Figure 1. Bean leaf beetles feeding on soybean pods
- There are several late season insects that are active in soybean fields
- Foliar and pod feeding can be problematic if economic thresholds are reached
- Make sure to continue to scout soybeans through R5
The soybean crop is well underway in the region, with most crops between R2-R4 depending on cultivar and other factors. Although many management decisions are usually applied by R3, we should continue to monitor our fields through crop maturity and assess potential late season insect damage to crops. This article will discuss the main insects to keep an eye out for, scouting, action thresholds, and management.
Bean leaf beetles
Bean leaf beetles are small, ¼ inch long insects with variable coloration. They are identified by the triangle shaped marking behind their head. Populations can build during the season as they have multiple generations during the year. Beetles can feed on foliage and green pods, potentially leading to yield loss and quality issues. Pod feeding by bean leaf beetle results in light green / cream-colored areas on the pod where the insect has removed the outer epidermis and left a thin membrane (Figure 1).
Scout from R1-R6 using sweep netting for beetles or visual assessments of damage. Assess at least 5 locations per field. The following action thresholds should be used as indicators for application of control: 20% defoliation per 16 ft row; 10% pods with feeding with beetles still present; more than 10 bean leaf beetles per 16 ft row with green pods.
Grasshoppers
Grasshoppers feed on a range of plants and tend to move into soybean fields when their initial food sources have depleted. They tend to be more problematic in dry years as drought reduces natural vegetation, forcing these insects into field crops. There are several grasshopper species that feed on soybeans. Pod damage is dramatic, with the insect consuming the pod and seed (Figure 2).
Scout from R1-R6. Remember that this insect moves into the field from edges, and therefore these areas should be assessed first. First walk field edges and assess grasshopper populations. If populations appear to be severe, assess populations across the field using sweep netting, visual assessment of grasshoppers jumping from foliage as you slowly walk 5 paces, or visual assessment of damage. The following action thresholds should be used as indicators for application of control: 25% defoliation from R3-R6; 15 nymphs or 8 adults on field margins per square yard at field margins (field margin treatments would be advisable); 10% of pods with injury and grasshoppers still present. In no-till fields planted behind grassy cover crops or small grains, the distribution of grasshoppers may be more uniform, and scouting should follow the typical 1 site per 10 acres, minimum of 5 sites per field rule of thumb.
Figure 2. Pod feeding from grasshoppers. Image J. Obermeyer, Purdue.
Stink Bugs
Several species of stinkbugs can potentially feed on soybeans, but green stink bugs typically are the most common and potentially problematic (Figure 3). Stink bugs are shield shaped, release an odor that smells similar to cilantro when aggravated (some find this scent offensive), and contain piercing/sucking mouthparts. Stinkbugs tend to move into fields from preferred plant hosts later in the season once these plants mature. Detection initially occurs around field edges bordering forests or weedy areas. Stinkbugs damage soybeans by puncturing soybeans and sucking juices from developing soybeans. Feeding results in shriveled, discolored grain. Often a tiny puncture mark and discoloration may be noted on the pod.
Scout from R3-R6. The following action thresholds should be followed: If the average number of stinkbugs exceeds an average of 40 per 100 sweeps (across 5 locations per field) and pods are still green in commercial fields. In seed fields that threshold is dropped to 20 stinkbugs per 100 sweeps when pods are still green.
Figure 3. Green stink bug.
Soybean Aphid
Soybean aphids are very small, soft-bodied, yellow-green insects with piercing sucking mouthparts (Figure 4). Aphids overwinter as eggs on buckthorn, where they hatch, and initially feed and reproduce. Typically, parthenogenic (fertile without mating), winged females move from buckthorn into soybeans in June. Aphids then will feed and reproduce as a wingless form when temperatures are mild (72-77°F) with a relative humidity of less than 78%. When aphid populations build to high levels or the quality of the soybean crop decreases, winged females again are formed, where they migrate to other fields or buckthorn. Aphids can reproduce very quickly as they are parthenogenic on soybeans, essentially borne pregnant with clones of themselves. A single aphid will give birth to 3-8 live young a day for 30 days, and have a generation time of 7-10 days. There are numerous natural enemies (e.g. wasps, ladybird beetles, minute pirate bugs) that often keep aphid populations in check.
Action thresholds are an average of 250 aphids per plant from R1-R5. Scout individual plants at each site with the use of a hand lens, focusing on leaf undersides. Often aphid colonies will be distributed in patches and infested plants are sticky due to accumulation of honeydew. A black / olive-green mold called sooty mold may also be associated with honeydew but is not of economic significance. Infections after R5 often do not cause yield loss.
Figure 4. Aphids
Notes and Considerations
There are several foliar insecticides that can be applied for the above insect pests. Ensure that applications are based on threshold levels to minimize failures and increase grower ROI. In addition, make sure to rotate insecticide modes of action if making more than one application in a season to minimize development of insecticide resistance. Lastly- keep an eye on product PHI as this may pose harvest issues to you if applied late in the season in some cases. If resistance develops and becomes widespread, we lose excellent tools. Remember that the label is the law. For assistance make sure to contact your FS Crop Specialist.
References
Hodgson, E. et al. 2012. Management recommendations for soybean aphid (Hemiptera: Aphididae) in the United States. J. Integ. Pest Mngmt. 3(1).
Obermeyer J. and C. Krupke. Green stink bug. extension.entm.purdue.edu/fieldcropsipm/insects/greenstinkbug.php . Accessed 8/1/2021.