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 Summer
2002
Fast-spreading
gypsy moths threaten trees, especially the oak trees so
precious to Chicago Wilderness. But the pesticide used to
slow their spread, Bt, also kills the caterpillars of many
butterfly species. Is it worth the risk?
Gypsy Moths & Bt:
A Double Scourge
By Arthur
Pearson
There is a crater on the moon named
for Ettiene Leopold Trouvelot. Were he alive today, many
people would like to send him there.
In the late 1860s, French-born Trouvelot
imported gypsy moth egg masses to his suburban Boston
home, seeking to develop a disease-resistant silkworm
and establish a commercial silk industry. With over a
million caterpillars in cultivation, the escape of several
gypsy moths was inevitable. The amateur lepidopterist
eventually shifted his attention to astronomy and became
famous for his detailed illustrations of the Sun and Venus.
He was given a faculty position in astronomy at Harvard
University before returning to France in 1882 — the
same year the first gypsy moth outbreak occurred on the
street where he lived in Medford, Massachusetts.
Now established throughout the Northeast
— from Ontario, Canada, to North Carolina, and well
into several midwestern states — gypsy moths defoliate
upwards of two million acres of hardwood forests every
year. Gypsy moth larvae (caterpillars) cause the damage,
not the adult moths. The caterpillars are polyphagous,
which means they eat almost anything. They feast on three
hundred different species of trees and shrubs, although
their hardwood of choice is any kind of oak tree.
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The mountain in the center of this photo has
been nearly entirely defoliated by gypsy moths.
Chicago Wilderness has not had a major outbreak,
yet. Photo by Tim Tigner, Virginia Dept.
of Forestry, forestryimages.org.
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During the 1980s, severe outbreaks
in the Northeast resulted in vast tracts of defoliation,
particularly in oak-dominated forests. Chris Bactel, Director
of Collections and Grounds at the Morton Arboretum, recalls
driving for fifty miles through a forested area near Johnstown,
Pennsylvania, in 1986 and seeing not a single leaf except
those on black walnut and tulip trees, two of the few
species distasteful to gypsy moth caterpillars.
The gypsy moth caterpillar's preference
for oak trees could make it an especially unwelcome presence
in northeastern Illinois. The Chicago Wilderness region's
forests, including its globally rare oak savannas, already
are suffering artificial succession caused by unchecked
deer populations and the absence of fire. Invasive species
such as buckthorn and green ash are choking out the oaks,
and with them scores of other oak woodland-dependent plant
and animal species. A major gypsy moth outbreak could
further damage the region's woodlands, to say nothing
of recreation areas and neighborhood streetscapes.
Dr. F. William Ravlin, assistant director
of the Ohio Agricultural, Research, and Development Center
at Ohio State University, has studied gypsy moths for
twenty years. He relates that in the Northeast, "the
composition of forests has changed as a result of the
gypsy moth. In some cases the changes are ones that would
have occurred naturally and the process is simply sped
up. In other cases, natural succession is sort of 'reset'
and a site may essentially begin all over again, with
open areas succeeding to shrubs and on to small trees,
etcetera." Dr. Ravlin cautions that definitive statements
about the effect of gypsy moths are difficult to make
because of the number of factors involved and the differences
between regions, yet indications are that tree species
less susceptible to gypsy moths may eventually come to
dominate wooded areas.
If this were to hold true in Illinois,
a gypsy moth outbreak could accelerate the loss of oak
savannas. However, a number of Chicago Wilderness conservationists
have been considering a different hypothesis. A paper
published in the March 1997 Journal of Arboriculture
estimates that in an outbreak situation, less than one-tenth
of one percent of the total number of the Chicago region's
trees would actually die due to gypsy moth defoliation.
With such limited mortality, there is hope that defoliation
will do little lasting harm to the region's forests. On
the contrary, periodic gypsy moth outbreaks might even
benefit the long-term health of the oak woodlands.
Oak woodlands are disturbance-driven
communities. In pre-settlement times, fire periodically
swept through them, which allowed the rich diversity of
plants and animals that depended on the open canopy structure
to thrive. Since the suppression of fire, the region's
oak woodlands have become overgrown and dark. Periodic
gypsy moth defoliation, although temporarily unsightly,
would allow extra light to reach the forest floor, mimicking
some of the benefits of historical fires.
