6 Scouting for Insects
Soybeans are inhabited by many kinds of insects. Most do not pose a
threat to profitable soybean production. A few kinds, however, can reduce
yields significantly if their numbers are high. Typically, insect problems
are low early in the season, until late July or early August. From that
time until the plants mature, soybeans may be invaded by large numbers
of foliage feeders or pod feeders.
Insect pest populations may differ greatly from
time to time and field to field. The only accurate way to determine whether
the insect infestation is great enough to reduce yields is by scouting
the fields. This guide describes the insects that may be encountered and
the prescribed methods for sampling insects in soybeans. For further information
on identifying soybean insects see Soybean Insect Pests: Field Identification
Guide (North Carolina Agricultural Extension Service publication AG-214)
and Insects and Related Pests of Field Crops (Publication AG-271),
available through the county Agricultural Extension Service office.
Recommendations for insecticides that are effective
in managing insect problems in soybeans can be obtained from the local
Extension agent or the North Carolina Agricultural Chemicals Manual,
also available from the county Extension office.
Seedling Insects
Many insects feed on soybean seedlings, but they are seldom abundant enough to be of concern. Under certain circumstances, however, insects may prevent a rapid grow-off of the crop, especially under stressful conditions; seedlings may even be killed in some cases.
Thrips
Thrips are tiny, slender insects that feed by rasping off the outer
layer of leaves and sucking out the juices. The immature stage (nymphs)
of these difficult-to-see insects are pale yellow. The winged adults may
be yellowish, white, tan, black, or even striped.
On seedling plants thrips may be found on the upper
and lower leaf surfaces as well as within folded leaves. This insect is
normally not a problem when moisture is adequate. During dry weather, however,
thrips populations are often high and their feeding, along with drought
stress, can severely stunt and occasionally even kill the plants. Thrips
cause crinkled, deformed leaves that often have a silvery appearance. Leaf
drop is also common.
Scouting Procedure. Sample only if damage
is readily noticeable. At each sample site randomly pick 10 leaves (a leaf
consists of three leaflets) and examine them for thrips damage - substantial
silvery scarring, deformed leaves, or both. Record the number of leaves
damaged. Sample a minimum of three sites (in small fields) or a maximum
of 10 sites (in large fields). After sampling the field, calculate the
percentage of leaves damaged. If the plants are stressed for any reason
(for example by chemical injury or drought) take special notice of thrips
activity because thrips damage and other plant stress factors are additive.
The number of thrips present can be checked by carefully examining the
leaves or by picking leaves and slapping them over a white card held horizontally.
Action Threshold. Treatment for this pest
can be considered if 75 percent of the leaflets are damaged, the plants
are under stress, and numerous thrips are present; all three conditions
should be met before treatment is applied. If the plants are growing vigorously,
it is likely that they will outgrow the damage. This insect is seldom a
problem after the first six weeks of plant growth.
Other Seedling Insects The bean leaf beetle
and beet armyworm become a problem on seedlings in isolated instances.
The beetles eat holes in the leaves; armyworms may consume the entire leaf.
If greater than 50 percent defoliation occurs and insect populations are
high, treatment is warranted.
Foliage-Feeding Insects
Foliage-Feeding Caterpillars
Many caterpillars feed on soybean foliage, but four kinds are most
common in North Carolina: the green cloverworm, corn earworm (before bloom),
soybean looper, and velvetbean caterpillar (Figures 7 to 10). The corn
earworm and green cloverworm are encountered much more often than the other
two. Accurate identification of these insects is important because the
type of treatment recommended depends on the type of caterpillar.
It is not difficult to identify these larvae by
general appearance, number of prolegs, and behavior. They have (from head
to tail) three pair of true legs and three to five pair of prolegs (the
temporary, fleshy legs toward the back of a caterpillar), counting the
pair on the last body segment. The following descriptions and illustrations
point out the major characteristics of each caterpillar.
Green Cloverworm (Figure
7). The larvae are light green and wiggle vigorously if disturbed.
Since the green cloverworm is the only caterpillar with four pairs of prolegs
it is easy to identify. Damaging levels of green cloverworm occur infrequently,
and a decision to treat should be carefully considered. NOTE: The green
cloverworm is close in appearance and behavior to the velvetbean caterpillar.
The green cloverworm, however, has only four pairs of prolegs.
Corn Earworm (Figure
8). Corn earworm larvae look
very different from loopers and cloverworms. They are usually darker
and appear "hairy." They have five pairs of prolegs. The small worms are
usually brown but large ones are green or yellow plus black. This species
is the only one that typically curls up when dislodged from the plant.
