Root and Stem Feeders - Soybean Insect Management

The importance of insect pests in Arkansas soybeans is extremely variable from year to year due in large part to environmental conditions. For example, hot, dry years favor many lepidopterous pests such as the soybean podworm and the beet armyworm; and when drought conditions occur, these pests usually are abundant. Many other lepidopterous pests, such as the velvetbean caterpillar and the soybean looper, may cause problems following migrations from southern areas, particularly in concurrence with winds out of the Gulf region where they are a common problem. Generally, insect pressure is greater in the southern part of the state compared to northern Arkansas due to warmer temperatures and closeness to the aforementioned migration sources.

Production practices also have an impact on the occurrence of pest insects in soybeans. For example, insects such as the Dectes stem borer and grape colaspis usually occur at damaging levels only in soybean monocultures. Row width can also affect insect pest pressure. Soybean fields which fail to achieve canopy closure by bloom are the ones most susceptible to damage by the soybean podworm. Planting date, tillage and adjacent crops can also have an impact on pest species occurrence. The Early Soybean Production System (E S P S), which is the planting of indeterminate varieties (M G III and IV) in April, has gained increasing popularity in the state. Fields planted to the E S P S are susceptible to pests such as the foliage-feeding bean leaf beetle and the pod-feeding stink bug complex. Because of the limited acreage in this system, such fields are a virtual "oasis" as a preferred host for these insects which normally would be found primarily on wild hosts.

A soybean field in Arkansas will contain millions of insects comprising a multitude of different species, both pests and beneficials, in a growing season. Proper insect identification and knowledge of the injury associated with pest species are the keys to any soybean insect pest management (I P M) program. Secondly, we must be able to determine the level of pest insects in the field by sampling and assessing the threat of the pest(s) to the crop using such strategies as percent defoliation, stand loss, etc.

Finally, it is important to determine what management tactics are available and whether or not they are economically feasible.

Insect Identification

The three types of insect pests found in soybeans in Arkansas are:

1. Foliage feeders, which comprise the biggest group of insect pests,
2. Pod feeders, which are probably the most detrimental to yield, and
3. Stem, root and seedling feeders, which are often the hardest to sample and are not detected until after they have caused damage.

Some insects, such as the bean leaf beetle, may feed on both foliage and pods but are primarily considered foliage feeders. The following information on individual insects is meant to provide the reader with basic information on some of the more common pests found in soybeans including the injury they cause, important descriptive information, relevant life history details and management considerations.

Insect scouting is essential in determining pest levels in soybeans. All of the thresholds used to make insecticide application decisions are based on the number of insects found and/or the extent of damage caused by insects. Soybean fields should be scouted weekly. The time period from the onset of bloom (Rl-R2) through physiological cutout (R7) is especially critical. When sampling, at least four areas in the field should be chosen at random that will provide adequate coverage of the field. Samples should be taken from each side of the field to adequately detect early insect infestations from one side of the field. Samples should be taken no less than 50 to 100 feet from the edges of the field. Remember, many insects such as grasshoppers, stink bugs and others often feed on wild hosts before entering a field and can often be detected on one side of the field which has suitable habitat before dispersing throughout the field.

In Arkansas two tools are used to sample the field: (1) the drop cloth or shake sheet and (2) the sweepnet. Each of these methods has advantages and disadvantages.

Drop Cloth

The drop cloth is also referred to as the shake cloth or shake sheet. The drop cloth is made of heavy cloth or plastic, 36 inches in length, with a one-half inch or larger doweling about 42 inches long at both ends. Samples are taken by extending the cloth with the dowels parallel to the soybean rows and shaking the 3 feet of plants on each row over the cloth. Insects that fall onto the cloth are then counted and the total divided by six to obtain the number of insects per row foot. Some keys to
taking good drop cloth counts are:

The drop cloth is very effective for sampling soybeans. It is easy to use and make. Sample uniformity is easily maintained. Late in the season, particularly when insect populations are high, the drop cloth is faster to use than the sweepnet and samples more of the plant. However, the drop cloth cannot be used effectively on narrow rows (less
than 19-inch rows), drilled or broadcast soybeans.

Sweepnet

Sweepnet sampling is conducted with a heavy-duty 15-inch diameter sweepnet. Swing the net briskly through the top 15 inches of the canopy. Some of the keys to taking good samples with the sweepnet are as follows:

The sweepnet has several advantages for sampling soybean insects. It can be used on any row width. It is more efficient to use than the drop cloth in short- to moderate-height soybeans, once the correct technique is learned. Also, the sweepnet is quicker than the drop cloth early in the season when insect populations are low. Disadvantages of the sweepnet are that sample uniformity is hard to maintain, less of the plant is sampled compared to the drop cloth, it is less efficient than the drop cloth late in the season, and sweepnets are not readily available.

