Contact: Jill Scheidt, agronomy
specialist
Tel: (417) 682-3579
E-mail:
scheidtjk@missouri.edu
LAMAR,
Mo. – The pods on soybeans are beginning to fill. With that stage in growth
comes an increased concern about prominent pests.
According
to Jill Scheidt, agronomy specialist with the University of Missouri Extension,
weather conditions are creating situations suitable for insects not commonly
seen.
“For
example, two-spotted spider mites are usually not a problem this time of year,
but with temperatures being hot and dry, they could pose a problem,” said
Scheidt.
Spider
mites are usually translucent in color and can be identified with a hand lens
by the two dark spots on the sides of the abdomen. Spider mites also have
sucking-piercing mouth parts and feed on the underside of leaves. If a leaf is held
up to the light, it is possible to see the webs glistening on the underside of
the leaf.
Threshold
levels for spider mites are 20 percent yellowing before pod set and 10 percent
yellowing after pod set when mites are present.
“A
high rate of insecticide must be used to kill spider mites. If a high rate is
not applied, the insecticide will kill beneficial insects that control spider
mite populations,” said Scheidt.
Thrips
are similar in size to spider mites and vary in color from yellow to dark brown,
but can be identified by horizontal stripes on the back of their lower body. Thrips
are thought to transmit soybean vein necrosis. Thrips are a rare pest in
soybeans, so threshold levels have not been determined yet.
Green
stinkbugs, both the nymphal and adult stages, attack primarily the seeds and
pods of soybean plants. They also will feed on soybean plant stems, foliage,
and blooms.
Green
stink bugs have piercing sucking mouthparts and punctures can be identified by
the presence of small brown or black spots. Green stinkbugs are bright green
and are identified by the dark bands on their antennae.
“Direct
feeding damage can lead to a reduction in seed quality and quantity. Young
seeds can be deformed, undersized or even aborted. Older seeds will be
discolored and shriveled. The germination rate also will be reduced for beans
produced as a seed source,” said Scheidt.
Indirectly,
feeding damage by stink bugs can delay plant maturity and cause the abnormal
production of leaflets and pods. Green stinkbugs can usually be found on the
edges of fields first. Threshold levels for green stinkbug are 1 per foot of
row during seed production.
Bean
leaf beetles feed on pods once seed development begins. Bean leaf beetles are
small insects about one-quarter inch long with black spots or stripes on their
back. Their most identifying characteristic is the black triangle located on
their forewings, just below the head.
Bean leaf beetles can clip pods and feed on
developing seeds. Threshold levels for bean leaf beetle are 10 or more bean
leaf beetles per foot of row and 20 percent defoliation or at least 15 bean leaf beetles per foot of row and at
least 10 percent pod damage.
Pod
worm moths, also known as corn earworm, like to lay eggs in an unclosed canopy
in soybeans. Identify moths by the black banding on the hind wings. Pod worm eggs
hatch 7-10 days after moth flights. Six weeks after moth flights is when pod worms
could stop foliage feeding and begin feeding on pods and become a problem.
“Don't
spray unless pod worms reach threshold, which is 1 pod worm/ft, because
beneficial clover worms carry a fungus that kill pod worms, and may take care
of the pod worms without a need to spray an insecticide,” said Scheidt.
Pod
worms come in all colors, from brown with yellow spots to white with black
spots and green. To distinguish pod worms from green clover worms, Scheidt says
to look at their legs. Pod worms have four pairs of large abdominal pro-legs
right in the center of the body while clover worm only has three pairs.
“Hero,
Warrior II and Mustang Max are effective insecticides to control all of these
insects in soybeans. Read the label for appropriate rates and water use for
each pest. Remember if multiple insecticides applications need to be made, use
a different mode of action to prevent insect resistance to insecticides,” said
Scheidt.
For more
information, contact any of these MU Extension agronomy specialists in
southwest Missouri: Tim Schnakenberg in Stone County, (417) 357-6812; Jill
Scheidt in Barton County, (417) 682-3579;
John Hobbs in McDonald County, (417) 223-4775 or Brie Menjoulet in Hickory
County, (417) 745-6767.