Soybean aphid (Aphis glycines) is an invasive pest in the United States. Outbreaks of soybean aphids are sporadic, so frequent scouting is necessary to make appropriate treatment decisions.
Egg: Eggs are not found in soybean.
Nymph: Nymphs look similar to wingless adult aphids but are smaller. White flakes may be present near aphids; these are cast skins from nymphs molting.
Adult: Adults are small, approximately 1/16 inch long, and have a pear-shaped body. Soybean aphids can be distinguished from other small green insects by the pair of black cornicles (“tailpipes”) at the end of the abdomen (Photo 1). Both winged and wingless aphids can be found in soybean at the same time; both are yellow-green in color. However, the winged forms will be slightly darker on the head and thorax, primarily a result of the muscles used to pump the wings (Photo 2).
Be aware of look-alikes: Soybean aphid is commonly confused with potato leafhopper nymphs (Photo 3). Both are light green in color and very small. Aphids are pear-shaped (small heads, large abdomens) with cornicles at the end of the abdomen. Potato leafhoppers are more triangular-shaped (large heads, tapered abdomens), and have hairy legs, white eyes, and no cornicles. Another important difference is in their activity: soybean aphids do not move when disturbed, but potato leafhoppers will move or jump away.
Soybean aphid is a relatively new pest of soybean in the United States. It is native to Asia and was first discovered in southeast Wisconsin in July 2000. Today, it is found all over the upper Midwest and into Canada. The lifecycle of soybean aphid is complex: soybean aphids are host-alternating (heteroecious) and undergo sexual reproduction during part of their life cycle (holocyclic). Buckthorn (Rhamnus spp.) is the primary host where cold-hardy soybean aphid eggs overwinter and adults undergo sexual reproduction (Photo 4). Soybean serves as the secondary host where soybean aphid feeds during the summer and reproduces asexually. Buckthorn is a very common shrub in the Iowa landscape. They tend to grow underneath trees in shelterbelts as shrubs or as small trees along fence lines or open field margins. Some homeowners grow buckthorn as ornamental or shelter plants around buildings.
In the spring, the overwintering eggs hatch, and the soybean aphid has two or three asexual generations on buckthorn before winged females move into soybeans in early June (Photo 5; Figure 1). The winged females are parthenogenetic (being fertile without mating) and give live birth to nymphs. This asexual reproduction continues throughout the summer on soybean, and up to 18 generations may be produced during the growing season depending on temperature. During the summer, aphids are all females and a mixture of wingless adults and nymphs, and occasionally winged adults. Aphid development is favored in late June to early July with temperatures of 72-77°F and relative humidity below 78%. During the fall, usually triggered by temperature and host quality, winged males and females are produced to migrate back to buckthorn, mate, and lay eggs.
Soybean aphids pose a significant risk to soybean production because of their tremendous reproductive potential. In the summer, the population in soybean is comprised of females that essentially clone themselves. All offspring are female, born pregnant, and give live birth. Their birth rate is 3-8 aphids per day for 30 days. The generation time is 7-10 days. The result is an exponential growth rate, where populations can double in 2-3 days under favorable conditions.
The factors that influence this population dynamic in soybean are numerous: the size of the colonizing population from buckthorn; soybean variety; mortality from predators, parasitic wasps, and fungal outbreaks; effects of environmental conditions on reproductive rate and survival; local redistribution by winged aphids among fields; and dispersal of winged aphids from fields. Winged aphids are commonly seen in late summer or early spring as aphids migrate between soybean and buckthorn. Overcrowding may force soybean aphids to produce winged forms, perhaps to enable them to colonize other less-infested soybean plants in the field. Additionally, deteriorating plant quality forces the production of winged females.
Soybean aphids can be found colonizing any above-ground portion of the soybean plant, including leaves, stems, and pods. The stem apices and young leaves of growing soybeans are colonized first. Later, the aphids appear on the underside of leaves and on stems and pods. Soybean aphids use a piercing-sucking stylet to extract plant sap. Soybean aphid feeding can result in yield losses up to 40%, but this extreme yield loss usually only occurs when heavy infestations result in sooty mold development on leaves (Photo 6). Sooty mold grows on the honeydew excreted by soybean aphids (essentially a sticky, sugary substance that is considered waste to the soybean aphid). Soybean aphids may also transmit viruses to soybean, although the role of soybean aphids in virus transmission in Iowa is not well studied.
Early in the season, fields near areas with abundant buckthorn or wooded borders may be at higher risk. Early planted fields are also attractive to soybean aphids migrating from buckthorn. Additionally, fields with coarse-textured soil and low potassium levels are at higher risk early in the season. Late-planted fields (usually double-cropped fields) may be at higher risk later in the season.
Soybean aphid populations fluctuate throughout the growing season, so it is important to scout for aphids on a regular basis to monitor populations. We recommend scouting every seven to 10 days as populations can double rapidly under ideal conditions. Prioritize scouting high-risk fields (early-planted fields or those with buckthorn stands nearby). Initial infestations will be on new growth, but later in the season, scouting should be done field-wide since colonies will form across the field and throughout the canopy. There are a few things that can indicate that aphids are in the field or high populations have developed:
- Early-season: Seeing lady beetles may be a clue that aphids are present (Photo 7).
