NewsFlash Logo Scout for Soybean Aphids

Soybean aphids on a single stem.
The soybean aphid (Aphis glycines Matsumura) was found in two southeastern Nebraska counties earlier this month. Only four aphids were found, but they serve as a reminder that farmers should begin scouting for this insect.

The soybean aphid is new to North America. It is an Asian soybean pest that was first discovered in the United States in Wisconsin in summer 2000. Since then it has spread throughout the north central United States and parts of Canada. We expect the aphid to spread to all soybean-producing areas of Nebraska.

Heavy infestations of this insect can cause significant damage and yield loss. Yield losses exceeding 25% were observed in Minnesota and Iowa in 2000. In addition, soybean aphids can transmit viral diseases, such as alfalfa mosaic, soybean mosaic, bean yellow mosaic, peanut mottle, peanut smut, and peanut stripe.

Soybean aphid description

The aphid is light green to pale yellow, less than 1/16 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 near the top of soybean plants or on the undersides of mature leaves. Later in the season the aphids can be found on all parts of the plant. It is the only aphid in North America that forms colonies on soybean.

Life cycle and injury to soybean

The seasonal life cycle of the soybean aphid is complex with up to 18 generations a year. Itrequires two species of host plant -- buckthorn and soybean -- to complete its life cycle. Buckthorn, a woody shrub or tree, is the overwintering host plant of the aphid. Soybean aphids lay eggs on buckthorn in the fall. 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/fall, when winged females and males are produced and migrate back to buckthorn, where they mate. The females then lay eggs on buckthorn, which overwinter, thus completing the seasonal cycle. Soybean aphid populations can grow to extremely high levels under favorable environmental conditions. Reproduction is fastest when temperatures are 72-77oF. Developmental time slows when temperatures exceed 81oF. When populations reach high levels during the summer (there were reports of up to 13,000 aphids per plant in Michigan), winged females are produced and migrate to other soybean fields. 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.

Soybean aphids injure soybeans by removing plant sap with their needle-like mouthparts. 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. 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 management

The aphid is very new to North America. As we gain more experience with it, the following management recommendations will be refined for Nebraska conditions.

    Scout 5 locations for every 20 acres of soybeans.
  1. Begin weekly scouting of soybean fields in late June. If possible, scout five locations for every 20 acres. At a minimum scout 10 locations per field. At each location, select five plants and estimate the aphid density per plant. Aphids are most likely to concentrate at the top of the plant, although in later reproductive stages they will move onto stems and within the canopy. As the season progresses, aphid numbers can change rapidly (populations can double in two to three days), so in July increase scouting to twice a week.
  2. Look for the presence of aphid natural enemies such as lady beetles, green lacewings, and other insect predators. These predators may keep low or moderate aphid populations in check. Often you may be able to find soybean aphids by examining plants where lady beetles are observed. The presence of "fuzzy" aphid carcasses indicates fungal pathogens are present, which can lead to dramatic reductions of aphid populations.
  3. Take note of winged aphids or "broad-shouldered" nymphs. Nymphs with broad or squared-off shoulders will become winged adults. These aphids will leave the plant, so if the majority of aphids are winged or about to become winged adults, the field may not have to be treated because the population will rapidly decline. A magnifying glass is helpful to see the "broad-shouldered" nymphs, but the winged adults are easy to see with the naked eye.
  4. Take note of plant condition. Plants under moisture stress are more vulnerable to economic damage.
  5. Closely examine late emerging soybean fields. It appears aphids damage late-planted soybeans more than early-planted soybeans.
  6. Consider insecticide treatment when soybeans are in the R1 to R2 stage (first bloom to full bloom) and there are over 200 aphids per plant, most of which do not have wings or "shoulder-pads".
  7. If the plants are covered with honeydew or sooty mold, or stunted, an insecticide treatment may still be of value but the optimum time of treatment is past.
  8. Initial research has indicated that the highest yield responses were obtained when insecticide treatment occurred between mid-July and the first few days of August (R1-R2 stages). Treatment after this period resulted in less yield response, however, 2002 field trials in South Dakota indicated treatment as late as stage R5 (beginning seed) resulted in significant yield protection.
  9. If fields are treated, leave an untreated test strip to compare against sprayed sections. This also provides a refuge for beneficial insects.
  10. Good insecticide coverage and penetration is required for optimal control of soybean aphid, as aphids feed on the undersides of the leaves and within the canopy. Use high water volume and pressure.
  11. Several insecticides are labeled for the soybean aphid (Chinese aphid on some labels). Alist of registered insecticides, rates, preharvest intervals, etc. is available on the UNL Department of Entomology Web site at http://entomology.unl.edu/instabls/soyaphid.htm
  12. Always read and follow label directions.

Current status in Nebraska

Although the aphid has only been found in about 15 Nebraska counties, it is likely established throughout eastern Nebraska. Common buckthorn, the aphid's overwintering host, has been found in much of eastern Nebraska. It is particularly prevalent along creek and river bottoms and also can be found as an understory shrub/small tree in wooded areas.

Because the soybean aphid is so new to North America, researchers are still determining soybean aphid biology, impact, and management options in North America. It is likely that the aphid will behave differently in different parts of the Midwest, so it is important that aphid infestations be reported to Nebraska researchers so Nebraska-specific studies can be initiated.

If you find aphids in your soybeans this season, please contact Tom Hunt, UNL Extension Entomology Specialist, Haskell Ag Lab, Concord, NE, (402) 584-2863, thunt2@unl.edu.

More information on the soybean aphid can be found at the UNL Entomology Web site and the Soybean Aphid Watch Web site.


 

Tom Hunt
Extension Entomologist
Haskell Ag Lab, NEREC