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Photos:
Corey Lindgren, Manitoba Purple Loosestrife Project
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