But what actually will happen in Illinois
has yet to be seen. Although gypsy moths have been present
in the state at least since 1973, they have yet to wreak
any major havoc. Part of the reason is that their reported
occurrence in 46 of Illinois' 102 counties mostly has
been isolated and minor, probably the result of "hitchhikers."
Gypsy moths naturally expand their range very slowly because
the adult female moths cannot fly. Geographic expansion
is largely dependent upon the caterpillars, which, in
search of additional food sources, launch themselves to
the wind on silken threads — a process known as "ballooning."
However, female moths, as if to make up for being earthbound,
lay their egg masses on just about anything, including
cars, trucks and RVs. This explains why isolated gypsy
moth outbreaks have occurred in such far-flung destinations
as Arkansas, California, Oregon, Utah, and Washington.
The non-aided spread of gypsy moths
into Illinois, however, was inevitable. As they ballooned
their way out of Michigan's Upper Peninsula and became
established in southeast Wisconsin by the early 1980s,
their populations began to spike in northeastern Illinois.
Still, the other reason significant gypsy moth damage
has yet to occur is because the Illinois Department of
Agriculture responded to gypsy moth "hot spots"
by spraying Btk.
Bacillus thuringiensis kurstaki,
commonly known as Btk, is the cornerstone application
in the National Gypsy Moth "Slow the Spread"
campaign. Conceding that the gypsy moth is all but impossible
to eradicate, the campaign is a unified management strategy
to slow gypsy moth expansion by nearly two-thirds each
year. Funded by the federal government and implemented
by designated state agencies, the campaign extends along
the entire gypsy moth "frontier" — a transition
zone between infested and non-infested areas that runs
for 1,200 miles from North Carolina through northeastern
Illinois, and into Michigan's Upper Peninsula.
Btk is a naturally occurring bacterium.
It contains a toxic protein that disrupts a caterpillar's
digestive system. Harmless to humans, birds, mammals,
and anything but leaf-eating caterpillars, Btk is one
of hundreds of varieties of Bt, some of which are available
through organic gardening suppliers to protect backyard
vegetable gardens from destructive caterpillars. On a
much larger scale, "Bt corn" — into which
the toxic Bt gene has been spliced to protect commercial
crops against the corn earworm — continues to cause
heated debate over its impact on non-target lepidoptera
such as the monarch butterfly.
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Gypsy moths are a temporary
threat to trees — but Btk is a permanent threat
to many rare native butterflies. The pipevine swallowtail
(above) is found in this region only at 'Waterfall
Glen, which was sprayed in May. Photo by
Randy Emmitt.
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Conservationists question Btk spraying
as a gypsy moth management strategy for the same reason.
A recent study published by the Illinois Natural History
Survey indicates that there may be one hundred different
species of lepidoptera in northeastern Illinois whose
larval stages coincide with that of the gypsy moth. Of
these hundred different species, seventeen are considered
conservative, which means that they require very specific,
high quality natural areas. Ron Panzer, conservation biologist
at Northeastern Illinois University and one of the state's
leading experts on lepidoptera, noting the stated limitations
of the study, estimates that there may in fact be as many
as seventy conservative species in northeastern Illinois
that could be impacted by Btk spraying.
Stan Smith, Manager for Nursery and
Quarantine Programs for the Illinois Department of Agriculture,
acknowledges that any caterpillar that ingests Btk during
the three to five days the treatment is effective in the
field will die. However, available studies, though far
from definitive, are less absolute. Lab test results on
a limited number of species, published in 1998 in Environmental
Entomology, revealed that most caterpillars in their early
instars (stages of development) die, but that caterpillars
in later instars are less susceptible.
As for long-term effects of Btk spraying
on populations of lepidoptera, there are again no definitive
studies. However, based on his research at Illinois Beach
State Park and elsewhere in the state, Dr. Panzer estimates
that 80 percent of all insects on any one site are fairly
common and widespread; and that four out of every five
species, if temporarily exterminated from a site, probably
would return and repopulate. But his concern, shared by
many conservationists, is for that one in five species
that probably won't return. "99.9 percent of the
Illinois landscape has been altered. What tiny percent
of a percent remains in a quality natural state is exceptionally
fragmented. Two-thirds of all conservative lepidoptera
species are found on fewer than eight sites statewide.
If we eliminate a conservative species from even one site,"
Panzer says, "how could it possibly return? It is
likely gone from that site forever."
