North Carolina Cooperative Extension Service
North Carolina State University

Corn and Soybean Insect Note (ENT/cs-05)

SOYBEAN INSECT MANAGEMENT

John Van Duyn, Entomology Extension Specialist

Soybeans harbor many species of insects. Several have the potential to be pests under high populations but most are either beneficial predators or parasites, or perform no function of direct interest to the soybean farmer. Those insects that do feed upon the plant have traditionally been classified as either foliage feeders or pod feeders, although a few may feed on both plant parts. Most of these plant feeding insects have chewing mouthparts (e.g. caterpillars and beetles) but some pierce the plant and suck juices (e.g. stink bugs and leafhoppers). A few species feed on the underground stems, roots, and nodules. Approximately 20 different plant feeding insects may be found with relative ease in soybean fields, but their individual or collective abundance or damage usually does not exceed the economic threshold. The corn earworm is by far the most important insect pest of soybean in North Carolina. Stink bugs, soybean looper, velvetbean caterpillar, green cloverworm, and bean leaf beetle infrequently occur at high populations but may sometimes reach damaging levels; soybean looper, stink bugs, and velvetbean caterpillar may occur with some regularity in coastal counties below the Albemarle Sound. Other plant feeding insects are rarely important to the soybean farmer. There are several readily available and excellent references on soybean insects and their management. For added information on the identification, biology, and other subjects, consult the publications listed at the end of this chapter in the "References" section.

The soybean plant has an amazing capacity to tolerate insect attack when grown under favorable conditions. Low levels of defoliation, root/nodule feeding, and flower/pod feeding will have no effect on yield. Whereas yield or quality of cotton, most vegetables, and other crops be can be adversely affected by low numbers of insects, the presence of insect pests in soybeans does not mean that yield loss has begun. Research based thresholds describe the level of pests or defoliation required to produce loss and should be used when deciding if control actions are necessary. Since soybean is a low value per acre crop, close attention must be given to producing the highest possible yield at the least cost. It is easy to unwisely increase the cost of insect control by not recognizing the plant's potential to tolerate some insect damage. On the other hand, a common problem that some NC soybean growers demonstrate is a lack of attention in detecting and controlling damaging insect problems when they do occur.

The following sections will familiarize the reader with the important insect pests of soybean and the techniques employed for their management.

INSECT PESTS OF THE SOYBEAN PLANT

Seedling Insect Pests

In the early season, insects begin colonizing soybean plants as they emerge from the ground. However, insects that occur early seldom pose any threat since the crop can tolerate loosing many seedlings without adverse effect and foliage injury is not of concern until the prebloom foliage loss threshold of 30% is met or exceeded (a "threshold" is an established level of damage or pest numbers describing when control action should be taken). Seedling insect pests include thrips, bean leaf beetle, grasshoppers and crickets, cutworms, and the grape colaspis. Several of these pests are common enough to warrant further discussion.

THRIPS
BEANLEAF BEETLE
GRASSHOPPERS and CRICKETS
CUTWORM
GRAPE COLASPIS
THREE CORNERED ALFALFA HOPPER

Root and Nodule Feeding Insects

SOYBEAN NODULE FLY
ROOT FEEDING BEETLE LARVAE

Late Season Foliage and Pod Feeding Insects

CORN EARWORM (soybean podworm)
STINK BUG
SOYBEAN LOOPER
VELVETBEAN CATERPILLAR
MISCELLANEOUS FOLIAGE AND POD FEEDING INSECTS

MANAGING INSECT POPULATIONS IN SOYBEAN

Managing crop pests involves a consideration of agronomic, economic, and biological factors. Soybean is a low value crop that can not support high cost pest management techniques. Therefore, low cost practices which avoid the build-up of insect pests and/or enhance the soybean crop's ability to tolerate insect injury are emphasized. These "avoidance practices" include several cultural techniques that: (1) help ensure a healthy, competitive, and compensating crop; (2) avoid infestations of pests by site selection; (3) seek to make the crop unattractive or unsuitable for some pests, and (4) preserve biological control organisms for pest suppression. Avoidance techniques can greatly reduce insect damage to soybeans but can not always be used in full force, or may not work as well as hoped. In those cases, damaging infestations may occur and "remedial techniques", such as insecticide use based on scouting and treatment thresholds.

Avoiding Insect Pests Through Crop Management

Although soybean is a crop of one plant species, differences caused by variety selection, planting date, cultural techniques, site, and season makes the crop highly variable in it's attractiveness to insect pests. In other words, all soybean fields are not alike, as far as attracting and building-up pest insects is concerned. If the soybean farmer recognizes these differences, he can actually plan to manage the crop for reduced insect pest numbers or, when this is not possible, he can predict which of his fields are attractive and may need more attention to prevent yield loss. Also, pest insects in soybean fields are greatly reduced by beneficial insects. If these predators and parasites are not adversely affected by unwise insecticide use, they will provide the grower with a large measure of free insect control. Important tactics used to reduce insect damage include the following (the opposite of the described practices may lead to increased pest insect infestation):

Site selection: Several insect and pathogen pests build-up if soybeans are not rotated, therefore, rotation helps insure reduced pest levels and improved crop health. Sites that have limiting pest, physical, or soil chemical problems should be avoided if these problems cannot be corrected.

