Published: July 24, 2008 12:25 pm            

Soybean aphid numbers continued to increase last week

Problem less predictable this year than in the past

Bruce Potter, integrated pest management specialist from the SouthWest Research and Outreach Center at Lamberton, reports that localized soybean aphid problem fields appear to be widely distributed through western Minnesota.

Several consultants have reported threshold numbers in fields that typically have early aphid populations (e. g. fields near rivers, creeks, lakes and other areas with buckthorn concentration) in south central, southwest and west central Minnesota. Most have reported that larger fields, away from buckthorn, typically have low populations. Again, this is not unusual.

At this time, economic levels of soybean aphid are not widespread. Fortunately, Potter reports that there are not any large geographic areas in the Upper Midwest with a high incidence of heavily infested fields to supply aphids downwind. Unfortunately, the wide geographic area with scattered fields near threshold may make this year problem less predictable than others.

While the fields examined have a less clear edge effect than previous years, it is still there. Most aphids are concentrated on the upper foliage of these early reproductive stage soybeans. This will change as new leaf development ceases and pod fill begins (R4-5). Extreme heat, heavy rainfall and wind or hail can lead to aphids using the lower portion of early reproductive soybean canopy to a greater extent.

Potter expects that aphid populations will increase more quickly in areas with less rainfall or in fields that hold less moisture. On a calendar basis, southwest Minnesota soybeans are behind in development but aphid populations are tracking similar to ahead of 2006. Hopefully, the recent rains and some moderate temperatures can help generate some fungal controls in more heavily infested fields.

Remember that the economic threshold for treatment of 250 aphids per plant, when 80 percent of plants in the field have aphids and populations are growing, is still valid, so field scouting is an important management practice.

        

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