- Early-season: Ants climbing soybean plants is a good indication of soybean aphid infestations; ants “tend” aphid colonies for their honeydew and provide protection from predators (Photo 7).
- High populations: Leaves that are shiny or black may indicate high populations; shiny, sticky leaves are coated in honeydew, and black leaves have sooty mold growth on the honeydew (Photo 6).
- High populations: Cast skins (white flakes) likely indicate high populations of soybean aphids (Photo 6).
Absolute counts: Randomly sample at least 30 plants per field, using a W- or Z-pattern to cover the entire field. Look at the entire plant and count the number of live soybean aphids (winged and wingless) on each plant, including stems and pods as well as winged and wingless aphids. Do not count cast skins (white flakes), diseased or parasitized aphids (see Biological in the Management section), or aphid look-alikes. If the average number of aphids across the field is below the threshold, return in one week to scout again. Continue to return weekly to monitor aphid populations through R6.
Insecticide applications are warranted until plants reach R6 when the average count is at least 250 aphids per plant AND at least 80% of plants are infested AND aphid populations are increasing. If plants are R6, aphid populations are very high, and plants are experiencing other stresses (e.g., drought), an insecticide may be warranted. Usually, yield loss will not occur after R5 since seed set is complete, and late-season applications are complicated due to the biology of the soybean aphid. This threshold has been widely studied and validated and typically allows one week for an application to be made before the economic injury level is reached. If an insecticide is applied, scout the field regularly after application to monitor for resurgence (second outbreak of aphids), replacement (outbreak of a different pest), or product failures (due to any reason, not just resistance).
Speed Scouting: Speed Scouting can save time when scouting soybean aphids and does not require absolute counts on soybean plants. However, it is difficult to monitor population dynamics over the season with Speed Scouting. With this method, plants are considered infested if there are 40 or more aphids (this is NOT a new threshold for soybean aphid but has been validated as a way to reduce scouting time). Use the Speed Scouting Worksheet to make treatment and scouting decisions.
- In each 50-acre area of soybean, scout for soybean aphids on 11 plants. Tally the numbers of infested (≥40 aphids) and uninfested (<40 aphids) plants. Using the Speed Scouting Worksheet, decide whether to treat the field, not treat the field, or continue scouting.
- When a treatment decision cannot be made and scouting must continue, continue the process on up to 31 plants. If you still cannot make a treatment decision, resample in three days.
- If treatment is recommended based on Speed Scouting, consider waiting and re-sampling in three days to confirm treatment is still recommended. Speed Scouting may recommend treatment when it is not necessary.
Tracking development: Soybean aphid has a lower developmental threshold of 50°F and an upper threshold of 95°F. Eggs begin hatching between 147-154 GDD accumulated since January 1. This coincides with buckthorn bud swell (165-171 GDD). You can track soybean aphid development on the Pest Maps and Forecasting page.
Cultural: Later planting may reduce the risk of early infestations on young plants. However, later planted fields are typically more attractive later in the season.
Biological: Since aphids are soft-bodied insects, many predators like to feed on them, including lady beetle adults and larvae, lacewing larvae, and pirate bug nymphs and adults. Soybean aphids also have parasitoids that turn aphids into “mummies”; mummies resemble aphids but are black or brown, depending on the species. Soybean aphids may also be infected with fungal diseases, turning discolored and fuzzy-looking. Soybean aphid natural enemies can be very effective in Iowa, so only making insecticide applications when deemed necessary through scouting can help preserve natural enemies in fields.
Host-plant resistance: Soybean aphid resistant varieties (Rag genes) can also be effective for management. However, there are a few challenges. First, current soybean aphid resistant varieties are limited in availability and genetics (herbicide tolerance has not been incorporated into soybean aphid host plant resistant varieties yet). Second, biotypes resistant to the different commercially available Rag genes, and combinations of Rag genes, exist and their distribution and frequency are not well understood. Soybean aphid-resistant varieties may be a good option for farmers that experience frequent soybean aphid outbreaks and do not use herbicide-tolerant soybean varieties in their operation.
Chemical: Neonicotinoid seed treatments are not reliable for soybean aphids because aphids typically colonize fields after neonicotinoid concentrations in plants are no longer effective. Many foliar insecticide products are labeled for soybean aphids and are highly effective if used according to the label. However, be aware that soybean aphid resistance to pyrethroids (e.g., bifenthrin, lambda-cyhalothrin) has been reported in Iowa, Minnesota, South Dakota, and North Dakota. Entomologists from these four states have a publication about managing insecticide-resistant soybean aphids.
If treatment decisions are made later in the growing season based on soybean aphid populations exceeding the economic threshold, consider the pre-harvest interval (PHI) of the insecticide product. This is the amount of time required by law between application and harvest.
For more information about soybean aphids in Iowa, go to the Soybean Entomology Lab website, which hosts a variety of publications.