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Soybeans are shown in a field early in July in Minnesota. (Jonathan Knutson/Forum Communications Co.)

Time to monitor soybean aphids in soybeans in southwestern Minn.

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Willmar, 56201
Willmar Minnesota 2208 Trott Ave. SW / P.O. Box 839 56201

HUTCHINSON -- There are soybean aphids in southwestern Minnesota and populations are rapidly increasing in a number of area fields. Scouting fields for soybean aphid pays.

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Perhaps even more than IN most years, growers should treat fields on an individual basis. Populations can be very different based on the location of the field, soybean growth stage and crop stage relative to neighboring fields.

Bruce Potter, integrated pest management specialist based at the University of Minnesota Lamberton Research and Outreach Center, recently reports that soybean aphid populations continue to increase. Potter indicates that alate (winged) aphids and alatoid (pre-winged) nymphs are common. Aphids are increasingly found in the lower canopy and we are starting to see some "white dwarf" aphids on lower leaves. Count these as any other aphid.

Any field in southwest Minnesota might have economic threshold numbers. An increasing aphid population is part of the 250 aphid/plant economic threshold. This does not mean all fields have a soybean aphid problem. In some areas, many or all do not. You have to scout each field to find out.

Planting date and soybean maturity is delayed this year. Aphid populations may be as well. Using research data from commercial soybean fields, University of Minnesota Extension recommends treating when rising aphid populations reach 250 per plant, with 80 percent or more of the plants in the fields harboring aphids.

Scouting soybean aphids, while time-consuming, is not difficult. Prior to soybeans flowering, aphids are most likely found on the top growing trifoliates of the plant. After flowering, the aphids will disperse lower onto the plant's stems, lower leaves and even pods. Where aphids feed doesn't seem to impact the amount of damage they cause. This is more a function of how many aphids there are and how long they've been feeding. Aphids suck sap, so the damage they cause is incurred over time.

One method of quickly estimating soybean aphid populations is to employ "speed scouting." This is a method of estimating aphid numbers using the presence or absence of aphids on a plant. Details on speed scouting are available at www.extension.umn.edu/go/1070.

The potential severity of a sustained level of soybean aphids can be measured by calculating cumulative aphid days, which is the mathematical basis for determining soybean aphid-induced yield loss in soybeans. Cumulative aphid days are an estimate of aphid pressure and a measure of aphid population density over time.

For example, the presence of 200 soybean aphids per plant for 20 days would equal approximately 4,000 cumulative aphid days (200 aphids multiplied by 20 days equals 4,000). Under stressful growing conditions, yield loss begins around 4,000 to 5,000 cumulative aphid days, and it may take many more.

These data are based on a large number of trials and produce repeatable results. The real point is that yield loss requires more than a couple hundred aphids per plant.

Fields with increasing aphid populations averaging more than 250 per plant are obviously outstripping natural enemies and more often than not will reach economic injury levels (greater than 4,000 aphid days). Thus, the 250 threshold provides a week of lead time for insecticide application.

The development of yield reducing aphid populations is much less certain if a field averages 50 or 100 aphids per plant. In other words, a threshold greatly below 250 would mean a high percentage of fields would be treated where aphid populations would not develop to yield reducing levels. It would also lead to treating earlier and in some cases a greater probability that the field would be reinfested and retreated.

David Nicolai is a crops educator with the University of Minnesota Extension Service Regional Center, Hutchinson.

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