Soybean Insect Management
Management Tip: Treatment levels for all foliage feeders (except
thrips) are at 40 percent defoliation prior to bloom and 25 percent defoliation
after bloom, with foliage feeders present.
Cloverworm - The green cloverworm larva is light green in color with white
stripes running down each side of the body. A full-size larva is approximately
one inch in length. Green cloverworms can be distinguished from all other
lepidopterous larvae because they are the only ones that have three pairs of
abdominal prolegs. When disturbed, larvae wiggle violently and fall to the
ground, similar to the velvetbean caterpillar. The green cloverworm is usually
the first foliage-feeding lepidopteran found in soybeans. Although it is
considered an important pest in northern states, it rarely reaches damaging
levels in Arkansas. Many entomologists feel the green cloverworm may be more
beneficial than harmful because it provides a feeding source for beneficial
insects, allowing them to build up for the time when more damaging larvae may
occur. When the green cloverworm does reach damaging levels, it can be
controlled with the lowest labeled rates of insecticides. Commercial
formulations of Bacillus thuringiensis (Bt.) are very effective for control,
even at the lowest labeled rates.
- The soybean looper and the cabbage looper are both commonly found in Arkansas.
Although the two species are almost indistinguishable, particularly in the
larval stage, control methods are extremely different. The cabbage looper is not
hard to control while the soybean looper is resistant to pyrethroids and
requires the use of more expensive insecticides to control.
The main characteristic for separation of loopers from other lepidopterous
larvae is that they have only two pairs of abdominal prolegs. The body is
thickest at the posterior end of the larva and tapers toward the head. The larva
is light to dark green with longitudinal stripes on each side of the body and
two stripes along the back. Soybean loopers often have black legs and markings
on the head and body; however, this is not a reliable technique for
identification because some soybean loopers do not have these markings.
The most reliable technique to identify larvae of the two species is
examination of mandibles. Soybean looper larvae have mandibles with ribs
terminating in an enlargement near the outer margins while cabbage loopers have
ribs which extend to the outer margins of the mandible.
Larvae of both species can become quite large, reaching almost 1.5 inches in
length. Larvae generally feed in the lower one-half to one-third of the canopy.
As the larvae develop, they eat irregular areas of leaves, leaving the larger
leaf veins. Loopers are voracious feeders, particularly the large larvae
(fourth-sixth instar) which consume 90 percent of the total food required by the
developing larvae. Soybean loopers have been observed to occasionally feed on
Generally, loopers do not reach damaging levels in Arkansas due to the
natural enemy complex of beneficial insects and pathogens. However, when they do
occur, it is usually late in the season and typically in areas where cotton is
also grown. Cotton nectar provides a carbohydrate source which can greatly
increase the egg production of the female moth.
Management decisions should be based on estimates of defoliation and the
number of larvae present. Also, when scouting, attention should be given to the
appearance of larvae. As previously mentioned, there are many natural enemies of
loopers. The ability to spot dead or diseased larvae can often mean the saving
of an expensive insecticide application. If treatment is warranted, consider the
use of a registered, effective formulation of Bt. in areas where resistance to
insecticides has been a problem. Finally, remember that loopers often occur with
other lepidopterous foliage feeders such as the green cloverworm and velvetbean
caterpillar. If the combination of these larvae produces 25 percent defoliation
after bloom, then control measures are warranted.
Leaf Beetle - The bean leaf beetle is a small beetle which primarily feeds
on leaves but will occasionally feed on pods. Adults are about one-fourth inch
long with color ranging from light yellow to red with four black spots on their
back and a black margin around the edge of the wing covers. Also, a black
triangle will be present just behind the prothorax or "neck." This triangle is
always present, but the four black spots may or may not be seen. The grub or
immature stage is found in the soil where it feeds on roots and nodules. It is
white with a black head and anal shield. Damage by bean leaf beetle adult is
characterized by small circular holes between leaf veins as opposed to jagged
leaf damage from caterpillars and grasshoppers.
The time of greatest concern with the bean leaf beetle is early in the
season when plants are small (growth stages V l - V 3). Defoliation levels
exceeding 50 percent on these small plants can occur in a very short time span.
In Arkansas, early planted (especially E S P S) soybean fields are particularly
vulnerable to attack. Late in the season, defoliation by bean leaf beetles in
conjunction with other leaffeeding pests can result in reaching the economic
threshold of 25 percent defoliation.
Current research indicates that the pod feeding of bean leaf beetles may be
even more important than the defoliation it causes. Beetles feeding on the pod
result in increased susceptibility to secondary pathogens, such as Alternaria,
damage to the seed and seed loss. Adults have also been observed to feed on the
pod peduncle causing loss of soybean pods. Also, the bean leaf beetle is known
to transmit bean pod mottle virus (B P M V). The earlier this disease is
transmitted to soybean plants, the more devastating the effects of the disease
can be. Yield losses can range from 10 to 17 percent. However, when plants are
infected with both B P M V and soybean mosaic virus (S M V), yields can be
reduced by 60 percent. S M V is often a seed-transmitted disease.
