Managing Soybean Aphids
Management decisions regarding soybean aphids are difficult due to the explosive potential of aphid populations and the interaction of aphids with climatic conditions and natural predators. Careful scouting is absolutely necessary to determine if treatment is needed and to time insecticide treatments to maximize their effectiveness.
Soybean aphids tend to develop most quickly on vegetative stage soybeans, and their development slows on flowering / reproductive stage soybeans. The recommended time to begin scouting is when soybeans are in the late vegetative stage (usually late June/early July in Midwest states). Growers should continue their watch through preflower and flowering stages and treat if aphids reach the economic threshold.
Abundance of lady beetles (first photo) or their larvae (second photo) are often indicative of soybean aphid populations. These and other natural predators help control moderate infestations.
An economic threshold (ET) for the R1 to R5 growth stages is currently established at 250 aphids/plant (average of 20 to 30 plants per field) if populations are increasing. If this threshold is reached, treating within 7 days is required to prevent populations from reaching the Economic Injury Level (EIL) where yield loss exceeds the cost of treatment. Scouting guidelines are shown in the sidebar. A "speed scouting" or sequential sampling method has also been developed that can save time when the decision is relatively clear cut: "Scouting for Soybean Aphid" from University of Minnesota Extension.
Soybean aphids colonizing leaves (left) and stem (right) of soybean plant are at economic levels in these pictures.
Research studies have shown that insecticide application can be a useful strategy for reducing yield loss due to soybean aphids. Furthermore, careful timing of applications was found to be important to help minimize losses. Results are reported below for studies conducted in the large outbreak years of 2001 and 2003.
Research on Insecticide Application and Timing
Field-sized insecticide trials were established in 2001 in 73 locations and again in 2003 in 103 locations, in several northern states where the soybean aphid had exceeded economic thresholds. When the results were grouped by spray date, the data reveal that highest yield increases were obtained when spraying occurred from mid-July through the first days of August in both cases (Figure 2). Treatments applied beyond this timeframe resulted in less yield response.
In 2001 less response occurred after August 10 while in 2003 the beneficial response lasted until August 20.
Figure 2. Soybean yield response to insecticide timing, 2001 and 2003 combined data. Source: Pioneer.
This demonstrates that soybean growth and development, weather patterns and aphid population dynamics can change from year to year. Growers should time spray applications based on soybean growth stage and careful scouting of each field, rather than on the calendar date. Differences in weather, including both temperature and moisture, will affect both the aphids as well as the growth of the plant and ultimately the population dynamics. Each year will be unique.
The soybean aphid actually grows and reproduces better under cooler temperatures; however, under hot, dry conditions, affected fields are at a much higher risk of yield loss due to soybean aphids. Drought places soybeans under stress, which can magnify the effects of aphid feeding. Just as importantly, pathogenic fungi may not establish on aphid colonies during hot, dry weather, allowing aphid populations to explode when cooler temperatures return. Spider mites may also establish on soybeans during hot, dry conditions, which further stresses the plant and compounds the aphid problem.
Aphid Reproduction Facts
- All spring and summer offspring are female, are born pregnant, and give live birth. Birth rate is 3 to 8 per day for 10 to 40 days.
- Generation time is typically 7 to 10 days. Populations can double in 1.4 to 1.9 days.
- Up to 15 generations may occur per season on soybeans. Aphids can disperse 3 to 6 miles per day.
- Entomologists suggest sampling 5 to 10 areas of the field (covering 80% of the field) and examining a total of 20-30 plants per field.
- See specific guidelines in your state.
Early Season Scouting Tips
Scout along tree lines and field edges. Look for:
- Aphids on the underside of the upper leaves
- Ants tending aphid colonies
- Presence of lady beetles and larvae
Midseason Scouting Tips
- Presence of honeydew and sooty mold.
- White "skins" remaining after aphids molt
- Crowding and movement into lower canopy
Conditions Favoring Aphid Reproduction/Survival
- Mild Winters
- Favorable Overwintering Sites
- Wooded areas with buckthorn species
- Favorable Summer Host Plants
- Soybeans - Alfalfa - All Clovers
- Moderate Summer Temperatures
- Optimum temperature for aphids is ~ 82 F (28 C) with temperature range from 68 F to 86 F (20 C to 30 C).
- Temperatures of 95 F (35 C) and above limit reproduction and survival
Conditions Favoring a Treatment Response
- Crop development stage at early flower to seed set
- Aphid levels increasing
- Weather favorable for aphid reproduction/survival
- Late-planted soybean fields
- Lack of beneficial insects
- Absence of parasitic fungi