Insects and Diseases

Figure 2. Sooty mold can easily be detected as the black film on the leaves.

Go to the following pages to study:

Description, identification, and the life cycle of soybean aphid

Agronomic impact of soybean aphids

Scouting soybean aphids

Management of soybean aphids

Symptoms and Signs of Soybean Aphid Activity

Plant symptoms

Figure 1. Leaf edges can show a yellow tint that is similar to potassium deficiency under heavy aphid breakouts.

Yield-damaging aphid populations can not be detected from the road. Early in the season, aphids tend to be located on new growth. Examining the upper trifoliate is a quick way to determine the percentage of plants with aphids, since sweep nets will not work. Upper leaves may exhibit a yellowing along leaf edges and in between veins that is reminiscent of potassium deficiency symptoms on lower leaves (Figure 1). Recently, research published from the University of Wisconsin identified that soybean aphid feeding and potassium deficiency both serve to impact soybean yield, but they could not find any correlation whether potassium deficient soybean plants actually promote soybean aphid outbreaks in the field.

Presence of natural enemies and parasites
The best indicator that aphids are present is cast skins, honeydew, ants, ladybugs, and sooty mold. All of these can be seen without a magnifying lens.

Cast skins are white, and litter the lower leaves and stem since soybean aphids go through 4 molts before reaching the adult stage. Aphids defecate excessive plant sap that drips on lower leaves, which is called honeydew. These drops glisten in the sun and make the plant look like it is sweating. Ants tend aphids for their honeydew. They move aphids between plants and protect them from ladybugs.

Sooty mold is caused by several species of fungi of various types (Figure 2). It appears on the leaves or stems of plants as a superficial, black growth of mycelium forming a film or crust on these plant parts. Sooty mold is not parasitic but lives off the honeydew. The fungal growth can be so abundant that it gives the leaf a black, sooty appearance and interferes with the amount of light that reaches the plant. This reduces photosynthesis and contributes to yield loss. The presence of the sooty mold fungi is usually not important to the health of the plant, but it does indicate the presence of aphids and may be a warning of a severe aphid or scale problem. Sooty mold can be diagnosed easily, because the black sooty mycelia growth can be completely wiped off a leaf or stem with a moistened cloth, paper, or hand. There are no control measures that apply to the sooty mold fungi. Since it grows on the excretions (honeydew) of the aphid, control of aphids with the appropriate insecticide or other means also results in the elimination of the sooty mold.

When heavy infestations occur during vegetative growth, plants may become stunted. In severe cases, height and node numbers on the stem may be significantly reduced. All of this will reduce canopy closure. Leaf expansion may be affected, especially during drought. It may cause the plant to produce cupped leaves, curled edges, or shortened veins that may be mistaken for herbicide injury.

Ladybugs or the multicolor Asian ladybeetle (Harmonia axyridis) can be a conspicuous sign of early infestations. Research on other species of aphid shows that an adult ladybug can consume up to 40 aphids per day and the larvae can consume 60-70 aphids per day.