Managing Soybean Aphid

Managing Soybean Aphid

Ken Ostlie
Department of Entomology
University of Minnesota

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The soybean aphid (Aphis glycines), an insect native of Asia, has rapidly invaded soybean in Minnesota and the U.S. Last summer the explosive buildup of aphids led to severe yield losses and unanticipated insecticide use in southeast Minnesota. The rapid spread of the aphid across Minnesota now places most soybean acreage at risk of infestation in 2002. Many growers and their advisors will be making aphid management decisions for the first time. This brochure provides basic information on the soybean aphid, its population dynamics, its damage to soybean, and management options.

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Risk for 2002

The soybean aphid spread rapidly across Minnesota in 2001, as revealed by MDA and U of M surveys. Thus in 2002 severe infestations are possible in much of the state; risk will be less in the northwest. Recent heavy rains have suppressed colonization of soybean.

Population Dynamics in Soybean

Aphids colonize soybean in early June from their overwintering host, a small understory tree called buckthorn (Rhamnus spp.). These spring immigrants deposit live young, called nymphs, under upper soybean leaves. Within a week, these nymphs mature into fully reproductive females that can deposit 2-3 young per day.

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Colony of aphids in transition: Note winged female (left side), nymph developing wings (lower right), and numerous cast skins (white) (H. Russell).

Populations build rapidly, doubling every 2 to 3 days, and may reach several thousand aphids per plant at their peak in early August. A majority of adult aphids are wingless; they reproduce on the same or adjacent plants. Some females develop wings and fly to other plants within the same or nearby fields to lay their nymphs.

Up to 15 generations may occur on soybean in the summer. The proportion of nymphs that become winged increases markedly as females respond to crowding, reduced host quality or decreasing daylength. In August, populations decline as winged aphids migrate away from fields or through natural control of the aphids (e.g., fungal disease outbreaks)

How do aphids damage soybean?

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Growth differences between untreated (right) and insecticide-treated soybean (C. Louis)

Aphids pierce the soybean's plumbing system and suck sap. Feeding impacts yield in three ways: removal of photosynthates (sap), reduction in photosynthesis, and transmission of viral diseases. Soybeans may become stunted with poor canopy development. Soybean growing in low potassium soils may also become chlorotic in upper leaves, an unusual deficiency symptom. Aphids may impact yield before plant symptoms become readily apparent.

Continued feeding during reproductive stages reduces pod and seed set (see graph), and potentially seed size, if pressure continues. The biggest effect is on pod set. Yield reductions may be significant, exceeding 50% in severely infested fields. Preliminary evidence suggests aphids affect photosynthetic rates as well as remove plant sap needed for growth and yield. Transmission of soybean mosaic or other viruses by soybean aphid further complicates losses and may affect seed certification. This disease dimension will develop over the next few years.

Yield impacts

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Soybean aphid effects on yield and yield components in five severely infested fields (K. Ostlie)

With populations reaching several thousand per plant, yield reductions may exceed 50%. The following graph illustrates the average impact on yield and yield components in five severely infested fields in southeast Minnesota in 2001. The primary effect of soybean aphids was on pod set, with lesser effects on number of nodes and seeds per pod. Yield benefits in insecticide-treated fields averaged over 8 bu/acre in 2001, but yield benefits varied widely between fields.

Management options

A comprehensive integrated pest management (IPM) approach, which blends cultural tactics, varietal resistance, biological control, and insecticides, frequently can't be used against newly invading pests. It takes time to examine the effects of soybean production practices on soybean aphid, explore the resistance or susceptibility of soybean varieties, and understand the impacts of natural enemies. Insecticides are the only readily available tools to manage soybean aphid until other alternatives are developed.

Scouting for aphids

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Scout for soybean aphids in the upper 3 leaves of vegetative soybean (B. Potter).

Aphids begin colonizing soybean fields at V1-2 (two or three nodes with open leaves). In early July, start scouting to become familiar with the aphid pressure in your fields. Aphids are small (1/16" or less), so a hand lens or other magnifier is needed.Check these locations:

During vegetative growth, look for aphids under young upper leaves, petioles, and stem.
During later vegetative stages also check new branches in the lower canopy.
During reproductive stages, aphids move to undersides of mid-canopy leaves and to stems.

