Scouting Soybean Aphids

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A Visual Guide to the Number of Soybean Aphids per Leaflet. Each soybean leaflet has a specific number of aphids displayed to help you count when field scouting this pest. From the University of Wisconsin IPM program. View pdf»

Aphids, winged aphids, and cast skins are visible in this photo.Photo credit: Michigan State University

Fungi called sooty molds grow on the honeydew, giving leaves a grayish look. Photo credit: Chris DiFonzo, Michigan State University

Parasitoid wasps are small and not easily seen. However, the results of their actions (aphid mummies ) are easily identified, here as the black structures. Photo credit:  Bob Ellingson, University of Wisconsin.

Aphids can infest a soybean field at any time during the growing season. Multiple scouting visits are needed to detect those infestations that come early by over-wintering aphids, or later in the season by migrating aphids.

• Scout once or twice a week beginning in late June or early July. Fields near buckthorn, the overwintering host, may be colonized at emergence and require earlier scouting. In areas without buckthorn, winged aphids migrate from other areas in mid-season.
• Aphid infestation is usually patchy. Check 20 - 30 plants in 10 different places, covering 80% of the field. Aphid infestation does not necessarily begin at the edge of the field. Examine the entire plant, particularly new growth. Consider speed scouting (see below).
• Aphid populations tend to be higher and more damaging in fields or parts of fields where plants are already stressed from dry soil, soil fertility factors, weather conditions, other diseases, or in fields that are planted late. Scout such fields most closely.
• To determine if an aphid population is actively increasing, check over several visits. Conditions that favor aphid population growth are cool temperatures, plant stress, particularly drought stress, and a lack of aphid predators.
• Use an action threshold of 250 aphids per plant if populations are actively increasing. This action threshold should be based on an average of 250 aphids per plant over 20 to 30 plants sampled throughout the field.
  
  In replicated research trials, this threshold has worked well in late vegetative (right at first bloom) to R4 soybeans. Spraying at or beyond R6 has not been documented to increase yield, especially if the crop has grown well through the vegetative stages. This threshold incorporates an approximately 7-day lead-time between scouting and treatment to make spray arrangements or handle weather delays.
  
  View Soybean reproductive growth stages at R1- R8 and soybean aphid action thresholds (a pdf file) for a good overview of this.

Early season scouting

• Look for the insects on the undersides of new leaves. They will be pale yellow and can be difficult to see.
• Use a hand lens. Aphids will be all different sizes, some with wings, others without.
• The presence of ladybug larvae at this stage (Figure 2) is a good sign aphids are present.
• Aphids might be confused with springtails or leafhopper nymphs. However, those insects will move quickly when disturbed, while aphids remain still.
• Assess the condition of the crop. Stresses such as dry soil, fertility imbalance, weather, and other pests and diseases can lead to a greater impact from aphid feeding than would occur under optimal conditions.

Midseason scouting - late V, early R

• Early bloom (R1-R3) is the critical stage in which peaking aphid populations pose the greatest risk to soybean yield. Be prepared to assess threshold levels.
• Check the entire plant. Aphids may now be moving down to new growth at lateral branches and the developing pods.
• Distinguish between wingless aphids and winged aphids. A large percentage of winged aphids means the colony will rapidly leave the field.
• Look for other signs of aphid infestation: plants are likely to be considerably above threshold if stems or pods are covered with aphids and honeydew, sooty mold covers the bottom leaves, or plants are stunted. Insecticide treatment is probably still of value, but the optimal time for treatment (greatest economic return) is past.
  • Cast skins are light-colored skins left from successive molting.
  • Plant symptoms: stunting, poor canopy closure, leaf distortion, symptoms of potassium deficiency in the upper leaves.
  • Aphids secrete a sugary solution called honeydew that attracts ants. Fungi called sooty molds grow on the honeydew, giving leaves a grayish look.
• Watch for natural predation at work:
• Predators and parasites such as the syrphid fly, multicolored Asian lady beetle larvae and adults, lacewings, and parasitic wasps.
• Check for mummies (parasitized aphids )and for winged females . Parasitized aphids slowly decline, turning tan, pink or brown. Do not spray if mummies are numerous, or if a majority of the aphids are winged or developing wings, an indication that the aphids will soon leave the field.
  

Speed Scouting References

University of Minnesota Aphid Speed Scouting webpage - overview, worksheets, examples, and FAQ

Hodgson, E. W., E. C. Burkness, W. D. Hutchison, and D. W. Ragsdale. 2004. Enumerative and binomial sequential sampling plans for soybean aphid (Homoptera: Aphididae) in soybean. Journal of Economic Entomology 97(6): 2127-2136.

Field_Validation_of_Speed_Scouting_for_Soybean_Aphid. May 2007. Plant Management Network. E.W. Hodgson, B.P. McCornack, K.A. Koch, and D.W. Ragsdale (MN), K.D. Johnson and M.E. O'Neal (IA), E.M. Cullen and H.J. Krauss (WI), C.D. DiFonzo (MI) and L.M. Behnken (MN).

Information on these pages is from the cooperative effort of researchers throughout the North Central states. Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: NCSRP Plant Health Initiative at www.planthealth.info. Please do not use copyrighted photos without permission.