Doug Taron, Curator of Biology at
the Peggy Notebaert Nature Museum, raises as an example
the pipevine swallowtail, whose host plant is the regionally
rare Dutchman's pipe. In the collar counties, there is
only one permanent colony of this butterfly, whose underwing
is emblazoned with a "c" of seven golden suns
upon a wash of metallic sky blue. The colony is located
at Waterfall Glen Forest Preserve in DuPage County, which
was sprayed with Btk in May. The potential loss of this
limited species of butterfly is particularly disturbing
to Taron and others concerned about biodiversity given
the fact that the purpose of Btk spraying is not to eliminate
gypsy moths.
"If all we can do is slow their
spread, why not do it in a more benign way?" he asked.
More benign ways do exist in the form
of natural pathogens. Nucleopolyhedrosis virus (NPV) frequently
results in the total collapse of gypsy moth outbreaks.
Entomophaga maimaiga (Em) is a fungus that is highly effective
in wet conditions. However, both pathogens usually move
into an area only after a major outbreak, and attempts
to introduce them preemptively remain problematic. Gypcheck,
a U.S. EPA-approved formulation of NPV, currently is difficult
to produce, and therefore, according to the USDA, cost-prohibitive
to manufacture in sufficient quantities. The only known
way to import Em spores is to collect spore-containing
soils or a sufficient number of infected gypsy moth caterpillar
cadavers and spread them around trees in infested areas.
One alternative treatment that has
proved both practical and effective against low-level
populations is the spraying of pheromone flakes, which
inundates an area with the scent that female gypsy moths
emit to attract a mate. With so much scent in the air,
the males become confused and have difficulty locating
a mate. Sensitive to the concerns about Btk, Stan Smith
reports that of the roughly twenty-six thousand acres
to be treated this year, 79 percent will be sprayed with
pheromone flakes, with Btk being reserved for those pockets
of high gypsy moth concentrations.
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Hairy gypsy moth caterpillars
munch on a leaf. At right, gypsy moths lay masses
of eggs on tree bark. Caterpillar photo,
USDA, forestryimages.org; moth photo by Ed Reschke.
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But for most conservationists, even
limited Btk use is too much if habitats of rare lepidoptera
are not avoided. Panzer warns, "It's the equivalent
of risking 20 percent of our rarest bird species."
Dan Griffin, director of operations for the Forest Preserve
District of DuPage County, worries about the cumulative
effect of spraying for different pests. "Say the
mosquito abatement district sprays a wetland and kills
all its mosquitoes to eliminate the one species of mosquito
that carries the West Nile virus; then the nearby woodland
is sprayed with Bt, and all the caterpillars are killed
to slow the gypsy moth. What effect does that have on
the entire food chain?"
Unfortunately, no one can provide
a definitive answer, and perhaps that is the real problem.
Gypsy moths have been in the United States for nearly
a century and a half, and in northeastern Illinois for
thirty years, and still there is insufficient information
regarding their impact and the effects of spraying with
Btk. In the absence of definitive information, management
decisions often have more to do with aesthetics and economics
than ecology. Federal dollars are now available for newly
invaded lands to keep unsightly defoliation from occurring,
so they are being used.
A number of Chicago Wilderness organizations
will be studying the impact of Btk on local ecosystems.
A new science task force considering alternative treatments
will draw on invaluable data collected by the Butterfly
Monitoring Network, a group of citizen scientists who
have monitored local butterfly populations over the past
fifteen years. Their data on local butterflies constitutes
the only baseline information on many natural areas that
were sprayed. These records will help biologists determine
whether Btk has lasting impacts on the survival of rare
lepidoptera. Chicago WILDERNESS will report on this data,
and its implications for the future of the gypsy moth
program, in future issues.
But major outbreak or no major outbreak,
gypsy moths will undoubtedly become a permanent part of
the Chicago Wilderness region, and the gypsy moth "frontier"
will move on. So, too, will federal funding, leaving municipalities
and counties to bear the cost of continued gypsy moth
management. And given the inevitability of the gypsy moth
infestation, do we truly want to risk wiping out many
populations of rare butterflies and moths — and whatever
specialized genes these populations may contain? That's
a permanent loss of biodiversity — a cost none of
us should be willing to bear.
Contact the Butterfly Monitoring Network
at (847) 464-4426 to get involved in efforts to save local
butterflies.
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