Management of soybean insects

Jeremy K. Greene and Jay W. Chapin

The keys to managing insect pests in soybean are to:
1. Scout fields during high-risk periods for your area.
2. Correctly identify insect pests.
3. Use treatment thresholds to make spray decisions.
4. Use the safest, most economical and environmentally sound insecticide and rate.
5. Accurately calibrate spray equipment and properly apply insecticides.

Scouting
Check soybeans weekly from mid-July to mid-September.  If velvetbean caterpillars or stink bugs are a problem in your area (mainly the southern Coastal Plain), continue scouting until October 15 or until leaves shed.  Place a high priority on checking fields in bloom from the last week of July through August.  Corn earworm moths are attracted to blooming fields and will lay more eggs in open-canopied beans on high spots and lighter soil areas.

Stink bugs can be difficult to scout for because they may not be found in all areas of a field.  Stink bug damage can occur from pod set to when pods begin to yellow, but greatest injury occurs during early pod-fill.

The most important consideration for any field scouting program is to get a representative sample.  If it is impossible to scout all fields, at least sample representative varieties and planting dates each week.  Do not treat all fields based on what is found in one variety or maturity group.

CONVENTIONAL ROW SPACING
Check in at least two different accessible areas of a field, such as opposite ends, or on a lighter and heavier soil type.  In both areas move in 20 steps and take at least two beat samples.  Take more samples if insect populations are not clearly above or below the treatment threshold level.  To take each sample, bend one row out of the way and place a 3 ft by 3 ft beat cloth (also called a ground or drop cloth) with dowel handles between the rows.  Bend 3 feet of one row over the cloth and beat down vigorously on the soybeans at least 10 times.  Move the beans back, and then count and identify insects.  Divide by three to get the number of pests per row foot.  Shake the cloth off thoroughly before taking another sample.

NARROW ROW BEANS
Soybeans that are drilled require insect scouting and treatment thresholds tailored for use in narrow rows.  There are several sampling alternatives - using a standard beat cloth or pan sampler to estimate pest populations per row ft, or using a sweep net.

BEAT CLOTH
A conventional beat cloth can be used to sample narrow-row soybeans if adjacent rows are pushed aside.  An effort has to be made not to disturb the sample row(s) prior to beating onto the cloth.  The narrower the rows and the ranker the canopy growth, the more difficult or impossible this is.

PAN SAMPLER
A pan sampler can be made out of sheet metal and used as an alternative to the beat cloth in narrow row beans.  A sampler 3 ft wide by 2 ft high, with a 4-inch deep trough on the lower edge can be slipped vertically between narrow rows.  When plants are beaten against the angled pan, dislodged insects accumulate in the trough, where they can be counted.

SWEEP NET SAMPLING
A sweep net is a more practical alternative for sampling insects in drilled soybeans.  Use a 15-inch diameter heavy-duty sweep net such that the upper edge of the net stays even with or slightly below the top of the canopy as you sweep it through the crop.  Sweep forcefully with a back-and-forth motion as you walk through the field.  Make one sweep with each stride.  You actually make an elongated "figure 8" motion with the net; each pass covering two 38-inch rows or the equivalent width of narrow rows.  Make 10 sweeps (each pass in either direction counts as a sweep); then count the number of insects in the net, being careful to sort through the leaves in the bottom of the net.  Take a minimum of two sweep samples in each of two different areas of the field, or more until you are confident of your estimates.  (Note: One source of sweep nets is Forestry Suppliers Inc., (800) 647-5368.  Ask for the heavy duty 15-inch insect sweep net.  Also be sure to order a replacement net.

Treatment threshold

DEFOLIATION
The general defoliation threshold for foliage-feeding pests or pest combinations is 30 percent leaf-area loss before bloom and 15 percent thereafter.  There is a tendency to overestimate foliage loss, in part because insects often feed in the upper, more visible part of the canopy.  In addition, there is a tendency for the eye to focus more on damaged leaves.  A technique to “calibrate” or check defoliation estimates is to remove a trifoliate leaf (three leaflets) from the top, middle, and lower part of the canopy without looking.  Then take an extra leaflet from the middle canopy, for a total of ten leaflets.

 Look at each leaflet individually and assign a score of 0 to 10 to each based on an estimate of the portion of leaf area that is missing.  For example, a score of 1 requires that at least 10% is missing; a 3 means that 30% is eaten; a 10 indicates that all or nearly all of the leaflet is gone.  Add up the score total for all ten leaflets to arrive at a defoliation estimate.  Calculate several such defoliation scores and compare the average to estimates made by simply scanning the canopy.

BEAT CLOTH OR PAN SAMPLER INSECT COUNTS

 SWEEP NET:
Sweep net thresholds in drilled soybeans are not as well-defined as those for beat samples.  The following thresholds should be considered guidelines until more research is available.  Use percent defoliation estimates as an additional treatment guideline for foliage feeders.  Prior to bloom, up to 30% defoliation is acceptable without economic yield loss, but once blooming begins, the guideline drops to 15% defoliation.

