Insect pests in soybean are sporadic but can be yield-limiting when their populations do build. It is difficult to predict when and where insects may become a problem in soybean, so regular scouting is important. Timely foliar insecticide applications at the recommended thresholds are usually effective for protecting yield from insect damage. For products labeled for soybean insects, see Ohio State University Extension Bulletin 545, Control of Insect Pests of Field Crops and Table 5-7. Insecticidal seed treatments have not been shown to be cost-effective for most forms of insect management and are only recommended under particular circumstances, such as fields transitioning to soybean production from pasture or CRP use. For more information see the multi-state extension fact sheet The Effectiveness of Neonicotinoid Seed Treatments in Soybean.
SOYBEAN APHIDS Soybean aphids are small (1/16 inch) pear-shaped insects ranging in color from pale to bright green or yellow-green. Aphids can have up to 12 generations per year, and can occur in both winged and wingless forms depending on conditions. They are often found in clumped colonies on the undersides of leaves. Early in the season they are most likely on new vegetation, and later in the season they are found lower in the canopy. Though the first colonists to soybean usually arrive in June (from buckthorn shrubs, where they spend the winter), populations usually do not start to build until mid-July or reach economic levels until August. Soybean aphids have piercing-sucking mouthparts and feed on plant fluids, so plant damage is not obvious until it is well-advanced, at which time plants may be stunted and covered with black sooty mold that grows on the aphid feeding waste. To avoid eco- nomic loss, populations should be managed before plant damage is apparent. Scout for soybean aphids starting at least 100 feet from the field’s edge. Examine 20 to 30 plants in widespread (not clumped) locations by walking a W pattern across the field. Count the number of aphids per plant and average the results. Before the R5 growth stage, treatment is recommended if numbers exceed an average of 250 aphids per plant with more than 80 percent of plants infested. After the R5 growth stage, an economic return on a spray is unlikely.
BEAN LEAF BEETLE The bean leaf beetle (BLB) is a small beetle that varies in color from golden brown to green, generally having four black spots on the wing covers, and always having a black triangle centrally behind the head and thorax. Larvae develop below ground and can be found feeding on soybean nodules, though this feeding is not economically relevant. The BLB overwinters in the adult stage and resumes activity in the spring. It can be found feeding on soybean foliage soon after soybean emergence. Bean leaf beetles pass through two generations in Ohio with the first generation of BLB appearing in early summer and the second generation appearing around late August or early September. The time of peak occurrence of BLB adults per generation may differ from field to field depending on the date of planting because the time of initial egg laying in a field depends on the time of initial emergence of the crop, which attracts the overwintering beetles to the site. If a soybean field is late-planted relative to other fields in the area, the first generation may not become established in the field and the probability of early season BLB damage is minimal. However, if planted late and missing the first generation, the likelihood of the field staying green in September enhances the chance of having a higher second BLB generation, where they may cause significant pod-feeding injury. A secondary concern with BLB is its ability to vector bean pod mottle virus (BPMV), which is a concern for seed quality.
Early-season foliar feeding is seldom economic. Foliar injury from the next generation will again appear in early July and continue until fall as a succession of first and second generation BLB adults emerge and feed on the crop. When pod set occurs, BLB adults will begin to feed more on the succulent pods, a more likely source of yield loss. Prior to pod formation, decisions to apply an insecticide rescue treatment are based primarily on the observed defoliation from all leaf-feeding insects combined. Rescue treatment is justified when defoliation exceeds 40 percent prior to bloom, 15 percent from bloom to pod-fill, and 25 percent after pod-fill to plant yellowing. Pod injury due to adult BLB feeding may be detected following pod set. Evaluation of pod injury should be based on inspection of all pods on 10 randomly selected plants. On each plant sampled, count the number of total pods and the number of pods exhibiting pod injury, and then determine the percent pod injury based on the 10 plants inspected. It is important to estimate percent pod injury on inspection of the entire plant. Treatment is justified if the percent pod injury is reaching 10 to 15 percent, and BLB adults are still present and still active.
