FRED
October 20, 1998
With Change in Diet, Beetles
Shake Up Corn Country
By JO THOMAS
URBANA, ILL. -- In the endless battle
between farmers and insects, the bugs
have won another round. A big change in the
behavior of the western corn rootworm beetle
is threatening one of the great success stories
in American agriculture, the use of crop
rotation instead of pesticides on millions of
acres of Midwestern corn.
The larva of the
beetle, first
documented in the
United States in 1867,
can devastate corn
by eating away the
roots of the young
plants, causing them
to keel over in a
strong wind. Forty
years ago, the beetle
became resistant to
the insecticides used
then. But farmers, in
turn, learned to
control the pest by rotating corn and soybean
crops in the same field.
This was the strategy: The adult beetles, which
eat corn silks and pollen, would lay their eggs
in a cornfield in midsummer. Their larvae, the
rootworms, would emerge in late May and
early June of the following year. When they
found themselves among soybeans instead of
corn, they starved to death because they could
not develop on soybean roots. By the following
spring, the soil would be free of rootworms,
ready for a new crop of corn.
But now, in a shift that has shocked farmers
and surprised entomologists, adult western
corn rootworm beetles have developed a taste
for soybeans and have added soybean fields
to their egg-laying destinations. When corn is
rotated onto these fields the following spring,
the western corn rootworms are waiting.
Farmers first discovered the rootworms in
rotated fields in 1986 near Piper City, Ill. "This
was unheard of," said Dr. Eli Levine, a research
entomologist at the University of Illinois at
Urbana-Champaign, who was called to take a
look.
He thought he might
see northern corn
rootworms, found in
relatively small
numbers here. In a
better-known shift in
behavior, its beetle
had begun laying
eggs that took two or
more years to hatch,
allowing some of its
offspring to bypass
crop rotation and
show up in young
corn.
But instead it was the western corn rootworm.
Levine theorized that the pyrethroid
insecticides used in seed corn had simply
forced the beetles into nearby soybeans at
egg-laying time.
For six years, the problem stayed in an area
less than three miles square. Then in 1992,
there were reports of infestations in
commercial cornfields that were far from any
pyrethroid-treated areas, including Lake
County, Ind., south of Gary. Now, the pests are
found in more than 10 counties in Illinois, the
northern half of Indiana, some counties in
western Ohio and lower Michigan, and they are
expected to reach southern Wisconsin in five
years. Their spread west is much slower, but
scientists expect them to reach Iowa in 10
years.
The use of soil insecticides has soared from 13
percent of farms to more than 90 percent in
some affected counties in Illinois. Nationally
these pesticides, widely used in agriculture
and the home, are the subject of scrutiny by
Federal officials concerned about water runoff
and health. If the problem spreads to all the
state's 7.9 million acres of rotated corn, the
additional cost of soil insecticides could reach
$104 million a year, crop scientists say. Yield
losses in untreated corn could reach $132
million a year in Illinois alone.
"A mere curiosity has emerged into a huge
problem," Levine said.
The problem, however, has been shaped in
part by human actions and agricultural
practices. The western corn rootworm beetle,
whose scientific name is Diabrotica virgifera
virgifera LeConte, originated in the tropics and
was only found in the southwestern United
States as a pest on buffalo gourds. In 1909, it
was found attacking corn on a farm near
Greeley, Colo.
The beetle got its path east to the corn belt
when irrigation made continuous corn crops
possible in Kansas and Nebraska, said Dr.
Robert Metcalf, a professor emeritus of
entomology and environmental studies at the
University of Illinois.
In 1961, the beetle
became resistant to
chlorinated
hydrocarbon
insecticides in
southeastern
Nebraska, Metcalf
said, "and it
immediately
migrated." By 1964, it
reached Illinois.
"It took a long time
to get growers off
pesticides," said Dr. Joseph L. Spencer, an
expert on insect behavior who has been
studying the beetles for the Illinois Natural
History Survey. Once farmers realized crop
rotation would control the rootworms,
pesticide use plummeted.