Look for other obvious insects that aphids attract: ants that tend aphids or ladybird beetles that eat them. Also check for signs of aphids, such as the white cast skins of molting aphids, honeydew (sap excreted by aphids), or sooty mold (a black mold that grows on honeydew).

The development of aphid populations in each field will be unique. Scout two times per week because aphid populations double every 2 to 3 days during this time period. Continue scouting through pod set.

Critical decision period

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Yield benefits of insecticide applications against soybean aphid in on-farm trials in 2001 (K. Ostlie)

Timing is critical. Insecticides only suppress soybean aphid populations for 7 to 14 days. Applications during the initiation of flowering (stage R1) and into full flower (stage R2) had the greatest benefit in 2001. Earlier applications risk rebound of aphid numbers just as pod set is occurring. Later applications may achieve less-than-desired benefits: damage to canopy development and pod set has already occurred, and aphid populations will soon decline from migration or fungal diseases. The critical period for insecticide decisions is from initiation of bloom to early pod set, when aphid populations are building rapidly, but there is still time to protect pod set. Monitor aphids after spraying for resurgence. WARNING: Many aphid species are resistant to insecticides. Multiple or poorly timed insecticide applications favor development of resistance. Scout to ensure insecticides are needed and well timed.

Insecticides for soybean aphid

Insecticides, especially foliar sprays, provide temporary suppression (7 to 14 days) of soybean aphid populations. The following insecticides are labeled for foliar application against soybean aphid, also called Chinese aphid on some labels. Although seed treatments with systemic activity show promising results, none are currently labeled against soybean aphid. Always read and follow label directions.

Insecticide	Rate per acre	REI¹	PHI²
*Asana XL	5.8 - 9.6 oz	12 hrs.	21 days
dimethoate	(see label)	48 hrs.	21 days
*Furadan 4F	0.50 pt	48 hrs.	21 days
*Lorsban 4E	1.0 - 2.0 pt	24 hrs.	28 days
*Mustang 1.6E	3.4 - 4.3 oz	12 hrs.	21 days
*Penncap-M	2.0 - 3.0 pt	4 days	20 days
*Pounce 3.2EC	4.0 - 8.0 oz	12 hrs.	60 days
*Warrior T	3.2 - 3.84 oz	24 hrs.	45 days

* Restricted-use insecticide

¹Restricted Entry Interval (REI): Entry into treated field within this interval requires protective clothing and gear as specified on label.

² Do not apply insecticide in this pre-harvest interval (PHI).

Soybean aphids present a challenging situation for foliar insecticides. Aphids generally feed beneath the leaves, move little, and have tremendous reproductive potential. Populations quickly rebound if insecticides are applied too early, rates are too low, or coverage is poor. For these reasons, good coverage is critical and best achieved through ground application at high water volumes (> 15 gallons per acre) and high pressures. Tank mixing with postemergence herbicides is not recommended because optimal timings for weed and insect control usually do not coincide. Application adjustments (high volume and pressure) for aphids are not compatible with herbicide applications.

Management Tips for Soybean Aphid

Begin scouting in early July to identify heavily infested fields.
Scout one to two times per week through pod set.
Look for aphids on the undersides of the upper three leaves in vegetative and flowering soybean.
Estimate aphid density per plant in at least 10 locations throughout the field.
Consider treating with insecticide if aphids are common and abundant (average more than 250/plant).
Insecticides may not be economical if:
1. Soybeans are setting pods,
2. Infestation is spotty,
3. Most nymphs are developing wings, or
4. Diseased nymphs are common.
Leave untreated yield strips and follow-up on aphid control if you use an insecticide.
Follow precautions to minimize bee kills and communicate treatment plans to beekeepers.

For further information on the soybean aphid and its management, consult the following web site: http://www.soybeans.umn.edu/

Funding provided by:
Minnesota Soybean Research& Promotion Council