- Several insects can cause early season damage to soybeans
- Proper scouting can help ensure effective in season management
- ROI is improved when pests are managed at the appropriate times and economic thresholds are reached
With soybean planting underway, it will be important to ensure that scouting for early season insects is on your to do lists. Some insects can be scouted preplant and doing this can ensure that the right treatments and seed are planted into the correct acres. Other pests can potentially be an issue after plants start to emerge, and being able to recognize these pests and know when or if an over-the-top insecticide treatment is needed will greatly improve overall productivity and maximize ROI,
Bean leaf beetle (BLB)
Adult beetles are approximately 1/5 inch in length and often have four black markings on the forewings (elytra). There are many color variants of this insect, but all contain a black triangle at the neck region. Heads are black. Adults overwinter in woodlots and soybean fields. Adults emerge in the spring and feed on available host plants, and as the season progresses, move into soybean fields. Adults mate and females will lay eggs at the base of the plants, and new adults will emerge in 3-4 weeks depending on temperatures, with the beetle having 2 or 3 generations a season. Overwintering beetles feed on foliage for about 3 weeks and may transmit bead pod mottle virus (BPMV) if they come in contact with the virus by feeding on an infected plant host. If BPMV is transmitted early in vegetative development, significant yield loss can occur. If BPMV is not present, early season damage is typically cosmetic. Late season pod feeding from second or third generation adults can be a concern.
Start scouting for BLB as soon as seedlings have emerged. Scouting methods and thresholds can be found here. The economic threshold for defoliation of soybeans prior to bloom is an average of 30% of leaf tissue removed with the beetle still present in the field. A foliar insecticide application may be required if economic thresholds have been reached at a given soybean growth stage. Seed treatments may help control initial BLB populations and therefore BPMV spread. However, keep in mind that overwintering BLB populations are substantially reduced following harsh winters where temperatures of 14° F or lower are observed. BPMV may also be seed transmitted.
Seed corn maggot
This insect is typically an issue in early plantings when conditions are cool and soil moisture is adequate. High risk fields are those that are heavily manured or had large amounts of plant material incorporated just prior to planting. Adult flies resembling small house flies are attracted to the volatiles emitted by fresh manure and decomposing plant tissue and lay eggs in these fields. Larvae that hatch from these eggs will feed on seed and germinating seedlings if present, reducing stands. Although this insect has several generations in a season, only the first generation causes economic damage. No rescue treatments are available for this pest, though several insecticide seed treatments can be applied and may be beneficial in high risk fields. If losses have occurred, replanting may be required.
Wireworm
When pre plant scouting indicates a potential problem, a seed treatment insecticide or in furrow / hopper box insecticide treatment may have utility. Make sure to check labels and rates.
White grubs
White grubs are scarab beetle larvae, and there are several species of white grubs that can impact soybean fields. Identification of the species involved in stand issues is important, as different species have different lifecycles and control strategies. For example, May/June beetles (the true white grubs) have a 2-3-year life cycle, whereas Japanese beetles, Asiatic garden beetle and masked chafer only have a one-year life cycle. Therefore, June beetles are potentially more damaging as they spend more time in the grub (feeding) stage, as compared to the other grubs that may be encountered. Pre plant assessments can be conducted by assessing at least 5 sites in a field. Dig up a soil sample 1 foot by 2 feet x 6 inches deep at each site and assess the number and species of grubs present. White grubs are white, comma shaped larvae, ¼ to 1 inch in length with a chestnut-brown head. The hair patterns on the rear ends of the grubs (raster pattern) can be used to identify the species (Figure 1.). Asiatic garden beetle grubs are smaller and have little white “pouches” near the head region. Damage caused by white grubs is usually evident by stunted, discolored, less vigorous plants, and careful removal of stunted plants and soil will uncover these larvae. High risk fields are those with sandy soils, following set aside acres or sod, or with a history of white grub issues.
If more than 2 grubs are present in pre-plant sampling, this may indicate a grub problem. Neonicotinoid seed treatments labeled for white grub suppression may be beneficial in these instances.
Figure 1. Hair patterns on the rear ends of white grubs can be used to identify the species. A 10x hand lens is perfect for this task. Image from extension.entm.purdue.edu
Soybean aphids
Aphids overwinter as eggs on buckthorn, a shrub/small tree common in landscape as an ornamental plant or fence lines, and hatch on these plants in the spring. Several generations will be produced on buckthorn and then winged female aphids will move to soybeans, typically in early June. These females are born pregnant and reproduce at a rate of 3-8 aphids per day for 30 days, with each of these offspring being ready to produce additional aphids within 7-10 days. The most serious damage to soybeans typically occurs during the reproductive phases, especially if soybean plants are under other environmental stressors (heat and drought). Consequently, scouting for this pest should intensify after R1 and continue through R6. Count at least 20-30 plants across 5 or more locations per field, focusing on the underside of leaves. At populations of > 250 aphids per plant a foliar insecticide would be warranted if plants are between R1 and R5 or if plants are at R6 and under significant drought stress.