Soybean aphids cause damage by sucking plant sap and transmitting viruses during the feeding process. Click on photo to veiw larger version. Photo © University of Wisconsin.

The soybean aphid is a recently introduced pest of soybean. It's current North American range includes the upper Midwestern states and three Canadian provinces.

Soybean aphids cause damage by sucking plant sap. Symptoms of feeding damage are plant stunting, yellow and puckered leaves, and reduced pod numbers. Yield loss is related to aphid density at specific crop stages and on the condition of the crop.  Risk of yield loss is greatest when aphid populations peak at beginning flower because of the impact on pod set.

Soybean aphids are also capable of transmitting soybean viruses such as Soybean Mosaic virus and Alfalfa Mosaic virus. We do not recommend spraying insecticide to control virus infection, however, because it is not effective. Aphids present at spraying are killed, but can transmit virus before they die. The field is quickly recolonized by winged aphids and virus transmission can resume. Planting seed clean of the virus is the best way to prevent virus disease.

Scouting Aphids

Start spot-checking seedling soybean and continue checking through pod set. Aphids are usually found on the undersides of new growth at the top and side branches. The insects are small (1/16 inch), soft-bodied, with wings or wingless. They are yellow in color early in the season. Winged adults have a black head and thorax. 

Aphids tend to build up more heavily on late-planted fields (see graph at left), so check late-planted fields closely. Moisture stress also favors aphid population growth and adds to yield risk.

Scout more intensively in July and estimate density on 20-30 plants per field, covering at least 80% of the field. Identify heavily-infested fields. Late vegetative and early reproductive stages are when soybean aphid populations tend to increase most. This is also the stage where yield return on treatment is the highest. Check the entire plant; at this stage aphids move from the top of the plant to the middle or lower areas of the canopy. Check the undersides of leaves, petioles and pods.

Take note of the presence of winged aphids and alatoid nymphs (with wing pads), high predator activity, and/or diseased aphids. These are all signs that the population is in decline or will leave the field shortly. Scout these same fields again within a few days to note If populations are increasing or decreasing.

UW entomologists are studying natural enemies that occur in Wisconsin, as well as working towards the release of additional aphid parasitoids collected from their native areas. Read more about soybean aphid parasitoids in Wisconsin»

Action Threshold

The timing of an insecticide application is crucial. The best treatment time is just at beginning bloom (usually late July to early August) to beginning pod stages if aphids are present at threshold and actively increasing. Use an action threshold of 250 aphids per plant. This action threshold should be based on an average of aphids per plant over 20-30 plants sampled throughout the field. Regular field visits are required to determine if aphid populations are increasing.

In replicated research trials, this threshold has worked well in R1 to R4 soybeans. Spraying at R6 or beyond has not been documented to increase yield. Look at Soybean Development Stages and Soybean Aphid Thresholds (a pdf file) for good photo close-ups of these critical growth stages.

This threshold incorporates an approximate 7-day lead-time between scouting and treatment to make spray arrangements or handle weather delays.

Caution must be used if treating aphids before mid-July. Early applications are not effective because they kill beneficial insects, and only temporarily reduce aphid populations, but do not provide full protection against yield loss.

Plants are likely to be considerably above threshold if stems or pods are covered with aphids and honeydew, sooty mold covers the bottom leaves, and plants are stunted. Insecticide treatment is probably still of value, but the optimal time for treatment (greatest economic return) is past. Review aphid management recommendations for 2007 »

Insecticides

Consider the product choices for your situation. Pyrethroids (Warrior, Mustang Max, Asana, Baythroid) and organophosphates (Lorsban) are two insecticide classes labeled for soybean aphid on soybean and commonly used in chemical control programs. Organophosphates exhibit a " fuming" action, which may work better in heavy canopies or at higher temperatures. Pyrethroids tend to provide longer residual than organophosphates or carbamates (Furadan) and are most effective at temperatures below 90°F.

Roundup Ready soybeans, soybean aphid, and the potential for soybean rust in Wisconsin have increased growers' interest in combining all pesticide products into a single spray application. While convenient, these tank mixtures are not recommended. Insect, disease and weed pests do not all appear at the same time at economically damaging levels so a single tank mixed application will not provide satisfactory control of all three pest types. Additionally, sprayer specifications such as water volume, nozzle type (droplet size) and pressure must be optimized for each pest situation. For example, medium to fine droplet sizes suitable for many fungicide and insecticide applications are not appropriate for herbicide applications where larger droplet sizes are necessary to avoid herbicide drift.

For soybean aphid, good coverage is important. Higher spray volumes (15 to 20 gpa) and higher pressure help to move the insecticide down into the canopy.

Adding insecticide to early-season glyphosate applications as "insurance"  is not recommended unless aphids are at threshold levels and actively increasing.

Common buckthorn (Rhamnus spp).

Life Cycle and Population Growth

The soybean aphid has a complicated life cycle that is completed on two very different plant hosts. Aphids overwinter in the egg stage on the leaves of  buckthorn. Two or three generations of wingless females are produced on the buckthorn in the spring, followed by a winged generation that leaves the buckthorn in search of soybean.

In the summer, many generations of mostly wingless females are produced on soybean until decreasing daylength triggers production of winged adults and a return to buckthorn in the fall.

Questions? Please email Eileen Cullen at cullen@entomology.wisc.edu