The soybean aphid, Aphis glycines, is a significant agricultural pest that can severely impact soybean crops. This insect, native to Asia, was first identified in North America in Wisconsin in 2000 and has since spread across most soybean-producing regions, particularly in the upper Midwest and Canada where cooler temperatures favor its development.
Identifying the Soybean Aphid
Soybean aphids are small, pear-shaped insects with a yellow body and distinctive black cornicles at the end of their abdomen. They can appear as either winged or wingless forms. These slow-moving insects are most frequently found on the undersides of new soybean leaves and stems.
Their presence is often indicated by a sticky, sugary substance called honeydew, which they excrete. A black, soot-like fungus known as sooty mold often grows on the honeydew. White “flakes” or shed skins can also be found near aphid colonies.
Impact on Soybean Crops
Soybean aphids cause harm to soybean plants by extracting sap. This direct feeding can lead to reduced plant vigor, stunted growth, yellowing leaves, and distorted foliage. Heavy infestations, particularly during the reproductive growth stages (R1 to R4), can result in reduced pod and seed counts, ultimately leading to significant yield losses.
The honeydew excreted by aphids provides a substrate for the growth of sooty mold on leaf surfaces. This black mold reduces the plant’s ability to photosynthesize by blocking sunlight. Soybean aphids also act as vectors for plant viruses, such as Soybean mosaic virus and Alfalfa mosaic virus, which they can transmit between plants as they feed. Early-season virus infection can pose a greater risk to yield and seed quality.
Life Cycle and Seasonal Patterns
The soybean aphid life cycle involves two different host plants: buckthorn and soybean. Aphids overwinter as cold-hardy eggs laid on buckthorn trees and shrubs. In the spring, these eggs hatch, and several generations of wingless female aphids are produced on buckthorn without mating.
As spring progresses, typically in late May to early June, winged female aphids develop on buckthorn and migrate to soybean fields, which act as their secondary host for the summer. Once on soybeans, these females reproduce asexually, giving birth to live female offspring. This rapid asexual reproduction allows populations to grow exponentially, with up to 15 to 18 generations possible during a single growing season under favorable conditions, and populations capable of doubling in just two to three days.
Winged forms can also develop on soybeans during the summer, particularly when populations become overcrowded, facilitating dispersal to other soybean fields. In late summer or early fall, usually triggered by changes in temperature and host plant quality, winged male and female aphids are produced. These winged forms migrate back to buckthorn, where they mate and lay the overwintering eggs, completing their annual cycle.
Integrated Pest Management for Soybean Aphids
Effective management of soybean aphids relies on an integrated pest management (IPM) approach, combining various strategies to minimize their impact. Regular monitoring or scouting of soybean fields is fundamental to this approach. Growers should begin checking fields once aphids are first observed in the spring, paying close attention to the undersides of leaves and new growth, and scouting every 7 to 10 days through the R6 growth stage.
Intervention thresholds guide when treatment is necessary, typically recommending action when aphid populations reach an average of 250 aphids per plant on at least 80% of plants, and populations are actively increasing, particularly before early pod fill (R4). Cultural practices, such as planting resistant soybean varieties, can also contribute to management by reducing aphid abundance and plant damage. However, the efficacy of these varieties may be challenged by the emergence of aphid biotypes capable of overcoming resistance.
Biological control plays a significant role, as numerous natural enemies attack soybean aphids. Lady beetles, including the multicolored Asian lady beetle, are particularly important predators, with both adults and larvae feeding on aphids. Other beneficial insects include lacewings, minute pirate bugs, and syrphid fly larvae.
Parasitic wasps can also parasitize aphids, turning them into characteristic “mummies.” Fungal pathogens, such as Pandora neoaphidis, can infect and suppress aphid populations, especially under conducive environmental conditions. The presence of lady beetles or ants on soybean plants can serve as an early indicator of aphid activity.
Chemical control, using insecticides, may be necessary when aphid populations exceed established thresholds. It is important to use labeled insecticides at full recommended rates and ensure proper spray coverage, as aphids often feed on the undersides of leaves. Rotating active ingredients helps prevent the development of insecticide resistance, as soybean aphids have shown resistance to certain pyrethroid insecticides. Insecticide applications should be timed carefully to minimize harm to beneficial insects and honey bees, with pyrethroids often preferred when bee activity is a concern, and late-day applications potentially reducing risk. Neonicotinoid seed treatments are generally not a reliable long-term solution for soybean aphid management due to the timing of aphid colonization.