It Is Time to Begin Scouting for Soybean Aphid

It Is Time to Begin Scouting for Soybean Aphid


The economic threshold for late vegetative through R5 stage soybeans is 250 aphids per plant with 80% of the plants infested and populations increasing.


Also see Managing Soybean Aphids for further information on thresholds, avoiding unnecessary treatments, general management guidelines, and why you still need to scout resistant varieties.



soybean aphid

Figure 1. Soybean aphid

soybean aphid

Figure 2. Soybean aphid populations can quickly build into the 1,000s under the right conditions.

Soybean aphid colonization has been slow to start this year, however, we are beginning to get reports of soybean aphids in southwest Minnesota and a few other states.  While we have had only one report of soybean aphids in Nebraska, and it was at a very low number, this is when we generally start finding them.

Start scouting now, as populations can start late and build fast. In 2011 we monitored a soybean field in Dixon County that was almost devoid of aphids on July 22, but by August 18 was over 2000 aphids per plant in areas of the field that were left untreated. 

Soybean Aphid Description

The soybean aphid is soft-bodied, light green to pale yellow, less than 1/16th inch long, and has two black-tipped cornicles (cornicles look like tailpipes) on the rear of the abdomen.  It has piercing-sucking mouthparts and typically feeds on new tissue on the undersides of leaves near the top of recently colonized soybean plants.  Later in the season aphids can be found on all parts of the plant, feeding primarily on the undersides of leaves, but also on stems and pods.

Soybean Aphid Life Cycle

The seasonal life cycle of the soybean aphid is complex with up to 18 generations a year.  It requires two species of host plant to complete its life cycle: common buckthorn and soybean.  Common buckthorn is a woody shrub or small tree and is the overwintering host. In the fall soybean aphids lay eggs on buckthorn. These eggs overwinter and hatch in the spring, giving rise to wingless females. These females reproduce without mating, producing more females. After two or three generations on buckthorn, winged females are produced that migrate to soybean.

Multiple generations of wingless female aphids are produced on soybeans until late summer and early fall, when winged females and males are produced and that migrate back to buckthorn, where they mate. The females then lay eggs on buckthorn, which overwinter, thus completing the seasonal cycle. Nebraska lacks significant and widespread buckthorn populations so early season soybean colonization by aphids appears to be limited.  

Soybean aphid populations can grow to extremely high levels under favorable environmental conditions. Reproduction and development is fastest when temperatures are between 70° and the mid 80°s when populations can double in two to three days. The aphids do not do well when temperatures are in the 90°s, and are reported to begin to die when temperatures reach 95°.

Like a number of other insect species (e.g., potato leafhoppers), these migrants can be caught up in weather patterns, moved great distances, and end up infesting fields far from their origin.   These summer migrants are likely the major source of infestations in Nebraska.

Soybean Aphid Natural Enemies

Soybean aphids have many insect predators.  The most visible predator is the multicolored Asian lady beetle, but the tiny (1/10-inch long) insidious flower bug (or Orius) is the most common and important predator. It feeds on a variety of small insects and spider mites. Naturally occurring predators, primarily the insidious flower bug, can significantly slow soybean aphid population growth, particularly during hot July weather. Other common predators include green lacewings, brown lacewings, damsel bugs or Nabids, and spined soldier bugs, among others.

Other groups of natural enemies include parasitoids and pathogens. The presence of aphid "mummies" (light brown, swollen aphids) indicates the presence of parasitoids. These mummies harbor immature parasitoids, which will become adults, emerge from the mummy, and parasitize more aphids. The presence of "fuzzy" aphid carcasses indicates fungal pathogens are present, which occasionally can lead to dramatic reductions of aphid populations. 


Symptoms of soybeans infested by soybean aphid may include yellowed, distorted leaves and stunted plants.   A charcoal-colored residue also may be present on the plants. This is sooty mold that grows on the honeydew that aphids excrete.  Honeydew by itself makes leaves appear shiny.  Soybean plants appear to be most vulnerable to aphid injury during the early reproductive stages.  Heavy aphid infestations during these stages can cause reduced pod and seed counts.

Soybean Aphid Occurrence in Nebraska

Soybean aphids have been reported in most soybean producing regions of Nebraska, although the highest and most economically damaging populations typically occur in northeast Nebraska.

It's usually mid-July before we begin to regularly find aphids, while soybeans are entering or in R3 (beginning pod stage). Nebraska aphid populations can reach economically damaging populations in late July, but most reach economically damaging populations in August, while soybeans are in the mid-reproductive stages (R4-R5).  In some years there are many fields where the aphid populations peak in late R5 (beginning seed) to early R6 (full seed). Of course, there are exceptions to any rule, so you should be watching for soybean aphid colonization and population increases.

Soybean Aphid Scouting

There are two methods to scout and determine if an insecticide treatment is warranted: a conventional scouting method using the 250 aphid/plant economic threshold and a speed scouting method. 

The Conventional, 250 Aphid/Plant Method

  • Begin scouting soybean fields once or twice a week in late June/early July. Check 20 to 30 randomly selected plants in various areas of each field. Aphids are most likely to concentrate at the very top of the plant, although they will move onto stems and within the canopy as populations grow and/or the plant reaches mid to late reproductive stages.  If a tree line or woodlot is adjacent to the soybean field, make sure and include a few sampling locations near these areas. Soybean aphids often are found first in fields near wooded areas.
  • Counting aphids is not as difficult as it may at first seem.  First, walk to a random spot in the field. Pull a plant, turn it upside down, and give it a quick scan to see where the aphids are located. Get a feel for what 10 or 20 aphids look like and count by 10s or 20s. 
  • The current threshold for late vegetative through R5 stage soybean is 250 aphids/plant with 80% of the plants infested and populations increasing (see above for details).   Thresholds for early R6 have yet to be determined, but are likely in the 400-500 aphids/plant range.  Insecticide treatment during or after mid-late R6 has not been documented to increase yield.

The Speed Scouting Method

This method uses a spreadsheet adapted from a sampling plan developed by the University of Minnesota. (See Enumerative and Binomial Sequential Sampling Plans for Soybean Aphid (Homoptera:Aphididae) in Soybean, E.W. Hodgson, E.C. Burkness, W.D. Hutchison, and D.W. Ragsdale, 2004, Journal of Economic Entomology 97(6): 2127-2136).  It is different from conventional scouting in that it relies on the number of "infested" plants. Plants are considered "infested" when there are 40 or more aphids on a plant. The scout does not have to count or estimate the number of aphids on a plant to determine whether it has reached the threshold of 250 aphids per plant. One simply determines if a plant is infested and enters this in the spreadsheet. The spreadsheet then recommends further scouting or treatment options based on the number of "infested" plants in a given area.

This tool is available on the Web in the UNL Extension Circular, Aphid Speed Scout (EC1582) or in a mobile app in both the Android and iTunes stores.

For treatment recommendations, including products and timing, see Managing Soybean Aphids.

Tom Hunt, Extension Entomology Specialist
Keith Jarvi, Extension Educator
Wayne Ohnesorg, Extension Educator


Online Master of Science in Agronomy

With a focus on industry applications and research, the online program is designed with maximum flexibility for today's working professionals.

A field of corn.