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 like 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 Podworms
Soybean Podworms, also known as Corn Ear Worm, and Cotton Boll Worm, can cause economic injury in late season soybeans. They are known to overwinter as far north as Arkansas, Tennesse, and North Carolina. Podworms will feed on soybean leaves but the main concern is pod feeding. Use a beat cloth to determine the number of larvae present. The threshold is 1 larvae per foot of row and 5-10% pod feeding.
Figure 4. Soybean Podworm – University of Kentucky, Dept of Entomology.
Green Cloverworm and Soybean Looper
Green Cloverworm and Soybean Looper have similar feeding habits and appearance. Green Cloverworm does not overwinter north of Southern Illinois and Soybean Looper overwinters in Mexico and Southwest US. They both feed on soybean leaves that cause a skeletonized appearance. As the larvae grow and develop, they will consume large portions of the leaf and leave the veins intact. Use the following thresholds for Green Cloverworm and Soybean Looper:
Figure 5. Soybean Pest Caterpillars. Hayden Schug, University of Maryland.
Fall Armyworms and Management Considerations
Fall Army Worm (FAW) is an erratic pest of relative concern to several crops throughout the Midwest and South. Most often we observe issues in forage grasses, alfalfa, and soybeans, especially those in double crop systems. FAW also feed on corn and can be problematic in late-planted fields and fields with hybrids lacking a labeled Bt trait for this pest. Damage typically is worse during droughty conditions. Large larvae are voracious feeders and approximately 80% of their feeding and damage within the last 3 days of their time as larvae, making early detection essential.
General overview
FAW do not overwinter in the Midwest. Adult moths migrate into the region from Southern areas each year, typically around mid-July, where they mate and deposit eggs in grassy areas. Once initial food sources are depleted, they “march” en masse into lush areas, which typically includes soybean fields, late planted corn, and forage grass / alfalfa fields, and potentially small grains and cover crops. Once their development is complete, they move to the soil and pupate. Usually only one generation per growing season is observed in the Midwest and several generations are possible in the Carolinas. Heavy FAW infestations in fields this year does not indicate an issue will occur next season as the insect cannot survive our winter conditions.
FAW like to feed on lush, green tissues. Damage includes mild to severe foliar feeding in most crops, and in soybeans pod feeding can occur. In some cases, plants can have leaf tissue completely removed. Feeding on corn ears similar to that of corn ear worm may be present on late-planted corn fields. Use the following threshold for soybeans:
- Averaging more than 10 FAW larvae in 25 sweeps across a field in small soybeans or 20% defoliation pre bloom.
- Post bloom- >15% defoliation