www.soybeans.umn.edu - Soybean Aphid Scouting and Surveys

Scouting for Soybean Aphids

Scouting for soybeans can be a
particular challenge for three reasons.

Soybean aphids are fairly small (<1/16”) and present a visual challenge, especially if a person is not accustomed to their appearance or if a person has bifocals.
Economic infestations may occur from mid vegetative stages through podfill. This long scouting window and the re-distribution of aphids among fields as the summer progresses makes repeated visits a necessity.
Aphids distribution on the plant changes during the course of the summer. While it would be nice to sample specific leaves, there is no stable sampling unit except the entire plant.

What to look for on soybean

Aphids and their signs on soybean

Aphids – Search the underside of new leaves on main stem and branches. Aphids on the stems indicate a heavy infestation that probably exceeds threshold.
Cast skins – Soybean aphids go through 4 molts before reaching the adult stage. The shed skins litter lower leaves and the stem like dandruff in heavy infestations.
Honeydew – Aphids defecate excessive plant sap that drips on lower leaves. These drips glisten in the sun. Walking through heavy infestations can be a sticky experience from the honeydew. Note: This honeydew will dry to give clothes a sticky, starch-like stiffening.
Sooty mold – A grey mold may eventually colonize the honeydew turning the surface of the leaf dark grey. In severe infestations, the field takes on an unmistakable grey cast.

Symptoms on soybean

Stunting – When heavy infestations occur during vegetative growth, plants may become stunted. In severe cases, height and the number of nodes on the stem may be significantly reduced.
Lack of canopy closure – As with stunting, leaf development and expansion can be retarded or drastically altered by heavy infestations.
Distorted leaves – Leaf expansion may be affected, especially during drought, to produce cupped leaves, curled edges, or shortened veins that may be mistaken for herbicide injury.
Potassium deficiency-like chlorosis – Upper leaves may exhibit a yellowing along leaf edges and in between veins that’s reminiscent of potassium deficiency symptoms on lower leaves. This situation occurs where soils are already low in potassium and it’s thought that aphid feeding robs new growth of potassium. Not a very reliable symptom with other causes.

Indicators that soybean aphids may be present on a plant

Ants tend aphids for their honeydew. They move aphids between plants and protect them from ladybugs.

Adult ladybugs, which feed on aphids, can be a conspicuous sign of early infestations.

Colonization of Soybean Fields: 2001 vs 2002

Soybean Aphid Shift Within the Plant
New Ulm - 2002