Insect identification
The four most common caterpillars found in soybean are the corn earworm, green cloverworm, velvetbean caterpillar, and soybean looper.  Since color and size are quite variable, the field key below can be helpful in pointing out distinguishing characteristics.

Corn Earworm.  Corn earworms have many color variations, but the presence of dark “warts” and more body “hairs” helps to distinguish smaller larvae from other common soybean worms.  Corn earworm also tends to curl up in a C-shape when handled.  They have a 4 + 1 proleg pattern, unlike green cloverworms or loopers.  The primary infestation period is from the last week of July to early September.  Corn earworm is a pest throughout the state.  Often called "podworm," this insect feeds on foliage, blooms, pods and even terminal stems.  Corn earworms and stink bugs are the most economically important pests of soybean because they feed directly on pods.	Corn earworm  M. Shepard
Velvetbean Caterpillar - 4 + 1 pair prolegs (3 + 1 when small); very active when touched; light green to black; causes problems around mid-August to late October; mainly found in southern Coastal Plain, especially Beaufort, Charleston, Colleton, Hampton and Jasper counties.  This late-season pest is often mistakenly called "armyworm” because it seems to appear overnight in large numbers and can rapidly strip a field.  The dark color of some specimens also causes confusion.  If you see large numbers of very small green worms on the shake cloth late in the season, be on the alert for velvetbean caterpillar defoliation beginning in the top of the canopy.	Velvetbean caterpilar - M.Shepard
Soybean Looper - 2 + 1 pair prolegs; looping movement; fatter at tail-end of body; usually causes problems from mid-August to mid-September, mainly in cotton production areas and southern coastal counties. The three pairs of thoracic (front) legs may be green or black.  Leg color can change on the same insect as it ages and has no effect on insecticide tolerance.  This pest prefers plants which are not drought-stressed.  Damage usually starts in the middle of a lush canopy.	Soybean Looper - M. Shepard
Green Cloverworm - 3 + 1 pair prolegs; looping movement; not fatter at tail-end; can be problem during July to September; seldom does significant damage by itself, statewide distribution.  This insect is often misidentified as looper due to the looping motion of small larvae.  This mistaken identity can be expensive.  High rates of pyrethroids and other insecticides are often wasted on cloverworms misidentified as loopers.	Green cloverworm
Stink Bugs - Green or brown shield-shaped insects as adults, immatures with same general shape, but no wings; mainly a September problem; mostly southern Coastal Plain.  Stink bug damage is much less obvious than worm damage but usually more costly.  Stink bug feeding causes shriveled seed with reduced germination and can cause small pods to abort.	Stink bug damage to soybean seed  Southern green sting bug eggs, nymphs, and adults
Lesser Cornstalk Borer - Green-blue or purple banded worm, up to three-quarters of an inch long; found at soil surface or tunneled into stem; body twitches vigorously when touched; builds a sand tube often found attached to the stem.  Can be serious pest during drought stress, particularly on soils with a sandy surface.  Burning and disking of wheat stubble prior to planting increases lesser cornstalk borer problems.  Reduced tillage reduces lesser cornstalk borer damage.  This pest destroys soybean stands by girdling or tunneling into seedlings.  The problem is usually misdiagnosed as poor germination or stand loss caused directly by drought stress.  The preventative treatment listed under control can protect stands in high-risk situations.	Lesser cornstalk borer sand tube and stem damage - J. Chapin
Soybean (Dectes) Stem Borer - Pale gray adults with very long black and gray banded antennae. Larvae are cream-colored legless grubs that tunnel stems of soybeans and other hosts (ragweed, cocklebur, etc.).  Larval stage is injurious to soybean by tunneling mainstems, producing seriously damaged plants and significant yield loss when infestation is early that often results in lodged or cut plants.  Associated with repetitive monoculture soybean, so crop rotation helps tremendously with prevention.  Insecticide sprays are ineffective in controlling larvae or adults, and cultural practices involving crop destruction and land preparation (i.e. disking, deep plowing) are primary modes of control.	Soybean (Dectes) stem borer

                                                           Chemical and rate selection
See the following section for insecticide recommendations.  A rate range is usually given for pest control.  Factors that influence the required rate are pest size, pest density, plant size, temperature and application method. The higher rates generally are needed for combinations of heavy populations, large worms, dense plant canopy, extreme temperatures (95 degrees F), and aerial application.

Use of broad-spectrum insecticides such as methyl parathion or Lannate can result in retreatment for late-season velvetbean caterpillar outbreaks.  In areas with annual velvetbean caterpillar problems, growers should consider adding dimilin to corn earworm, stink bug, or boron treatments to prevent retreatment.

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