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Soybean Aphid Surveys and What is beginning to Develop Regionally

It is that time of year when soybean aphid populations require monitoring. We have been scouting fields across the region for three weeks now. As aphids colonize fields and increase their numbers, it becomes a great time to begin scouting and start to track the population's progress over the next five weeks. The joint MN-ND soybean survey map provides a regional perspective on where early aphid activity is underway.
Activity in Minnesota has been reported in portions of Central, West Central, and to a lesser extent South Central. Currently, even the fields where plants with aphids are greater than 75%, the aphids/plant average is less than 40. However, as fields approach the 100% infested plant level, populations can increase to treatable levels.
The next MN survey map illustrates aphid numbers for each production field scouted as a pie chart. Scouts estimate populations on each plant sampled, placing a tally mark in the estimated # of aphid / plant range.The pie chart indicates the % plants observed within a range. 

Any pie charts that begin to show orange and red indicate those fields had plants with aphids into the 100's. The more orange to red, the greater the percent plants with larger numbers of aphids.This map was created Friday, July 13. More fields will be added later as all the reports for this week are submitted and mapped. Also, note that the symbols overlap each other. The symbol for the more heavily infested fields will always be layered on top in the graphic and not hidden.

Some of the fields sampled along that southern edge (Counties of Stearns, Meeker, Kandiyohi, Pope and Douglas) have some fields with plants in the 61-100 aphid per plant range, but still represent a smaller proportion of the plants sampled. 

Aphid updates and other important crop updates can also be found at Southwest Minnesota IPM Stuff Newsletter prepared by Bruce Potter, UMN Extension IPM Specialist. Subscribe or follow the link to read his updates. 

Be sure to read the statewide perspective on aphid population development in the MInnesota Crop News, as well.

The updates for both these resources are listed and linked in the right hand column for quick reference.

Your Monitoring Program

Yield damaging soybean aphid populations can occur in any field. However, you may have noticed that some of your fields tend to have more consistent problems with aphid infestations from year to year. Several factors increase the likelihood of aphid problems. 

In spring, aphids are often found first in geographic areas with abundant buckthorn. Smaller fields with wooded borders are often the first to develop high populations. In addition, early-planted fields and fields with slightly droughty coarser textured or lower potassium testing soils often see aphid populations develop sooner. Early planted soybeans and perhaps uncontrolled volunteer soybeans serve as a source of aphids for other fields. Later in the season, full-maturity soybean or late-planted soybean are often reported to have larger populations.

Rigorous scouting of high-risk ("indicator") fields can provide valuable information on when aphid populations are beginning to increase in your area, which should trigger scouting of other fields. 

Treatment Decisions:  Pyrethroid Resistance Complicates Decisions 

Be sure to scout fields BEFORE making a treatment decision. Avoid the temptation to treat below threshold numbers because it is "cheap", or for convenience or ‘insurance’; this is bad management. Insecticide resistance means management will no longer be easy or cheap. 

These non-threshold-based approaches have put us in a difficult position. You should have heard or read plenty of information this winter about Pyrethroid Resistance in soybean aphid. Insecticide treatments place selection pressure on the population to find survivors. Treating frequently increases the likelihood that you find those survivors sooner. 

Resistance was documented for soybean aphids in 2013, 2014, 2015, 2016 and 2017. In 2016, fields in Polk County, MN required 4x the bifenthrin and 28x the lambda-cyhalothrin for control when compared to a lab-reared, susceptible colony of soybean aphids. Populations tested in 2017 were much more hardy. In laboratory screening, populations survived a dose selected that should have killed 99% of the aphids. Some populations had as little as only 10% of the aphids die.

Soybean aphid population screening for mortality when exposed to an LC99 dose for two representative pyrethroid insecticides. LC99 stands for the "Lethal Concentration expected to kill 99% of the exposed population." Populations highlighted by the asterisk were significantly different from the populations with 90% or greater mortality. Source: Dr. Robert Koch, Soybean Entomologist, U of Minnesota.

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