SW MN IPM STUFF 2017 Issue 13


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This newsletter and the advice herein are free. You usually get what you pay for.

Crop weather

Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University of Minnesota Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN, can be found at SWROC Weather.

As of Aug 7th, the ROC was at 12.76 inches of precipitation accumulated from May 1st compared to an 11.86 historical average. Warm nights have helped with degree-day accumulations for the same period with 1643 degree days accumulated in 2017 compared to a 1695 degree-day historical average. 

Much of the area had a welcome shot of rain on the 9th. The SWROC recorded 0.73”. The moisture was a week late for some fields but will help many.


In drier areas, you might already be observing some pullback on the ears.

Common rust is the most commonly observed foliar disease. It is at low severity in field corn. Goss’s wilt symptom development has been slowed by dry weather in some areas.

Generally, northern and western corn rootworm and corn borer populations are low but there are individual fields with economic levels. In the case of western corn rootworm, fields with a several year history of no Bt-RW trait are where we are seeing the most root damage and beetle activity.

Pay attention to corn borer in refuge hybrids, particularly in central and WC MN.


In much of the SW part of the state, the soybean canopy is not as tall and lush as in 2017. However, depending on seed fill, soybeans could still yield well.

When good bugs butterflies go bad

Caterpillars on front of four-wheelerFigure 1. Another example of using your four wheeler as a sweep net. In this Northeast SD photo, selected for the high larval density and species diversity. In addition to the large number of thistle caterpillars, there are several loopers, potato leafhoppers, aphids, lady beetles, and a 1st generation bean leaf beetle. Photo: Jordan Hill.

The second MN generation of thistle caterpillars continue to cause concern in some soybean fields. I have been receiving photos of individual and large aggregations of larvae. Most of the excitement is in SW, SC, C, and WC Minnesota and evidently parts of eastern SD.

Adults and egg-laying are tapering off now but a two-week or so period of painted lady butterfly activity means that there is a wide range of larval sizes out there. 

There are several action thresholds for thistle caterpillar control. On larger soybeans, like those we have now, I would still use the 20% defoliation guideline rather than numbers of larvae. That said, numerous larvae/plant might cause me to pull the trigger at a bit less defoliation.

While the larvae are very abundant, I have not been in any fields that were approaching economic thresholds. There are scattered reports of insecticide treatment in heavily infested fields. Defoliation damage from multiple insect species can be combined when making a treatment decision. On the other hand, I would not treat soybean aphids at a lower threshold just because you are finding a few thistle caterpillars.

The University of Nebraska recently developed a nice illustration on how to assess insect defoliation in soybeans. It helps prevent focusing on the worst plants and leaves. Once you get yourself calibrated you can determine potential problem fields very quickly.

guide for assessing soybean aphid defoliationFigure 2. A guide for assessing soybean defoliation. Don’t focus on the worst leaves. With permission from University of Nebraska Extension.

Some things to consider when deciding on an insecticide treatment for thistle caterpillar in soybean:

  1. Size of the larvae – Mature larvae are approximately 1.25 inches long. Most of the feeding is done by the large larvae; however, if most of the larvae are full grown, or if you see chrysalis in the field damage, may be ending. Make sure insects are still present in the rolled and webbed leaves.
  2. The thistle caterpillar’s preference for the upper canopy and webbed leaves makes them very visible, and it is very easy to overestimate defoliation.
  3. Area affected – If the larvae are mainly on field edges or in a particular area of the field, you do not need to treat the whole field.
  4. Larvae are usually concentrated on the upper part of the plant and not that difficult to control. Use a labeled insecticide at a labeled rate.

Soybean aphid watch 2017

Aphid treatment continues and has expanded to some new areas. Fields in the southern part of the state that have higher aphid populations are related to planting date and moisture. The earlier planted, wetter parts of the state as well as some of the extremely dry areas have lower aphid populations. Remember, there are always exceptions.

Some of the plot areas at Lamberton are at threshold. We should have all our soybean aphid insecticide trials treated at threshold populations by the end of the week.

Late season soybean aphid information

Don’t give up on scouting too early. Late season aphid scouting is complicated by dense canopies that are hard to walk through, early morning dew, and aphids inconveniently scattered through the canopy. We have been through this before.

  • It is very easy to underestimate soybean aphid populations spread through the canopy of large soybean plants:
  1. Calibrate your eye by closely counting a few leaflets of varying aphid densities.
  2. Learn what 10, 20, 100, etc. aphids look like on different leaf sizes.
  3. Aphid numbers need not be counted individually but instead, estimated with the goal of deciding whether the field is averaging well below, close to, or at/above a 250/plant average.
  • Use this skill to quickly estimate aphid populations through the foliage of plants you pull while moving through the field. Re-calibrate your estimating skills occasionally.
  • Shaking the plant before estimating aphid populations will help remove water droplets and dead aphids - seems to makes things easier for me anyhow.
  • Don’t hesitate to enlist help if you have vision problems or otherwise cannot effectively scout fields.
  • Unless you are doing soybean aphid research, lighten up. For practical purposes there is not much difference between 200 and 300. There is a difference between 50 soybean aphids and 250 or more though.

