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Now is time monitor soybean for soybean aphid.

Soybean aphids spend their winter as eggs on buckthorn plants. After emerging from their eggs aphids spend a bit of time on buckthorn. Winged soybean aphids then soon find their way into adjacent soybean fields. Earlier planted soybean fields and the edges of fields nearest wooded areas inhabited by buckthorn are likely locations each year to first find soybean aphids.

Extension entomologists –and the students that worked with them- spent many an hour over multiple years working to better understand the relationship between soybean aphid populations and the soybean crop. This research helped to establish treatment thresholds. Thresholds were established to allow plenty of time for an insecticide treatment to happen before economic yield loss occurs.

Scouting soybean fields for soybean aphids is important for several reasons, including: avoiding unnecessary insecticide applications, preserving the efficacy of insecticides for when they are truly needed and knowing when to apply an insecticide once thresholds have been reached.

Economic threshold.

Now and until soybeans reach the R5 (beginning seed, but before seeds fill the pod), treat soybean fields with an effective insecticide if the following three criteria are met:
  • more than 80% of plants are infested with aphids
  • there is an average of 250 aphids per plant,
  • the aphid population is growing.

Scouting 101.

There are two different ways to scout fields for soybean aphids and make a threshold-based treatment decision. One way is to count the number of soybean aphids on each plant. The other method is called “speed scouting” because, rather than having to count all of the aphids on each plant, one can simply train their eyes to determine whether a plant is infested (there are more than 40 aphids) or uninfested (there are fewer than 40 aphids).

University of Minnesota Extension entomologist Dr. Bob Koch and integrated pest management specialist Bruce Potter put together some great information about how best to tackle scouting soybean fields for aphids, regardless of your preferred method. This information can be found here.

University of Nebraska Extension developed an iOS app to help people that are practicing the speed scouting method. The free Aphid Speed Scout app can be found on the Apple App Store here.

Soybean aphid survey program.

Similar to other recent years, University of Minnesota Extension has student interns scouting soybean fields this summer looking for pest and disease issues. Interns are stationed out of regional Extension offices in Crookston, Moorhead, Morris and Rochester. This survey program is sponsored by both the Minnesota Soybean Research & Promotion Council and the University of Minnesota IPM program.

Regional Aphid Update.

The scouting protocol that student interns are using was developed by now retired Extension educator and entomologist, Dr. Phil Glogoza. The protocol has student interns visit random soybean fields, walk a zig-zag pattern, stopping multiple times along the way to estimate aphid population densities on 31 plants in each field. This number of plants is based on the speed scouting protocol that has people assess a maximum of 31 plants to make a treatment decision for a particular field.

University of Minnesota Extension is collaborating with the NDSU IPM program – under the direction of Extension entomologist Dr. Jan Knodel- to provide the Minnesota soybean aphid data used to populate bi-state maps that personnel at NDSU produce.

UMN student interns began scouting for soybean aphids beginning last week, finding no aphids on the plants that they assessed in the 40 fields they visited (Figure). While this survey program is no substitute for scouting your own soybean fields, it provides to us a snap-shot in time aphid population dynamics throughout the region.

Why so few aphids so far?

While one cannot know with certainty why aphid populations remain undetectable in surveyed fields at this point in the 2019 growing season, one can speculate:

Theory #1: Few eggs going into 2018/2019 winter.

Unlike in 2017, soybean aphid population densities in 2018 did not reach treatment thresholds in most of surveyed fields in northwest and west-central Minnesota. This meant that once the soybean crop began to mature there weren't as many winged soybean aphids flying to buckthorn plants to mate and lay eggs.

Theory #2: Polar vortex!

After Minnesota experienced the extremely cold temperatures associated with a polar vortex this past winter UMN Extension post-doc Dr. Anthony Hanson and UMN Extension entomologist Dr. Bob Koch wrote an article projecting what this might mean for survival of insect pests. They projected just how much these extremely cold temperatures might have impacted soybean aphid survival. Mortality was projected to be greater than 95% throughout the northern third of the region covered by this summer’s aphid survey.

Aphids are winged and can travel – so stay alert!

While the aphid population growth has started out slowly in 2019, this can change over a very short period of time should aphids begin to move away from a region with a larger infestation and get sucked into a large weather system. It is important to stay vigilant until soybeans reach full-seed.

If your fields are located in southwestern Minnesota, timely, regional pest management information can be found in the weekly newsletter written by Extension IPM specialist Bruce Potter. A link to Bruce’s newsletter can be found here.
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