Scout for soybean aphids in fields now

By David Nicolai, University of Minnesota Extension

Soybean aphid numbers have increased to treatable levels (more than 250 aphids per plant) in some southern and central Minnesota soybean fields. However in other fields the populations are stable to low in many cases. Recent temperatures have been hot but this does not mean that soybean aphid populations will decline. As temperatures increase, aphid reproductive rate goes down while time between generations is shortened. The optimal temperature for soybean aphid is in the low 80s. Soybean aphids can reach more moderate temperatures by moving lower into the canopy even during hot air temperatures.

Because of potential movement of winged aphids in and out of soybean fields in August it is important to continue to scout soybean fields closely to determine accurate population levels. In August the threshold of 250 aphids per plant, when 80 percent of plants have aphids and populations are growing is still valid.

If over 70 percent of the plants are infested, use speed-scouting found at www.soybeans.umn.edu/home.htm to rapidly assess whether the field should be treated. To ensure the population is actually increasing, we recommend that the field be re-checked in three to four days.

Soybean aphid biology and weather

Insect natural enemies, rainfall and disease cause mortality and slow the rate of population increase. Typically, doubling times across the Midwest average about four to five days but during outbreaks maybe reduced to two to three days. Dr. Ken Ostlie, University of Minnesota entomologist reports this year indicate faster doubling times than usual from across Minnesota.

Ken also provides the following updates and aphid information: Soybean aphids are re-distributing among fields this time of year with young soybean fields and previously low fields preferentially colonized by winged aphids. Soybeans planted after pea harvest are particularly attractive and because of limited insecticide residual (three to 10 days at best) and rapid vegetative growth, may need to be sprayed more than once.

On very young plants, heavy downpours and hail can significantly reduce aphid numbers locally. Depending on local reproductive rates and immigration, these reductions may be temporary.

Prominent features of the soybean aphid life cycle include:
• After eggs hatch on buckthorn in spring, aphids build up numbers before colonizing soybean.
• Aphids may invade soybean anytime after emergence and colonization may continue into August.
• Populations build rapidly (only females present, give live birth, young mature in three to seven days, doubling time two to five days under favorable conditions).
• Winged forms appear when mothers are crowded or plants stressed and disperse to other fields.
• Sexes appear in the late summer and fall, fly to buckthorn, mate and lay eggs.
• Eggs overwinter under bud scales.
Females give birth to live young nymphs at a rate of three to five per day. These female nymphs mature in five to seven days and begin producing their own young. The result: an exponential growth rate where populations can double in two to three days under favorable conditions.

What about fungal outbreaks?
Heavy dews, fueled by recent rainfall and high dew points, may be increasing the prospects of fungal outbreaks as canopies begin to close. Fungal diseases on aphids require prolonged high humidities for successful sporulation and cuticle penetration. These ingredients plus cooler weather were implicated in the widespread collapse of soybean aphid populations last August.

What about insurance treatments against low-level infestations?
Insurance treatments for soybean aphids appeal to growers for several reasons: previous losses with tardy aphid control, anxiety about waiting for inevitable, avoidance of later-season infestations, protection of plant health, convenience of tankmixing with post-emergence herbicides, such as glyphosate. Typically there is little utility for insurance treatments against soybean aphid. Aphid populations vary widely from field to field, as does the timing of infestations.

Applying insecticides to unscouted or low-level infestations is a recipe for problems. First, fields may not need treatment so growers waste their money.

Second, early applications do not prevent subsequent colonization since any residual toxicity or repellency is gone in less than a week. There is only a temporary reprieve in scouting effort. Don’t walk away from any sprayed field for the rest of the season.

Third, early applications eliminate natural enemies and may actually increase the chances of needing to treat later in the season.

Fourth, unnecessary insecticide use enhances the chance that insecticide resistance will develop in aphids or other soybean pests, such as two-spotted spider mites.

Summary: Once a field is sprayed, don’t walk away from fields assuming a single spray will provide protection for the rest of the summer. There’s a lot of time left for re-colonization and aphid populations are reaching a point in the season where long-distance dispersal is more likely. Insecticide residues last less than a week, regardless of insurance or assurance program claims of 30 or more days. Check fields roughly two weeks after spraying to see if colonization and resurgence of the population has taken place. Aphids arriving in the field will find a field cleared of predators by the insecticide.

(David Nicolai is a crops educator with University of Minnesota Regional Extension in Hutchinson.)