Columbia, Missouri
July 31, 2009
Infestations heaviest in
Bootheel counties
Dectes stem borer, an insect that tunnels in soybean plants, is
now found in significant numbers in soybean fields surveyed in
12 counties in southeast Missouri.
Some fields have 85 percent of the plants infested, said Kelly
Tindall, entomologist at the
University of Missouri Delta Research Center, Portageville,
Mo.
The insect can cause mature plants to fall over, or lodge, at
harvest time in the fall.
Adult stem borers, also known as longhorn beetles, started
emerging in late July from overwintering burrows in the dried
soybean stems left in crop fields after harvest last year.
Tindall reported on the pest at the MU Crop Injury Diagnostic
Clinic at MU Bradford Research and Extension Center, Columbia.
Extension specialists and commercial crop advisers from across
the state attended two two-day training sessions starting July
28.
Surveys by MU entomologists found stem borers in low numbers in
major soybean counties along the Missouri River from St. Charles
to St. Joseph and as far north as DeKalb and Lewis counties in
northern Missouri. The entomologists surveyed random fields in
all soybean-producing counties in the state last fall.
Tindall said the economic impact of the stem borer is unknown.
“Sometimes the Dectes causes the plants to fall over. And
sometimes not.”
After adult longhorn beetles emerge from their burrows, they
deposit eggs into leaf stems to restart the life cycle. After
hatching, the larvae tunnel up and down the main stem of the
plant.
Egg laying and hatching can cause yellowing of soybean leaves.
Growers often assume that change in leaf color is the first
stage of sudden death syndrome, a soybean disease, Tindall said.
When the larvae reach soil level inside the stem, they hibernate
as partially grown larvae. Sometimes they girdle the plant,
causing lodging. They go into pupation for about two weeks in
late spring or summer before emerging as adult beetles.
So far, the main countermeasure is early harvesting when the
soybean ripens, before the plants fall over.
“Insecticides kill adult beetles. However, the next day beetles
can move back into a field,” Tindall said. “The day after
spraying, you will find dead beetles in the field, but you will
also find new live beetles.”
Prolonged emergence over the growing season makes insecticide
control difficult. Larvae and pupae inside the stem are
impossible to reach with spray.
Confounding the issue of damage is research that shows some
soybean plants, with only one larva in a plant stem, produce
more seed. However, the survey found up to six larvae in some
plants. “It is likely those plants will have lower yields,”
Tindall said.
The adult Dectes stem borer is gray, ranging from dark to light.
The beetles are about 3/4-inch long. Black-and-white antennae
longer than the insect’s body give the common name of longhorn
beetle.
While many soybean pests come from China, the Dectes is native
to the United States. Traditional host plants are weeds such as
cocklebur, wild sunflower and both common and giant ragweed.
“When we survey and find these weeds in a soybean field, we
often find more larvae in the weeds than in soybean plants,”
Tindall said.
Wayne Bailey, MU Extension entomologist, said earlier that the
beetle might have adapted to soybeans as the crop became more
available than their weed hosts.
Dectes stem borers caused alarm among Bootheel growers in 1968
when they were first found in soybean fields south of
Portageville. The insect caused damage for a few years and then
seemed to disappear.
This behavior baffles entomologists. “We have no idea what
happened to them,” Tindall said. “Changes in tillage practices
may play a role.”
In today’s no-till systems, stems from the last crop, which may
be infested, remain on the soil surface. Early recommendations
were to plow stems under at least two inches of soil. Few
farmers still plow.
“This insect behaves differently from what was reported in the
1960s,” Tindall said. She spends a good part of her time
tracking and studying the pest. She has found the beetles to be
active longer during the growing season than what the original
researchers found.
Dectes stem borers also are found in central Kansas, she said.
They are a major concern in commercial sunflower fields, where
girdling causes crop loss.
Sunflowers, which have heavy seed heads, fall over more easily
than soybeans, she said.
Dectes seems to thrive on sunflowers. “The longhorn beetles grow
to almost twice the size of the beetles we find in soybean
plants,” Tindall said. While Dectes stem borers have been found
in fields in southwest Missouri, the borers are not common in
eastern counties of Kansas. The populations in the two states do
not overlap.
An MU Extension guide sheet co-authored by Bailey and based on
research from the 1970s was distributed at the field day. To
download a copy, go to
extension.missouri.edu and type “Dectes” in the search box.
The crop clinic at MU Bradford Farm provides annual updates for
farmers and advisers to learn new crop threats and controls.
Certified Crop Advisers earn required continuing-education
units.
“The program may sound like it is the same old thing every
year,” said Tim Reinbott, superintendent of Bradford Farm. “But
the content always changes.”
The MU Extension guide “Soybean Pest Management: Dectes Stem
Borer” (G7152) is available for purchase or free download at
extension.missouri.edu/publications/DisplayPub.aspx?P=g7152 |
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