NOTE: The corn earworm feeds not only on foliage but also on pods. If many
pods are present, little foliage feeding may take place (see the discussion
of the corn earworm in the section on pod-feeding insects).
Soybean Looper (Figure
9). The soybean looper has only three pairs of prolegs and thus can
be quickly identified. This light green caterpillar walks in an "inch-worm"
manner (but note that this movement is also characteristic of the green
cloverworm). The soybean looper is largest in girth at the rear and tapers
toward the head.
Velvetbean Caterpillar (Figure
10). These caterpillars move into the coastal plain and tidewater areas
of the state in late August and September but seldom reach economically
important levels. They are normally light green (although they may be green
with black stripes or entirely black with light stripes); they wiggle vigorously
if disturbed. They have five pairs of prolegs, although on small larvae
the first pair may be hard to see. The last pair of prolegs gives the appearance
of a forked tail.
Other Caterpillars. The beet armyworm, silver-spotted
skipper, salt marsh caterpillar, yellowstriped armyworm, and a few other
caterpillar species are occasionally found feeding on soybean foliage.
These larvae almost never cause economic damage by themselves but can add
to the defoliation in a field.
Foliage-Feeding Beetles
Several beetles eat soybean foliage, and one kind or another may inhabit
a soybean field throughout the growing season. Beetles can cause considerable
defoliation by themselves but more commonly feed along with caterpillars.
Bean Leaf Beetle. These beetles are yellowish
buff to reddish and usually have black margins and four black spots on
the back (Figure 11). The spots
are sometimes missing, but this insect always has a triangular mark on
the front of the wing covers. It usually inhabits fields with soils that
have a high moisture-holding capacity. Only the adults eat foliage, producing
rounded holes in the leaves.
Mexican Bean Beetle. Adults and larvae of
this insect eat soybean foliage. The beetles are copper to yellow and rounded,
with 16 black spots (Figure 12).
The larvae are yellow, oval, soft-bodied, grublike insects with dark, branched
spines. Foliage damage from both stages is netlike in appearance and usually
progresses upward from the bottom of the plant.
Other Beetles. The Japanese beetle, grape
colaspis, blister beetle, and spotted cucumber beetle are often encountered
in soybeans. The Japanese beetle in particular may become abundant, but
it occurs during June and July when substantial defoliation can be tolerated
and is therefore seldom a problem. The other beetles are commonly seen
but seldom are abundant.
Scouting Procedures
Scouting for foliage-feeding insects involves estimating the percentage
of the total leaf surface that has been removed by the insect. The method
for taking a sample is to look up and down the row, as far as can be comfortably
seen, and then to estimate the percentage of defoliation at that site.
Beginners often overestimate the extent of defoliation.
One way to help "calibrate" the eye is to look at a plant and estimate
the percentage of defoliation. Then pull all the leaves from the plant,
estimate the percentage of area eaten from each leaf, and calculate the
average defoliation (add the individual defoliation estimates and divide
by the number of leaves). This leaf estimate should be the most accurate.
The first "whole plant" estimate should be close to the second "leaf" estimate.
If not, continue to practice until proficient.
Each field should be sampled no less than three
times at representative sites. For larger fields (those of 25 acres or
more) it is recommended that 10 samples be taken. If the defoliation level
is clearly well above the threshold or is near zero after the first three
samples have been taken - and if casual observations between sampling sites
confirm the sampling results - further sampling is unnecessary.
Action Thresholds
Different treatment thresholds are used during the vegetative and reproductive
stages of plant growth. Before soybeans bloom, they can tolerate 30 percent
loss of foliage. However, a foliage loss of 15 percent is used as a threshold
beginning two weeks before bloom and throughout the blooming and pod-filling
stages.
Pod-Feeding Insects
Corn Earworm
The corn earworm often functions as a foliage feeder and is described
under foliage-feeding caterpillars. However, it prefers to eat blooms and
fruit (pods). Therefore, when plants are in the reproductive growth stage
this caterpillar is sampled as a pod feeder. The corn earworm is the most
damaging insect found on North Carolina soybeans. Its feeding can reduce
yields and delay plant maturity.
Scouting Procedure. The sampling method used
for the corn earworm after bloom depends upon the crop row width. For widths
greater than 30 inches, a shake cloth (sometimes called a beat
sheet or ground cloth) is used. (Figure
13 and front cover.) The cloth
is unfolded between two rows, and plants along the row for 3 feet on each
side are struck downward with the hand and forearm to dislodge caterpillars
onto the sheet. Care must be taken to prevent the plants from sweeping
across the sheet. Earwornis that fall onto the cloth are counted, making
sure that the worms are properly identified and not confused with other
commonly found species of worms. The size of the larvae should be noted,
as this information is needed in making control decisions.