Table 1. Treatment Levels for Various Larval Insect Pests of Soybeans for Different Row Spacings Using a Drop Cloth.
Row Spacing (inches)	Larvae per Row foot		Comments
	C E W	S L/C L/ B A W/V B C
38	4	6 - 8	Treat when worms are 1/2 inch or larger.
30	3	4.5 - 6	For loopers and other defoliators, the number of larvae is in addition to 25% defoliation after bloom.
19	2	3 - 4	For drought-stressed fields, reduce C E W threshold levels by one for 30-38 inch rows and by 1/2 for 9-19 inch rows.
C E W=corn earworm; S L=soybean looper; C L=cabbage looper; B A W=beet armyworm; V B C=velvetbean caterpillar

Table 2. Equivalent Economic Threshold Conversion Between Drop Cloth and Sweepnet.
	Drop Cloth	Sweepnet
Insect	Number per Foot of Row	Number per 25 Sweeps	Number per 50 Sweeps	Number per 100 Sweeps
Stink Bugs	1	9	18	36
S L, C L, V B C, G C W¹	6	29	58	116
C E W²	4	15	30	60
¹S L = soybean looper; C L = cabbage looper; V B C = velvetbean caterpillar; G C W = green cloverworm. Threshold numbers in association with 40 percent defoliation before bloom and 25 percent after bloom. Number represents medium and large larvae. ²C E W = corn earworm. Since C E W are difficult to sample with a sweepnet, sweep deeper into the canopy using extra force. Supplement with visual checks for bloom and pod feeding.

Insect Management

Integrated Pest Management (I P M) is the use of all available control tactics to effectively keep pests from reaching population levels which will cause economic crop injury. These control tactics may include cultural control, biological control, host plant resistance and chemical control.

Cultural control of insects involves agricultural practices such as crop rotation, planting dates, tillage practices, row patterns, etc., which may help in the control of a pest. It is important to remember that such practices must be in harmony with agronomic practices that promote maximum economic yield.

Tillage such as disking, chisel plowing or other practices can expose many soil insects to an unsuitable environment. Insects such as wireworms, grape colaspis larvae, bean leaf beetle larvae, Dectes stem borer and others may be affected by stirring the soil. The effect of no-till or minimum tillage on insect pests is not well understood. However, it is theorized that no-till cultural practices may aggravate problems of soil-dwelling insect pests.

Crop rotation is one method of cultural control that has been proven effective for control of diseases, nematodes and weeds. However, little is known in regard to insect pests. Experience in Arkansas seems to indicate that problems with Dectes stem borer and grape colaspis are worse in fields with no crop rotation.

Planting date can also impact the outbreak of an insect pest. However, planting dates are generally determined by climatic conditions, varietal and economic considerations. Insect management normally should not play a major role in determining when to plant. As mentioned earlier, the early planting of MG III and IV varieties often provides insect pests such as the bean leaf beetle and stink bug with a food source which is not usually available and may result in pest populations of damaging levels. Late planting may help in avoiding problems with pests such as the bean leaf beetle and grape colaspis. However, late planting can extend the growing cycle of soybeans, resulting in greater potential to pod and seed damage by corn earworm and stink bugs. By avoiding the extremes of planting too early or too late, it may be possible to avoid some insect problems.

Row width can have a definite impact on insect problems. For agronomic as well as pest management concerns, it is critical that canopy closure be achieved by bloom (R 2). It has long been realized that soybean fields which do not reach canopy closure by bloom are more susceptible to damage by the corn earworm.

Biological Control

Biological control in soybeans is, for the most part, the conservation and utilization of natural enemies of insect pests to keep them from reaching damaging levels. In the soybean I P M system, the major objective is to allow natural enemies to do their work without disruption from insecticides. Growers who make insecticide applications only when they are absolutely necessary take full economic advantage of the natural enemies. Also, when insecticides are needed, consideration should be given to products that are less disruptive to beneficial insects. Bacillus thuringiensis may be used for lepidopterous pests such as velvetbean caterpillar, green cloverworm and the looper complex and can provide good control.

Chemical Control

Insecticides should be thought of as a "last resort" to prevent insect damage when cultural and biological controls have failed to keep insect pests below economically damaging levels. Then insect pest levels reach economic thresholds and action
must be taken to avoid economic losses, conventional insecticides have been proven to provide effective and economical control. The only way to determine if an insecticide application is necessary is by scouting the field to determine pest population levels. Never assume that if one field is at treatment level; all fields should be treated. Differences in planting date, growing conditions, stage of maturity and other factors often influence pest population levels. Scout every field.

Correct insect identification is critical to ensure the use of a labeled and effective insecticide. Always read the label of any pesticide before use. Insecticides are an important component of any soybean I P M program. The careful and judicious use of the proper insecticide in accordance with instructions on the pesticide label is crucial for maintaining an effective insect management program.

Identification Guide to Caterpillars on Soybeans Using Number of Ventral Prolegs and Description

General Insect

Physical Appearance

One Proleg

Geometrid Larvae various colors

Two Prolegs

Soybean Looper Green colored, often black true legs, white stripes

Three Prolegs

Green Cloverworm Green colored, wiggles violently when touched.

Four Prolegs

Corn Earworm Various colors, often balls up when touched.

Velvetbean Caterpillar Green to black colored, white stripe, wiggles violently when touched.

Fall Armyworm Usually brown, smooth appearance, prominent white to creme colored inverted "y" on front of head.

Beet Armyworm Green to black, prominent black spot above second true leg.

Yellow Striped Armyworm Double row of triangular shaped markings on back, with bright yellow stripe on side.

Pictures of chickens, flowers, wheat, a boy looking through a magnifying glass, irrigation pipe, soybean pods, and fruits and vegetables.	Cooperative Extension Service





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Insect Management
Soybean Insect Management