Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu
Soybean aphids are starting to show up in soybeans. Populations are still extremely low in many fields but populations can quickly explode, especially with the cooler temperatures. The soybean aphid is small (less than 1/16” in length), yellow in color, with black cornicles (the dual tail pipes coming out the back end). A hand lens will be necessary to see the cornicles which can be used a key characteristic to distinguish aphids from other small insects.
Soybean aphid highlighting cornicles.
Soybean aphids have piercing-sucking mouthparts that they use to remove plant fluids, feeding on the pods, stems, and leaves. Their feeding damage stresses plants and under heavy infestations, yield losses in the Midwest have been reported as high as 10-15%.
The economic threshold is an average of 250 aphids per plant, on two consecutive field visits spaced about 5-7 days apart. Aphid populations can quickly crash due to natural enemies and fungal pathogens, so the purpose of the two visits is to determine if populations are increasing or decreasing. When scouting, sample the entire field using a “Z” or “W” pattern, examining 30 plants per field. As you can imagine, counting 250 aphids is time consuming so here is a link with information on how to “Speed Scout” for soybean aphids: http://ento.psu.edu/publications/speed-scouting-form%20for%20PA.pdf
The threshold was developed for plants through growth stage R5 (seed at least 1/8 inch long in one of the pods in upper 4 nodes). Once the plants are R6 (full seed), treatment is only necessary if the plants are drought stressed. Treatment is no longer necessary once plants reach R7 (beginning maturity).