NOTE: The corn earworm is more attracted to open
areas in soybeans and to field edges. Therefore, make sure that samples
are collected away from the borders and in average areas of the field.
Using a shake cloth in narrow-row soybeans (less
than 30 inches) is not practical. In these fields a rigid beat cloth
(Figure 14) or sweep net (Figure
15) must be used. The rigid beat cloth, which measures 2 feet by 3
feet, is not available commercially and thus must be constructed. Plans
are available from county Extension agents. Sweep nets can be purchased
from suppliers, six of which are listed in the box. Nets must be 15 inches
in diameter and strongly built.
To take a sample with the rigid beat cloth, place
it gently between the soybean rows at about a 45-degree angle. Consult
the table on the next page to determine the number of rows that should
be sampled for a given row width. Then gently bend that number of rows
over the open side of the device and shake the foliage firmly, although
not vigorously enough to "trampoline" the caterpillars off the screen.
(The procedure is similar to that used with a stardard beat cloth in beans
planted at conventional row widths.) Count the corn earworms and compare
the average number of worms to the economic threshold listed for the row
width sampled (see the table below.)
Each sweep net sample consists of 15 sweeps done
continuously, one after another. Each sweep is made by briskly thrusting
the net downward in an arc about 3 feet long perpendicular to the plant
rows (Figure 15). Make the sweeps from one side to the other and parallel
to the ground. Thrust the net about 15 inches deep into the foliage on
each sweep. Each sweep should cover five, three, or two rows of 7-inch,
14-inch, or 2 1 -inch rows respectively. While sweeping, pace down the
row taking one sweep per step for 15 paces. The sweeps must be vigorous
enough to dislodge insects. Separate the corn earworms from the leaves
and count them.
Take a minimum of three samples per field, and take
an additional sample for every 3 acres in fields over 6 acres - for example,
in a 6-acre field take three samples; in a 9-acre field, four samples,
and so on. If the action threshold is greatly exceeded or if no caterpillars
are found after three samples, no further sampling is needed.
Some Suppliers of Sweep Nets
Cajun Ag Services
Carolina Biological Supply Co.
612 Eucalyptus
2700 York Road
Port Allen, Louisiana 70767
Burlington, North Carolina 27215
Clo Wind Company
Johnson Sweep Nets, Inc.
827 Congress Avenue
1510 South Brawley
Pacific Grove, California 93950 Fresno, California
93706
Oakfield Apparatus, Inc.
Ward's Natural Science Est., Inc.
P.O. Box 65
5100 Henrietta Road
Oakfield, Wisconsin 53065
P.O. Box 92912
Rocheqter- New York 14697-9017
Action Threshold. The threshold used depends
upon the row width and the sampling device used.
In 30-inch to 40-inch rows sampled with the standard shake cloth (beat
sheet), use an action threshold of two earworms (3/8 inch or longer) per
row foot.
Narrow rows sampled with a rigid beat cloth or sweep
net will require one of the following action thresholds:
Row No. of
Threshold
Sampling
Width Rows per
(worms per
Device
(inches) Sample*
sample)
Rigid beat cloth 7
2
1.4
14
1
1.2
21
1
2.0
Sweep net
7
5
4.0
14
3
3.8
21
2
5.0
*The number of rows shaken or swept across on each sample.
Stinkbugs
Stinkbugs can cause significant yield and quality loss if their population
is above the threshold. The green stinkbug (Figure
16) and brown stinkbug are those most commonly found. These shield-shaped
bugs are similar in appearance but, as their names indicate, differ in
coloration. Young bugs (called nymphs) do not have wing covers and are
greenish or brownish with black, orange, and white markings.
Stinkbugs lay their eggs in clusters, and groups
of immatures may be highly localized. Also, stinkbugs are strong fliers,
and an infestation may appear or disappear in a short time. Therefore,
be sure that an active infestation is present shortly before treatment.
Stinkbug populations do not usually exceed the treatment threshold.
Scouting Procedures. Scout for stinkbugs
when checking for corn earworms using the same procedures. Small nymphs
(less than 1/2 inch long) should be counted separately. Large nymphs (longer
than 1/2 inch) should be counted together with adults.
Action Thresholds. Stinkbug thresholds are
one-half the corn carworm thresholds given in the table above.