Insects In Soybeans Are A Concern That Should Be Addressed Says Extension Specialist
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. ∆