Early planting: Planting before May 20 usually insures that few late season caterpillars will develop in the field, however, some early planted fields may be more attractive to thrips and bean leaf beetle, which are minor pests, and occasionally stink bugs.

Fertility and pH maintenance: Thin and short stands often have more corn earworms, but good growth reduces attractiveness and enhances the plant's ability to compensate for damage. Reducing plant stress from low pH, poor fertility, or inadequate moisture will enable plants to tolerate insect feeding to a high degree.

Variety selection: If planted early, maturity Group V and earlier maturing varieties will bloom and harden-off before the corn earworm moth flight and, therefore, will be unsuitable for infestation. Also, early maturity can greatly reduce soybean looper, velvetbean caterpillar, and late stink bug infestations; in rare situations stink bugs can be trap cropped by early maturity fields.

Narrow rows (21" or less): A complete canopy allows a higher level of biological control by insect predators, parasites, and diseases. Also, narrow-row soybeans seem to be less attractive to egg laying corn earworm moths.

Avoiding unwarranted insecticide use: Both foliar sprays and Temik used at-planting may reduce the build-up of beneficial insects and lead to more severe later season caterpillar infestations. Early season insect pests rarely require insecticide and, if used, may produce more negative consequence by removing predators and parasites than benefits from insect control. In areas that have soybean looper and velvetbean caterpillar, spraying for corn earworm or stink bug may lead to high infestation by these later season pests. Therefore, avoiding unwarranted corn earworm treatments will tend to reduce other late season caterpillar numbers. Also, insecticide intended for corn earworm but applied too early can cause a severe corn earworm infestation. If early season (before corn earworm) spraying is necessary, a low rate of a carbamate insecticide (Sevin, Larvin, or Lannate) will cause the least disruption to the important bug predators and will usually reduce beetles or worms to below threshold if properly timed and applied.

Remedial Insect Management Techniques

In spite of the grower's wish and best attempts at pest prevention, crops cannot always be fully protected from insects by cultural and biological management techniques. Fields may become infested and economic yield loss may occur. Scouting, thresholds, and insecticides are used to discover and identify damaging insect infestations and to reduce the level of pests before yield loss has happened.

Scouting Soybeans: Scouting is a general field survey designed to measure the level of pests or their damage. In soybean, scouting is used to identify fields that may get economic loss from foliage feeders (from several types of insects) or pod feeders (corn earworm or stink bug). The technique used is also influenced by the row width and sampling device employed, when sampling corn earworm and stink bugs. For added information on soybean insect scouting see "Integrated Pest Management Soybean Scouting Manual" (NC State University publication AG- 385).

Scouting for foliage feeding insects. Scouting for foliage- feeding insects involves estimating the percentage of the total leaf surface that has been eaten by insects. The method for taking a single sample is to move to the sample site, look up and down the row at nearby plants and leaves, and estimate the percentage defoliation on the plants observed. Record this estimate and repeat the procedure for all samples. After the last sample is taken, average the individual defoliation estimates over all the samples taken and compare this average estimate to the defoliation threshold.

Beginners often overestimate the extent of defoliation. One way to help "calibrate" the eye is as follows: (a) look at a single plant and estimate the percentage of defoliation -- record this estimate, (b) remove the plant from the ground and pull all the leaves from the plant, (c) estimate the percentage of area eaten from each leaf -- folding the leaves into quarters will help, and (d) calculate the average defoliation (add the individual defoliation estimates and divide by the number of leaves). The individual leaf estimate should be most accurate and the first whole plant estimate should be compared to the leaf based score to judge the accuracy of the first estimate. Use this procedure for practice until the scout becomes proficient at estimating whole plant defoliation.

When scouting fields, each field should be sampled at a minimum of three sites (one sample per two acres with a minimum of three samples and maximum of 10 samples). Extra large fields should be divided into units not larger than approximately 30 acres (e.g. a 45 acre field would be divided into two units of approximately 22 acres each). If the defoliation level is clearly well above the threshold, or if it 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 usually unnecessary. When foliage injury is encountered, the scout should determine which insect(s) is responsible and if population will cause further damage if not treated.

Each field should have a significant portion of the area scouted. Patterns of scouting are usually dictated by the travel pattern between fields, access, and field size and shape. Many patterns can be successfully employed. A "U" or "Figure 8" are good patterns to use. Samples should be widely spaced within the pattern used.

