When the soybean aphid first appeared in the United States in
2000, Wisconsin farmers saw what it could do. Where infestations
were severe, the aphids and the viruses they transmit cut soybean
yields by 10 to 15 percent.
A year ago even the experts at the University of
Wisconsin-Madison didn’t know if the outbreak in 2000 was an
anomaly. But last summer the aphid was back, packing just as big a
Now that the aphid is here to stay, growers need information on
the pest, which is native to China. A team of researchers from the
College of Agricultural and Life Sciences is building a picture of
how the aphid operates in Wisconsin. The researchers want to know
what other crops the aphid feeds on, when it colonizes fields, and
what growers can do to manage aphid/virus damage.
"We're starting to learn what to expect from the aphid/virus
complex and to identify steps growers can take," says plant
pathologist Craig Grau. "We hope to put together a complete
integrated pest management package for growers in the next few
Members of the team presented some of their early findings at
this year’s Wisconsin Fertilizer, Aglime and Pest Management
Conference in Madison on Jan. 17. In addition to Grau, the team
includes entomologists Dave Hogg, Tom German, and John Wedberg, weed
scientist Chris Boerboom, agronomist John Gaska and their students.
Entomology graduate assistant Robb Alleman reported that the
soybean aphid can feed and reproduce on several kinds of clover in
the laboratory and on red clover in the field. It also will feed on
snap beans, which is causing problems for growers because the aphid
transmits viruses to snap beans. Alleman found that the aphid fed
very little on alfalfa and not at all on peas.
In southern Wisconsin scattered aphids appear in soybean fields
about June 1. Researchers say that the aphid population can then
spread across an entire field within two weeks. In late July — when
their numbers may exceed 5,000 aphids per plant— the population
produces winged adults. Grau and his colleague Mary Lee found that
those winged aphids are extremely effective in moving viruses from
diseased plants to healthy ones.
To manage viral diseases, growers are usually told to minimize
sources of viral infection, plant resistant varieties and control
insects that spread viruses. But they don’t have all those options.
"It’s difficult for growers to minimize sources of the problem
viruses because aphids can acquire at least two of them from red
clover and other forage legumes," Grau says. In addition, soybean
seed may carry several of the viruses aphids transmit. Planting
virus-free seed would be another way to manage viruses.
But producing virus-free seed will require a coordinated effort
within the seed industry.
"We don't have highly resistant soybean varieties available right
now," he says. "There is a flicker of good news. We are starting to
learn that there are a few varieties that are at least partially
resistant to soybean mosaic virus."
That leaves managing the insects that transmit the diseases as
the best option for growers.
Hogg and his graduate student Bob Ellingson are looking for ways
to control the aphids without pesticides. They’re studying the
aphid's major predator, the multi-colored Asian lady beetle. The
beetle, also an Asian immigrant, kills and eats tremendous numbers
of aphids, according to Hogg.
In a preliminary field study, Ellingson found that the beetles
controlled the aphids in screen-enclosed soybean plots when he added
one beetle larva for every seven plants. However, adding beetle
larvae at that density did not control aphids on plants outside the
Insecticide sprays can decimate aphid populations, according to
Wedberg and his coworkers. However, aphid populations rebounded
within two weeks, often rising to numbers that exceeded pre-spray
levels. Even though aphid populations increased after spraying,
there was a yield gain of 8.2 bushels/acre. Wedberg believes that
repeated insecticide applications would be prohibitively expensive
for growers and would still not control the spread of viruses.
Wedberg is more encouraged by preliminary results with a systemic
insecticide applied to soybean seed before planting. Plants grown
from treated seed appeared to be healthier and had fewer aphids than
plants grown from untreated seed. The systemic insecticide also
didn't harm the predators that eat aphids. While the trial is
encouraging, Wedberg cautions that the researchers haven't yet
analyzed the yield results or determined if the systemic insecticide
reduces viral problems.
The CALS researchers found that planting soybeans early — before
May 10 — played a key role in minimizing aphid-related losses.
Soybeans that grew rapidly suffered less damage than those that grew
more slowly, according to the team's findings.
Practices that result in a rapidly forming soybean canopy also
appear beneficial, according to Grau. The researchers found that
planting at high densities and avoiding herbicide injury — which may
thin the canopy — appeared to reduce the damage caused by aphids and
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