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Soybean Aphid Update

University of Minnesota Extension, Stearns County News
August 12, 2015         
Source:  Dan Martens, Extension Educator
University of Minnesota Extension
Stearns, Benton & Morrison Counties

Soybean Aphid Update
By Dan Martens, University of Minnesota Extension

FOLEY, Minn. (08/07/2015) — Here’s an update on soybean aphids posted at the U of M Extension Crop News Website on Wednesday, August 5. You can keep up with U of M Extension perspectives on crop issues by doing a Internet search “Minnesota Extension Crop News;” and you’re welcome to call the county office. This information is from Extension Entomologists, Robert Koch, Ken Oslie, Bruce Potter, Ian MacRae, doing work across the soybean growing areas of Minnesota. Here are some key points.

Soybean aphid populations in many areas of Minnesota have been increasing. A number of factors make population trends and management less predictable than in the previous couple of years:

  • Late summer dispersal of winged of soybean aphids is currently occurring, bringing higher numbers of aphids to colonize fields; sometimes fields that were previously treated (maybe treated too early).
  • Forecasted weather conditions for the upcoming week look favorable for aphid population growth. (I’ll add that aphid populations can double in 3-5 days when daytime high temperatures stay close to 80 degrees with somewhat dry conditions. Heavy and/or frequent rain can slow them down a notch. Temperatures pushing much over 90 would slow them down a bit too.)
  • Bruce Potter reports some fields in southwestern Minnesota have reported unexplained poor performance of recent insecticide treatments and will require additional applications to control existing populations. (That might be interesting to learn more about.)

Our Extension Insect crew says these factors point to the importance of weekly scouting for soybean aphids and only treating when populations reaches the threshold of 250 aphids per plant on 80% of plants checked around the field. At this threshold, yield loss is not yet occurring, but yield loss can occur if aphid populations rise to 1000 aphids per plant. 

If a field needs to be treated more than once in the same year, remember the potential for developing insecticide resistance. Do not reapply the same insecticide mode of action. Ask if you need some help sorting that out. Do not spike or tank mix with a below-label rate of another insecticide.  Use of rates below label rates can increase chances of developing resistance. As the season progresses, be aware of the pre-harvest intervals of the various insecticides.

Failure of an insecticide does not necessarily mean the pest is resistant to the insecticide. Some other factors could be: misapplication, such as incorrect rate or poor coverage; unfavorable weather conditions (wind, rain, temperature); and recolonization by aphids moving in from other areas.

ADDED NOTES: Best management practices do NOT include spaying because the neighbor sprays. Even fields on the same farm may not all be the same. Making decisions based on scouting and threshold protocols is good for the pocket book, water, pollinators and other natural resources…and maintain farmers’ reputations in taking care of land, water and other natural resources.

From previous information, as we get later in the growing season we consider the following:

  1. Scouting should continue until R6.5 (pods and leaves begin to yellow), regardless of calendar date.
  2. Through R5 (seeds developing, but pod cavity not filled), use the economic threshold of an average of 250 aphids per plant AND aphids on more than 80% of plants AND aphid populations increasing.
  3. Yield loss can occur into early R6 (pod cavity filled by seed). At this stage consider insecticide application if aphid populations are high and plants are experiencing other stress.

My congratulations and admiration goes to 4H and FFA members, parents and families, and other youth and adults who are participating in county fairs - and working and learning together around homes and farms in other ways. It takes commitment, time and effort to share work and learning experiences. The tangible benefits are evident when people work together for constructive purposes. Some rest and recreation count too. 


Daniel Martens
Extension Educator, Ag Production Systems
(320) 968-5077
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