Scouting for corn earworm. The corn earworm often functions as a foliage-feeder and is described under foliage-feeding caterpillars for this situation. 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. It's feeding can reduce yields and delay plant maturity.

The sampling method used for the corn earworm after bloom depends upon the crop row width. For rows greater than 30 inches, a shake cloth (a 3 ft. X 3 ft. cloth sometimes called a beat sheet or ground cloth) is used. The cloth is unfolded between two rows, and plants along the row for 3 feet on each side of the shake cloth (6 row feet) 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 and displacing insects. 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. Calculate the average number of caterpillars per six foot sample and compare this number to the appropriate threshold given below.

Using a shake cloth in narrow-row soybeans (less than 30 inches) is not practical. In fields with rows less than 30 inches a rigid beat cloth or sweep net must be used. The rigid beat cloth, which measures 2 ft. high X 3 ft. long X 4 in. deep (a shallow box), is not available commercially and thus must be home built. Sweep nets should be 15 inches in diameter and strongly made; see the list of potential suppliers at the end of this chapter.

To take a sample with the rigid beat cloth, place it gently on the ground between the soybean rows at about a 45^o angle, gently bend the plant(s) over the open side of the device, and strike the foliage sharply with the forearm or stick, although not vigorously enough to "trampoline" the caterpillars off the sampler surface (the procedure is similar to that used with a beat cloth in soybeans planted at conventional row widths). Sample two 7" rows simultaneously (side by side rows) or one row if at or above 14 inch row width. Count the corn earworms and compare the number of caterpillars averaged over all samples to the economic threshold listed for the row width sampled and anticipated selling price.

Each sweep net sample consists of 15 separate sweeps done continuously, one after another; this is one sample. Each of the 15 sweeps per sample is made by briskly thrusting the net downward in an arc of about 3 feet and perpendicular to the plant rows. Make the sweeps from one side to the other and parallel to the ground. Vigorously thrust the net about 15 inches deep into the foliage on each sweep; with the top of the net opening even with the top of the plant canopy. Each sweep should cover 5, 3, or 2 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 the insects and a properly done sample will contain some leaves. Separate the corn earworms from the leaves and count them. Average the number of caterpillars over the samples taken and compare this average to the threshold for the appropriate row width and selling price.

With any sampling device and any row width, the number of samples taken is dictated by field size. 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; in a 21 acre field, seven samples; and so on. If the threshold is greatly exceeded (e.g. 4 times higher), or if no caterpillars are found after three samples, no further sampling is needed.

Scouting for stink bugs. Scout for stink bugs when checking for corn earworms in August, using the same procedures. However, most stink bug infestations will occur later than corn earworm, in late August through early October. The beat cloth (wide rows), rigid beat cloth (narrow rows), or sweep net (narrow rows) can be used. Follow the guidelines for sampler use as described for corn earworm. However, due to the more varied distribution of stink bugs, a greater number of samples must be taken. Take 10 shake sheet or rigid beat cloth samples per field, or 12 sweep net samples per field. For fields below 5 acres 50% fewer samples may be taken. Groups of small nymphs (less than 1/2 inch long) should be counted as one adult or large nymph. Large nymphs (longer than 1/2 inch) should be counted together with adults.

Applying Thresholds: When scouting, some level of insects and their damage will always be observed. However, most situations will not result in yield loss since the insect numbers present are too low to produce injury beyond the plant's compensation ability. Thresholds are levels of pest injury or insect numbers (that are assumed to cause injury) that will result in yield loss beyond the cost of a remedial action (usually spraying with an insecticide). Current thresholds for soybean insect pests in North Carolina are:

Defoliation by any insect or group of insects -- 30% foliage loss up to two weeks prior to blooming or 15% foliage loss from two weeks prior to flowering and until pods have filled.

Corn earworm thresholds -- for differing sampling techniques, row widths, and soybean selling prices.

Stink bug Thresholds -- for differing sampling devices and row widths.

Insecticide Application: Most insecticide application in North Carolina soybeans is for corn earworm control. Therefore this discussion focuses on this insect. Application methodology used for corn earworm is applicable to the other insects found in soybean.

Research has shown that the optimum timing for treating corn earworm in soybean is when most caterpillars are mid-size (1/2 to 3/4 inch). In this case almost all earworms will be killed if an effective insecticide is applied correctly and at a suitable rate. When caterpillars are very small (less than 3/8 inch) they are often found within flowers or flower clusters, or between the folds of new leaflets. Treatment when most earworms are quite small will often result in poor control. Large caterpillars are easily killed in soybeans, however, most damage (approx. 96%) is done during the last stages of growth and waiting to treat until most caterpillars are large may lead to damage before spraying is done. Since the objective of insect control is to prevent economic damage or expensive treatment, either treating too early or when caterpillars are large is unacceptable and treatment of corn earworm should be directed against mid-size caterpillars.

