The soybean aphid feeds on the phloem fluids (sometimes referred to as "sap") by inserting piercing-sucking mouthparts directly into the phloem vessels that carry products of photosynthesis from the leaves to other parts of the plant. Prior to feeding, aphids "taste" the sap to determine if the plant is a suitable host species and if the quality is acceptable. Once they settle and begin feeding, the injury from soybean aphid infestations can reduce plant growth, pod number, seed number, seed weight and seed oil concentration (2, 24). Early and prolonged aphid infestations can affect all yield components, while later infestations tend to only reduce seed size (2). In addition, soybean aphids decrease photosynthesis rates of soybean plants (11).

Direct yield loss from soybean aphid feeding does not occur when the first (or five or ten) aphids begin feeding. Today’s soybean varieties are equipped to handle minor challenges, including a few aphids. Yield loss from soybean aphid is related to how many soybean aphids are present and for how long the aphids are present and feeding. The amount of aphid population pressure over time is calculated as aphid-days. Simply put, this is the average number of aphids on a plant multiplied by the number of days they are present. A single soybean aphid on a plant for 10 days is equal to 10 aphid-days, 200 aphids on a plant for 20 days is equal to 4,000 aphid-days, and so on. This aphid-day concept proved to be a good indicator of how soybean yield responded to aphid populations (23). 

Feeding by aphids does not cause the plant to “leak sap”. Soybean aphids require specific nitrogen-rich amino acids that are present in plant fluids at low concentrations (16,25). Therefore, aphids must consume large volumes of sap to acquire enough nutrition. Excess water and sugars from the sap are excreted by the aphids as waste. This is called honeydew. It is the sticky, shiny substance that accumulates on leaves of aphid-infested plants. The sugary honeydew is sometimes fed on and used by other insects as an energy source. A fungus called sooty mold also utilizes aphid honeydew and results in a dark coating on soybean leaves which, may block sunlight and interfere with photosynthesis (12, 10, 8). 

Soybean aphids are not known to transmit fungal or bacterial diseases to soybean. Because soybean aphids and soybean pathogens are associated with certain environments, some people may wrongly assume that the presence of a fungal disease (such as charcoal rot) means that the disease was transmitted by aphids or that the disease entered through the wounds caused by aphids. 

However, aphid feeding can transmit disease-causing viruses from one plant to another. The soybean aphid has been associated with the transmission of several viral diseases of soybean, such as soybean mosaic virus (6), alfalfa mosaic virus (17), and others (26). Because these viral diseases are not currently considered significant threats to soybean yield in the North Central region, they are not directly accounted for in general aphid management recommendations. The “tasting” or probing of plants by soybean aphids can also transmit viruses in plants that are not soybean aphid hosts, such as dry beans (18) and potato (3, 4). This effect in other crops is particularly pronounced when soybean aphid populations are high.