A guide to Soybean Aphids in South Dakota Adam Varenhorst Assistant Professor & SDSU Extension Field Crop Entomologist Patrick Wagner SDSU Extension Entomology Field Specialist Amanda Bachmann SDSU Extension Pesticide Education & Urban Entomology Field Specialist
Soybean aphid (Aphis glycines) agronomy Description Very small, approximately 1/8 long Bright green to yellow Black cornicles (tailpipes) on the end of the abdomen (Fig. 1) Can have wings or be wingless Causes shiny soybean leaves due to honeydew excrement Sooty mold growth causes soybean to have dark appearance Figure 1. Wingless soybean aphid Figure 2. Winged soybean aphid Lifecycle Overwintering (primary) host is buckthorn species. Summer (secondary) host is soybean Populations may double every 1.5 to 13 days depending on environmental conditions (temperature, precipitation, etc.)
Scouting methods agronomy Traditional method Randomly select 20 plants throughout the soybean field. Examine all parts of each plant, and count the number of aphids present. Record the populations for each plant and average the 20 counts to determine the average population in the field. Compare the calculated value to the economic threshold of 250 aphids per plant. If populations exceed 250 aphids per plant, management should be implemented to avoid potential yield loss. Speed scouting method Print off a speed scouting worksheet for each field, or download the Aphid Speed Scout phone application. To scout, randomly select 11 plants spread throughout the field. Examine all parts of each plant, and count the number of aphids present. When 43 soybean aphids are observed, record a plus (+) for that plant. If 43 aphids are not observed, record a minus (-). Continue this process for the 11 plants. If all 11 plants are positive, the field needs to be sprayed. If 6-10 plants are positive, additonal plants must be scouted until a decision can be made. If 5 or fewer plants are positive, the field does not need a treatment. Figure 3. Soybean aphid colony
Management agronomy Economic Threshold The economic threshold for soybean aphids is 250 aphids per plant. This allows for a seven day window before yield loss will be observed. Injury Plant sap removal with piercing sucking mouthparts Honeydew promotes sooty mold which inhibits photosynthesis Vector of soybean mosaic virus (typically very minor issue) If left unmanaged, soybean aphids can reduce yields by 40% Management Broad spectrum foliar insecticides Use Rag (Resistant to Aphis glycines) gene soybean The most important natural enemies of the soybean aphid are minute pirate bugs (Fig. 4) and lady beetle adults (Fig. 5) and larvae (Fig. 6) Other natural enemies include: hoverfly larvae (Fig. 7), green and brown lacewing larvae, and damsel bugs Figure 4. Minute pirate bug Figure 5. Lady beetle adult Figure 6. Lady beetle larvae Figure 7. Hoverfly larvae
agronomy Soybean aphid look-alikes The following insects are commonly mistaken for soybean aphids: Clockwise from top left: Figure 8. Tarnished plant bug nymph, Figure 9. Thrips, Figure 10. Whitefly nymphs, Figure 11. Potato leafhopper nymph, Figure 12. Minute pirate bug nymph, Figure 13. Whitefly adults.
agronomy Acknowledgements This publication was developed through funding from SDSU Extension. South Dakota State University, South Dakota counties, and USDA cooperating. South Dakota State University adheres to AA/EEO guidelines in offering education programs and services. Photographs Figure 8: Scott Bauer USDA Agricultural Research Service, Bugwood.org; Figures 9, 10: Frank Peairs, Colorado State University, Bugwood.org; Figure 11: John C. French Sr. Retired, Universities: Auburn, GA, Clemson, U of MO, Bugwood.org; Figure 12: Adam Sisson, Iowa State University, Bugwood.org; Figure 13: Clemson University USDA Cooperative Extension Slide Series, Bugwood.org. All other photos courtesy of Adam Varenhorst. Publication: 03-2007-2017