Biocontrol of Purple Loosestrife and Other Invasive Plants
It’
like something out of a sci-fi movie. Plants that quietly invade then
completely take over whole forests or swamps or prairies. It’s not sci-fi
– it’s real. Termed invasive species, there are already numerous species doing
just that in Wisconsin natural areas and backyards. Purple loosestrife is one
such invasive plant that is partial to wetlands, streams, creeks and even
roadside ditches. Buckthorn is another. One species of this small tree prefers
woodlands and another, bogs. Garlic mustard is an herbaceous plant that thrives
in the light shade of a forest.
Invasive
is a pretty strong word to apply to the habits of a mere plant. It is an
appropriate term, however, when applied to plants with pretty strong
characteristics which they usually have in common. In all cases, these plants
are not native to the areas they are invading. That is, they are exotic.
Generally, they were brought here by people, mostly our ancestors, for various
desirable purposes.
They invade rather than just survive because their natural enemies were not
brought along with them. In their native environments, several organisms,
including insects and diseases, were present and acting to control their growth
and spread. This balance is part of the natural order of life in all
ecosystems.
Other traits invaders have in common include a very high reproductive rate,
an aggressive root system, and an ability to adapt to a wide range of growing
conditions. Consequently, just cutting them down does not kill them. They
either re-sprout from their roots or their seeds survive and germinate over a
long period. Or both.
They
also adapt well to the changes in the habitat that they themselves have
created. Changes in light, soil moisture and nutrition, and space.
Herein lies the problem. As these newcomers grow and spread, they make the
site inhospitable to the native species that once called it home. As natives
are overcome and their populations die out, the ecosystem is severely altered.
Since every ecosystem is a complex web of relationships among species of plants,
animals, insects and fungi, diversity is diminished. The result is an overall
loss of biodiversity.
The trump card in this dicey little problem is control. It is beyond
difficult.
Conventional
control measures include pulling, cutting or digging and then removing the
plants. Another is using chemical weed killers. These are all extremely labor
intensive due to the large areas usually involved by the time the problem is
apparent. Treatment must also be repeated regularly for years to come if not
indefinitely due to the re-growth from roots left after cutting and because of
new seedlings which grow from the enormous volume of seeds produced by each
individual plant. Seeds also move in from surrounding infested areas with the
help of birds, animals and water. Cut stem ends can be treated with herbicides
which will reduce but not eliminate re-sprouting. It also doubles the work when
cutting.
These controls carry inherent environmental complications. Herbicides
present obvious concerns. Plants that have been pulled, cut or dug need to be
disposed of somehow. Composting does not kill the seeds if they are present at
the time of cutting. Chipping or shredding their branches also leaves viable
seeds. Landfilling is the only other solution but that has its own undesirable
aspects.
Hope for the future is two-faceted, public education and biocontrol. People
must become aware of the problem and learn to identify them. They must also
stop purchasing, planting and cultivating these alien invaders as well as all
other plant species deemed to be even potentially invasive. Furthermore, we all
must take responsibility for invasives growing on our own property. We can also
get involved to help with citizens’ efforts to control them on public land. In
Milwaukee County, the Park People sponsor “Weed Out” programs to control garlic
mustard and buckthorn. MGs in other counties have invasive plant projects.
Biocontrol is the other face of this double-sided coin. Biocontrol
utilizes one organism to control a pest organism. The control can be a
parasite, a predator or a disease. Scientists clearly understand that safety of
the introduced organism is absolutely of the highest priority. Obviously, it is
crucial that the organism being introduced has been extensively researched so as
not to become a pest itself. Luckily, science has advanced to a point where
that is possible with great certainty. Here are some of the considerations and
guidelines used.
Natural enemies from the pest plants’ native habitat are the most likely
candidates because they are already adapted to feed on that plant. Ones that
feed almost exclusively on that plant are the best. That is to say, they are
extremely host specific. They also cannot have effects of any kind on other
organisms. Neither can they harbor any internal parasites or diseases that will
be introduced simultaneously that could become pests.
Other considerations are related to the feasibility of successful rearing,
transportation, dissemination and perseverance of the control organism.
Purple
loosestrife (PLS) is an invasive plant for which biocontrol is currently being
implemented. The process began in the 1980’s with a search for suitable
organisms. Over 100 insects were found to feed on PLS in its native countries
of Europe and Asia. After years of research and evaluation, only 5 met the
safety and other criteria. Introduction of 4 of these began in 1992. The most
promising are the Galerucella beetles, fondly referred to as “Cella” by
biologists involved in the rearing and release program in Wisconsin and other
states. These beetles feed on PLS leaves until flowering begins when they switch
to feeding on the flowers, thereby preventing seed production. This is a
critical means of preventing spread of PLS seeds which can be washed downstream
for miles.
Release sites are continuously monitored for their effects. Success has been
documented in most infested sites where the beetles have been released.
Furthermore, there has been no evidence of beetles feeding on alternate plant
species.
Citizen involvement is welcomed for rearing and releasing beetles in infested
wetlands. Contact the WI-DNR at (608) 266-7012 and ask for the loosestrife
control publication. To help with the biocontrol project, contact the Purple
Loosestrife Biocontrol Project, DNR Research Center, 1350 Femrite Dr., Monona,
WI 53716, (608) 221-6349 or (608) 267-5066. The program is being managed jointly
by the Wisconsin Wetlands Association, the UW-Extension and the DNR. For
program information visit
www.wiscwetlands.org or
www.dnr.state.wi.us.
– Sharon Morrisey,
Consumer Horticulture Agent, Milwaukee County
UW-Extension
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