Treatment of soybeans can be successfully done by ground sprayer or aircraft. If by ground, a minimum volume of six gallons per acre applied through hollow cone nozzles (on row or broadcast spacings) should be used; pressures of about 60-70 psi or higher are preferred. Ground sprayers - high clearance or tractor mounted - can be operated in wide row soybeans without much equipment damage to the crop. Research has shown approximately 2% yield loss within the wheel lanes attributed to plant injury in large soybean plants from a high clearance sprayer and a slightly greater loss for a tractor sprayer. Equipment damage potential is highly influenced by soybean plant height and erectness, and the sprayer boom width.

Damage from ground sprayers in narrow row width soybeans can be significant. As shown in the table below, predicted yield loss per acre can be over four (4) bushels of soybeans per acre for both the high clearance and tractor mounted sprayers. Yield loss would be expected to be proportionally greater or less as yield increased or decreased from 35 bushel per acre used in the table. Also, yield loss would be greater if soybean plants were lodged and tangled. Although damage from ground sprayers may be undesirable, such equipment should be used to avoid spraying in drift sensitive areas and when waiting for an aircraft will allow much greater damage from the insects.

Predicted yield loss in 35 bu/A narrow row (7 inches) soybean after insecticide spray simulation with high-clearance or tractor mounted ground sprayer. *

* average of three tests; C. D. Heim, J. W. Van Duyn, and J. R. Bradley, Jr.

Aircraft are often used to apply insecticide to soybean. Airplanes and helicopters normally apply from one to three gallons of water based spray per acre. If approximately one gallon per acre is used, the carrier often contains water plus an emulsified vegetable oil to reduce evaporation. All of these techniques may perform well if field size and shape, obstacles (e.g. power lines), and weather conditions are satisfactory. However, aircraft spraying may be undesirable if fields are small or close to houses or businesses, have significant road frontage, or are in areas adjacent to waterways (especially with restricted pyrethroid insecticides). Aircraft can be especially important in periods of corn earworm outbreak, when significant acreage must be treated quickly, or when wet field conditions make ground spraying impossible.

Insecticide Selection: Treatment with insecticide should only be made when insect numbers or damage exceed the economic threshold, as determined by scouting. Be sure that the target insects are present when treatment is applied, since several pests (e.g. corn earworm) can develop and leave the plants in a relatively short period in hot summer weather. Lower rates can be used with ground application and on lighter infestations. Mid rates to high rates may be warranted by air, when infestation is high, and/or in periods of rainy weather. Consult a current copy of the North Carolina Agricultural Chemicals Manual for specific insecticide suggestions to use against insect pests occurring on soybean.

SELECTED REFERENCES

1. Handbook of Soybean Insect Pests. 1994. Published by the Entomological Society of America, 9301 Annapolis Road, Lanham, MD 20706-3115. (Phone 301-731-4535 & Fax 301-731-4538)



2. Insects and Related Pests of Field Crops: Some important, common and potential pests in North Carolina. 1982. North Carolina Cooperative Extension Service publication AG-271. Available from, Department of Agricultural Communications, NCSU, Raleigh, NC 27650. (Phone 919-515-3173 & Fax 919-515-3135).



3. A Guide to the Identification and Biology of Soybean Arthropods in North Carolina. 1980 (revised). North Carolina Agricultural Research Service technical bulletin # 238. Available from, Department of Agricultural Communications, NCSU, Raleigh, NC 27650. (Phone 919-515-3173 & Fax 919-515-3135).



4. North Carolina Agricultural Chemicals Manual (current year copy). College of Agricultural and Life Sciences, North Carolina State University. Available from, Department of Agricultural Communications, NCSU, Raleigh, NC 27650. (Phone 919-515-3173 & Fax 919-515-3135).



5. Integrated Pest Management Soybens Scouting Manual.1999. North Carolina Cooperative Extension Service publication AG-385(revised). Available from, Department of Agricultural Communications, NCSU, Raleigh, NC 27650. (Phone 919-515-3173 & Fax 919-515-3135).

Source of heavy duty, 15" diameter sweep nets:

1. Gempler's
   P. O. Box 270
   211 Blue Mounds Road
   Mt. Horeb, WI 53572
   1-800-874-4755 (Technical service number)
   1-800-382-8473 (order number)
   

Other Resources

1. Plant Disease Notes
2. North Carolina Agricultural Chemicals Manual
3. North Carolina Cooperative Extension Service

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This page (http://www.ces.ncsu.edu/plymouth/pubs/soybeanInsectMgt2.html) was created by John W. Van Duyn Ph D. Extension Entomologist and Wayne Modlin, Res. Tech. III.

Date Created 12/10/98.
Last revised on 2/11/00.

1. Published by North Carolina Cooperative Extension Service

Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.