TWO-SPOTTED SPIDER MITE Two-spotted spider mites are arachnids (related to spiders), not insects, but are scouted and managed in the same way. They tend to be more of a problem under hot, dry conditions―typically later in the summer, though economic infestations can occur earlier under the right conditions. Spider mites are very small (< 0.002 inch) and difficult to spot, so the easiest way to scout for them is to look for telltale signs of their injury―yellow spotting or stippling on the upper side of leaves. This damage usually begins in the lower canopy and progresses upward as the mite population increases. Heavily infested leaves may also have light webbing similar to spider webs. Vegetation can be tapped over a black sheet of paper (black construction paper works well, often better than white paper); dislodged mites will resemble fine grains of sand or motes of dust. Spider mite infestations often begin at field borders and progress inwards. There are no number-based thresholds available for mites, in part because counting them is not practical in a scouting context. Populations can increase rapidly so scouting every four to five days is recommended during drought conditions. Walk a broad pattern in the field and examine at least two plants in each of 20 locations. Use the following scale developed by the University of Minnesota to evaluate spider mite damage in soybean, with treatment recommended at level 3:
- No spider mites or injury observed.
- Minor stippling on lower leaves, no premature yellowing observed.
- Stippling common on lower leaves, small areas on scattered plants with yellowing.
- Heavy stippling on lower leaves, with some stippling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common and some lower leaf loss. (Spray Threshold)
- Lower leaf yellowing readily apparent, leaf drop com- mon. Stippling, webbing, and mites common in middle canopy. Mites and minor stippling present in upper canopy. (Economic Loss)
- Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stippling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.
There are relatively few products available for the treatment of two-spotted spider mites and some pyrethroid insecticides may actually “flare” spider mite populations, making them worse. Common choices for spider mite control in soybeans are products containing chlorpyrifos, dimethoate, bifenthrin though other miticides exist. It is important to re-scout five days after treatment as many products will not kill mite eggs, which will hatch to form a new generation of mites.
STINK BUGS A number of stink bug species may be found in Ohio soybeans, including green, brown, red-shouldered and brown marmorated stink bugs. These are relatively large, shield-shaped insects that often appear at field edges first. Stink bugs have piercing-sucking mouthparts which they insert into developing soybean pods, feeding on the developing seeds. This damage can be subtle from the outside, but results in shriveled or aborted seeds which decreased yield but also reduces seed quality (a particular concern for seed or food-grade beans). Adults will lay egg masses in soybean starting in mid-July and the immatures (nymphs), and later the adults, will feed on pods. Sample for stink bugs with a sweep net by taking five sets of 10 sweeps at different parts of the field. Count all stink bug species and life stages together. Treatment is recommended at an average of four stink bugs per 10 sweeps for grain soybeans, and two per 10 sweep for food grade or seed soybeans. Brown marmorated stink bugs are difficult to capture in a sweep net, however, so if this particular species is present visually scan vegetation for them and treat at one to two per row-foot.
A number of insect species feed directly on soybean leaves and are sporadic pests or occur in low numbers. But collectively, their feeding may add up. These insects include Japanese beetles, grasshoppers, green clover- worms and various other caterpillars. A general defoliation threshold can be collectively used for leaf-feeding insects, with treatment recommended at 40 percent defoliation prior to bloom and 15 percent from bloom to pod-fill. These percentages refer to whole-plant defoliation, not just a few leaves.
Table 5-7: Insecticides Labeled for the Control of Soybean Insects.
Bt - Bacillus thuringiensis
BLB = Bean leaf beetle MBB = Mexican bean beetle JB = Japanese beetle
GCW = Green cloverworm PLH = Potato leafhopper GH = Grasshoppers
SA = Soybean aphid
SM = Spider mites
PHL = Preharvest limitation, waiting period required (in days) prior to harvest or foraging.
* Use is restricted to certified applicators only.
† These compounds are highly toxic to bees exposed to direct treatment or residues on blooming crops or weeds. Do not apply these products or allow them to drift to blooming crops or weeds if bees are visiting the treatment area.