Robert Koch, Extension Soybean Entomologist, Eric Yu, Extension Educator – IPM Crops
Midsummer heat helped keep soybean aphid populations in check across much of Minnesota, as temperatures above 90°F can limit aphid reproduction and survival. However, more moderate temperatures have recently created favorable conditions for aphid population growth, and numbers are increasing rapidly in some areas. Rainfall has also been uneven across the state. While some areas have received much-needed moisture during pod and seed development, other soybean fields remain under significant drought stress. These dry conditions can also favor two-spotted spider mites, making careful, field-by-field scouting important.
Below are several considerations for scouting and managing soybean aphids as fields approach the later reproductive growth stages.
The use of trade names is for educational purposes only. No endorsement is implied and no discrimination is intended.
Midsummer heat helped keep soybean aphid populations in check across much of Minnesota, as temperatures above 90°F can limit aphid reproduction and survival. However, more moderate temperatures have recently created favorable conditions for aphid population growth, and numbers are increasing rapidly in some areas. Rainfall has also been uneven across the state. While some areas have received much-needed moisture during pod and seed development, other soybean fields remain under significant drought stress. These dry conditions can also favor two-spotted spider mites, making careful, field-by-field scouting important.
Below are several considerations for scouting and managing soybean aphids as fields approach the later reproductive growth stages.
Scouting strategies
Continue scouting based on soybean growth stage rather than the calendar. Aphid populations can continue to increase late in the season, particularly in less mature fields, and soybean aphids may still cause yield loss into the early R6 growth stage.
- Know the growth stages: At R5 (beginning seed), seeds are at least 1/8 inch long in a pod at one of the four uppermost nodes. At R6 (full seed), a green seed fills the pod cavity at one of the four uppermost nodes. By approximately R6.5, leaves and pods begin to yellow, leaving progressively less time for insect feeding to affect yield. For additional information on identifying soybean reproductive growth stages, see this soybean growth stage guide.
- Continue scouting through early R6: Scout fields at least weekly through early R6. Continue monitoring fields for insect problems until approximately R6.5. Do not use a calendar date to determine when scouting should end.
- Pay attention to less mature fields: Any soybean field can be colonized late in the season, but late-planted, less mature or later-maturing fields may remain attractive to aphids longer and are often at greater risk of late-season population increases.
- Inspect the entire plant: Late-season aphids are often distributed throughout the canopy rather than concentrated on new growth at the top of the plant. Check lower branches, non-senescing leaves, stems and pods.
- Know what to count: Include live adults, nymphs and small, light-colored aphids, sometimes called "white dwarfs." Do not count white cast skins, parasitized aphid mummies, diseased aphids or look-alikes such as potato leafhoppers.
- Rescout previously treated fields: Fields treated with insecticides earlier in the season can still be recolonized by aphids later in the season. Populations can increase rapidly in such fields if the earlier insecticide applications eliminated predators and other natural enemies. Also, scouting fields a few days after an application is essential to verify efficacy and check for secondary pests such as two-spotted spider mites.
Action thresholds
Treatment decisions should consider both aphid population levels and soybean growth stage. The established economic threshold remains the best guide through R5, while decisions become more situational as soybean approaches R6 and approach maturity. Population trends, crop maturity and environmental stress should all be considered when determining whether an insecticide application is warranted.- Through R5: Treat when fields average 250 aphids per plant, with more than 80% of plants infested and aphid populations actively increasing through the R5 growth stage. This threshold is not the point at which economic damage occurs; it is intended to provide time to make an application before populations reach damaging levels.
- Avoid treating below threshold: Populations below 250 aphids per plant are less likely to reach economically damaging levels and should continue to be monitored. Unnecessary applications of insecticides are unlikely to provide an economic return, can reduce beneficial insect populations and increase selection pressure for insecticide resistance.
- Early R6: A formal economic threshold has not been established for R6 soybean. However, extremely high populations, often reaching thousands of aphids per plant, may still cause economic yield loss during early R6, particularly when plants are experiencing additional environmental stress. These decisions should be made on a field-by-field basis.
- Do not treat declining populations: Aphid populations can decline rapidly due to weather and natural enemies. Cooler, wetter conditions can favor fungal pathogens, while predators such as Asian lady beetles may become abundant late in the season. From late August into autumn, as soybean plants mature, aphids may also begin moving from soybean to their overwintering host, buckthorn. If populations are clearly declining, do not treat.
Insecticide selection and application
When treatment is warranted, carefully consider insecticide resistance, effects on beneficial insects and spider mites, preharvest intervals and the need for adequate coverage in a dense late-season canopy.- Check preharvest intervals: Preharvest intervals (PHIs) for commonly used soybean insecticides typically range from 18 to 30 days. Always check the product label and ensure the PHI is compatible with the expected harvest date.
- Avoid standalone pyrethroids: Pyrethroid resistance is widespread in Minnesota soybean aphid populations. Avoid relying on a product containing only a pyrethroid for soybean aphid management.
- Consider selective aphidicides: Products containing sulfoxaflor (Transform), afidopyropen (Sefina) or flupyradifuron (Sivanto) can provide effective control of soybean aphids, including pyrethroid-resistant populations, while being less disruptive to beneficial insects. Preserving natural enemies can help suppress surviving or recolonizing aphids.
- Rethink unnecessary premixes: Adding a pyrethroid to a selective aphidicide may provide little additional aphid control when the selective product is already highly effective. For resistance management, alternating individual effective modes of action is generally preferred over mixtures, particularly if a pyrethroid-containing mixture is planned to be used against an aphid population that might have pyrethroid resistance. Using two modes of action in one application can also reduce options for subsequent applications.
- Consider spider mites when selecting an insecticide: Hot, dry conditions favor two-spotted spider mites. Many pyrethroids, except bifenthrin, can flare spider mite populations. Carefully evaluate both pests and select products accordingly when aphids and spider mites occur in the same field.
- Prioritize canopy coverage: Aphids can be located deep within dense, late-season soybean canopies. Use the full labeled rate and follow label recommendations for pressure, nozzle selection and carrier volume to ensure adequate canopy penetration and coverage.
The use of trade names is for educational purposes only. No endorsement is implied and no discrimination is intended.
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