Caterpillar - The velvetbean caterpillar is usually not a problem in
Arkansas. However, every five to ten years this pest is found in damaging
levels, usually only in the southern region of the state. The velvetbean
caterpillar is a voracious feeder and can strip a soybean field of leaves in a
When larvae are very small, first to third instar, they can be misidentified
as green cloverworms. However, when the larvae reach the third instar
(medium-size larvae), dark longitudinal lines with alternating lighter colored
stripes are visible. Larvae typically range in color from pale green to dark
green or even brown or black. Larger larvae are easily distinguished from green
cloverworms and loopers because they have four pairs of prolegs. When disturbed,
they exhibit violent wiggling behavior much like the green cloverworm which
helps separate them from other species with four pairs of prolegs (such as the
corn earworm or any of the armyworm complex). The adult is characterized by
being dark brown with a darker line running laterally across the middle of both
This pest is susceptible to many natural enemies, particularly the fungus
Nomuraea rileyi, in soybeans which can decimate a population of velvetbean and
caterpillar in short order. Numerous insecticides are effective for control,
including several commercially available B. t. formulations.
- Several species of armyworms may be found in soybeans during the growing
season including the yellowstriped armyworm, the fall armyworm and the beet
armyworm. The yellowstriped armyworm occasionally occurs on seedling
soybeans in large enough numbers to cause damage, but usually the plants
can recover with no loss to yield potential, and control is seldom necessary.
The yellowstriped armyworm larva is dark to black with a yellow stripe running
down each side of the body. Also, there is usually a black spot on each side of
the first abdominal segment. As with the fall armyworm and beet armyworm, the
yellowstriped armyworm has four pairs of prolegs.
fall armyworm populations occur late in the season and do not build up to
damaging levels. However, the fall armyworm has occasionally infested, been
observed to be present in the early spring on seedling soybeans at levels high
enough to cause damage. Control is usually not required. The fall armyworm
larvae can vary from tan to green in color and have black bumps with dark black
hairs on the body. On the eighth abdominal segment there are four distinct black
spots on the upper half of the body. Also, the fall armyworm has an inverted "Y"
on front of the head.
recent years the beet armyworm has been observed to develop large populations
late in the season. Larvae feed on blooms, pods and foliage and have caused
significant yield loss in isolated incidents. Mature larvae are green in color
with prominent lateral stripes. Unlike the fall armyworm, the beet armyworm has
no stout black hairs on the body, and there is usually a black spot on each side
of the body on the second thoracic segment, just above the middle pair of true
legs. The beet armyworm and fall armyworm have shown resistance to pyrethroids
and can be difficult to control.
Blister Beetles - The margined blister beetle and the striped blister
beetle are both common in Arkansas. The adults are elongate with a broad head,
narrow neck and long, slender legs. The margined blister beetle is dark gray to
black, while the striped blister beetle is yellowish orange with brown stripes
on the wing covers. Adults usually feed in groups in the field and can virtually
strip all the leaves in spots in a soybean field. If enough areas in a field are
spot treatment may be required.
Grasshoppers - The redlegged grasshopper and the differential
grasshopper are two grasshopper species that are common in soybeans in Arkansas.
Grasshoppers are rarely a problem. However, when grasshopper populations build
to damaging levels, it usually occurs in fields with undisturbed pastures or hay
fields close by. Typically they are found first along the edge of the field in
large numbers early in the season and then disperse throughout the field as the
season develops. Often treatments can be made along the field edge to control
grasshoppers before they disperse if necessary. Grasshoppers will feed on leaves
and pods, if they are available. Grasshoppers are favored by drought conditions
and are often associated with two or more consecutive years of drought
Garden Webworms - Larvae of the garden webworm usually appear early in
the season. They are green with black spots on every body segment. Webworms are
easily distinguished from other larvae by the silken webbing they produce. Also,
when disturbed the garden webworm will back away from the disturbance. Control
is normally not required. However, in situations of high populations or in
conjunction with other foliage feeders, severe defoliation can occur. Garden
webworms are generally found first on pigweed, and localized field infestations
are generally associated with this weed.
Thrips - Thrips are one of the most abundant arthropods found in
soybeans. They are very small, less than one-tenth of an inch in length. The
most common species found in soybeans is the soybean thrips. Adults of this
species have characteristic transverse bands of brown and white on the abdomen.
Larval stages are yellow to orange in color. Thrips injury is characterized by a
silvery appearance to the leaves, blackening of the terminal and a general
reduction of plant vigor. Plants are most susceptible to thrips injury during
drought conditions or other stress situations that result in stunted growth.
Usually thrips are not a problem. However, when they occur in large numbers
(especially under stressful conditions), seedling mortality can occur. In these
situations, control measures may be necessary. Under normal conditions, plants
outgrow any injury and can withstand very high thrips populations (even up to
100 per plant).
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