But crop rotation, despite its benefits in
restoring nutrients to soil, has limitations as a
pest-control tool. In the spring of 1996,
Spencer took a lawn chair and went out into
the soybean fields to watch rootworm beetles.
In the nearby cornfields, they were yellow,
but "the further I went into the soybeans,
more and more females were green," he said.
The yellow beetles were full of corn, the green
ones full of soybean leaves. Many beetles had
eaten some of each, cycling between the
fields. Laboratory tests have since shown that
"if they just eat soybeans, they'll die," Dr.
Spencer said, but if they are given a choice
between the two, they will eat some of each.
Larry Bledsoe, an entomologist with the
Purdue University extension service, said the
beetles had, in a way become resistant to crop
rotation. "Planting a soybean field where
rootworm eggs were laid is similar to putting
on a powerful insecticide," he said. Faced with
insecticides, almost all the insects die, but a
few of the very resistant will not. By killing all
but the resistant ones, "we have changed the
distribution," Bledsoe said. The insects left to
reproduce are the ones that are
insecticide-resistant -- or, in this case, the
insects that are less choosy about laying their
eggs only in corn.
The exact cause of the behavior shift is not
known, Spencer said. It may be that the
changes can be attributed to weather or some
environmental factor. But he considers it more
likely that there has been a genetic change in
the beetle.
Dr. Robert J. Novak, a professor of medical
entomology at the University of Illinois, who
has started studying the DNA of the beetles,
said he had seen "a marker that looks very
good" for differentiating between the beetles
that like soybeans and the ones that do not.
Dr. John N. Thompson, an evolutionary biologist
at Washington State University in Pullman,
pointed out that, "Every time a genetically
based shift is found, it's always a situation
where a local plant or an introduced plant has
gotten very abundant relative to the usual
host plant. That's the situation you have with
corn and soybeans. Every other year, you
have a tremendous universe of food to be
used."
Farmers in the affected region got a respite
last summer, a tenfold decrease in the number
of rootworms, Dr. Levine said. He believes they
were drowned by heavy rains that caused
flooding just as they emerged. But this
development "in no way diminishes the
long-term seriousness of the problem," he
said.
Fifteen miles south of Piper City, where it all
began, Greg Pool said he did not intend to take
any chances. He farms nearly 1,400 acres in
Melvin, Ill., and still rotates soybeans with
corn. Only now he spends $16 to $17 an acre
for insecticides.
For three years Pool has set aside a 10-acre
plot to participate in a University of Illinois
study, leaving untreated corn in scattered
patches. The scientists are watching the
damage to the corn roots. He is watching the
effect on his yield per acre.
"The damage is still on the way up," Pool said.
"In the first year, there was a two-bushel
difference. Last year, we were in the
8-to-10-bushel difference. This year, it was 17
bushels.
"I guess I've learned my lesson," Pool added
"Next year I won't have the plot, and I'll treat
all my acres."
Dr. Kevin L. Steffey, an extension specialist in
entomology in the department of crop sciences
at the University of Illinois, who has made
some of the estimates of possible damage, said
he understood the sentiments of farmers like
Pool: "They don't have alternatives."
Hoping to prevent the unnecessary use of soil
pesticides, Steffey and scientists at Purdue are
trying to establish some threshold for the
number of beetles that indicate insecticides
may be necessary the following spring.
A number of seed companies are hoping to
introduce genetically engineered corn that
protects itself from rootworm. Monsanto,
which last year began marketing corn
protected against borers, plans to introduce a
product that resists rootworms in 2001 or
2002.
Spencer points out that this would provide only
temporary relief.
"It's another magic bullet," he said. "We've
fired them before. It may kill one insect, but a
resistant variant may arise in its place. It's like
a phoenix. In agriculture, problems are not
solved forever."
*******************************************************************************
Fred Magdoff
Northeast Region SARE Program
Hills Building
University of Vermont
Burlington, VT 05405
tel:802-656-0472
fax:802-656-4656
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