Aphids on a soybean leaf Photo credit: Michigan State University

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 to 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.

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. View current soybean aphid management recommendations» (link to the NCSRP Plant Health Initiative website)

Early season scouting

The presence of ladybeetle larva is a good indication that aphids are present.

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 is a good sign that 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. View photos of soybean growth stages at R1- R8 and soybean aphid action thresholds» (pdf)

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.

Brown lacewing - a common aphid predator Photo credit: Dan Mahr

Watch for natural predation at work. Learn to recognize predators and parasites such as the syrphid fly, multicolored Asian lady beetle larvae and adults, lacewings, and parasitic wasps. Fungal pathogens can also reduce aphid numbers quickly in warm, wet weather.

An aphid infected by by a fungal pathogen. Aphid populations can decline rapidly in warm, wet weather from fungal pathogens. Photo credit: Bill Stoneman

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.

Aphid mummy (a shell left after parasitism by wasp larvae) Photo credit: Bob Ellingson

Source: NCSRP Plant Health Inititative, Soybean Aphids: Scouting