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 Chemical Manual, also available from the county Extension office.
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 scaring, 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.
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.
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 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).
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 at least 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.
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 is used (Figure 13). (The shake cloth is sometimes called a beat sheet or ground cloth.) 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. Earworms 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 (30 inches or less) 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, several are are listed below. 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 page 00 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 standard 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 on page 00.)
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 21-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 of field 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.
Action Threshold. The threshold used depends upon the row width and the sampling device used.
Row Width No. of Rows Threshold
Sampling Device (inches) per Sample* (worms/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
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*The number of rows shaken or swept across on each sample.
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 earworm thresholds given previously (see page 0).
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