For treatment scheduling purposes, you can estimate when fields will hit economic threshold by doubling the observed population every 2 days to every 5 days. Scout to verify the results of your calculations.

Most agriculturalists are starting to get a bit punchy by this time of year. The 2017 aphid season will soon be history.

A research-based and field-validated soybean aphid economic threshold and a satirical look at alternative and supplemental thresholds

The correct economic/action threshold for soybean aphids was developed from numerous trials over numerous years and has numerous environments behind it. This threshold is conservative and works even when crop prices are high and treatment costs are low. There is considerable lead time (usually a week or more) between reaching economic threshold and yield loss. This threshold allows beneficial organisms a chance to control aphids for you and helps preserve remaining insecticide effectiveness to boot:

  • 80% or more of the plants with aphids
  • An average of 250 soybean aphids/plant.
  • Soybean aphid populations are increasing. Indications of a declining population include most plants with:
    1. A large percentage of the aphid population consists of nymphs with wing pads and winged aphids
    2. Very abundant predator and parasitoid populations and numerous dead/dying aphids
  • Use this threshold until the R6 (full seed) stage of soybeans

Some may choose to use the speed scouting method for assessing aphids. The speed-scouting threshold is not 40 aphids/plant. Rather, this binomial sampling method uses the same economic injury level and economic threshold data. The percentage of plants that have 40 or more aphids corresponds to a probability that the field is averaging 250 aphids/plant.

If the field is uniformly infested with aphids near 40/plant, speed scouting can indicate treatment early. Re-scout these fields to make sure populations are at or above 250. Speed scouting might under-estimate populations when there are low aphid/plant populations with small pockets of heavily infested plants.

Of course, there are several alternative and often used soybean aphid thresholds and I have been compiling these for years. Results when using these alternatives are typically very inconsistent. These do not often provide a positive return on investment (ROI) for the grower. I have listed some of the most common of these in no particular order.

1. The inventory control threshold

This threshold is sometimes used when it is determined that you might have left over insecticide inventory at the end of the season. 

This threshold can be applied by a wide spectrum of the agricultural system with insecticide on hand (ag-chem retailer, custom applicators, and farmers).

Related to the quota threshold.

2. The paranoid threshold

This threshold commonly triggered in people who watch too many infomercials.

Much lower than the 250 aphid/plant threshold.

Aphids will be bad – you just know it! There is some other stuff out there too so you should spray earlier. There might be a spider mite so you should change your insecticide choice. Your beans probably could have a fungal disease. 

In its ultimate form, the paranoid threshold triggers the first soybean aphid insecticide application when seed is purchased. Later season, the action threshold shifts to spotting an Ag Cat or other aerial application device.

This threshold is often combined with the schizophrenic threshold.

3. The schizophrenic threshold

This is the threshold used by those who can’t make up their mind.

When this threshold is used, the treat and don’t treat decisions are usually both wrong.

When not employing the published 250 aphid/plant threshold, most academics default to using the schizophrenic threshold.

4. The calendar threshold

Used by those who are used to timing herbicide and fungicide applications by calendar and crop stage.

Insects are not weeds or fungi. Unlike less evolved organisms, they have the equivalent of a rudimentary brain and can move or otherwise react to their environment. Some soybean aphids are probably busy right now plotting which soybean field to fly to and infest (see the paranoid threshold).

5. The quota threshold

The urge to use this threshold increases near the end of the month and fiscal quarters.

This threshold predominates when visits from ag-chem reps coincide with an urge to go fishing.

This is the threshold that is often described on TV and other popular media June through August.

6. The imminent vacation threshold

More commonly used by those farmers and their advisors who can’t wait until September for vacation.

Strangely, some growers have the uncanny ability to pick vacation dates that fall immediately after aphids reach a 250/plant average.

7. The tired and bored threshold

This threshold is often deployed in mid to late August about the same time as the imminent vacation threshold.

This and thresholds 2, 3, 5, and sometimes 9 are often combined with an herbicide application.

8. The bifocal adjusted threshold

Adds 50% to 300% to the number of aphids observed.

Use of this threshold should increase with age of the soybean scout.

Those who consistently use this threshold might want to consider hiring a reliable scout.

9. The spray when your neighbor sprays threshold

For this threshold to be effective, your neighbor must be psychic and not using any of the previously listed supplemental thresholds.

Having an agronomic genius for a neighbor is critical for maximum ROI with this threshold.

If you are planning on using this threshold, you should plant the same varieties on the same days as your neighbor. Following the exact same fertilizer and weed management program as your neighbor helps fine tune this threshold, and why wouldn’t you - they’re an agronomic genius.

Folks that gravitate toward this threshold are often found in the local coffee shop or cruising the backroads critiquing neighborhood crops.

10. The I just do what my crop advisor tells me to do threshold

This works very well when your advisor uses the 250/plant threshold.

Advisors using alternative thresholds 1-9 will definitely provide an improved ROI… but it may not be yours. 

Learn biological basics and work with an advisor you can trust.

Happy Trails,