Wednesday, August 13, 2014

Soybean aphid thresholds demystified - a satirical examination

prepared by Bruce Potter, IPM Specialist SW MN
University of Minnesota Southwest Research and Outreach Center, Lamberton


Before anyone gets too bent out of shape over this... Yes, I know this is a stressful time for some soybean growers, most of their advisers and for aphid researchers too.   It is not illegal to apply insecticide to any population of soybean aphids as long as it is within label restrictions.  Finally, I am not the police and it's the soybean farmer's money, not mine, at risk. Insecticide resistance does tend to spread itself around though.

Based on emails and phone calls, there seem to be several economic (action) thresholds for soybean aphids in use now.

  1. The widely adopted threshold endorsed by research and Extension entomologists in the North Central States is: 250 aphid average /plant threshold for soybeans with increasing populations is valid for soybeans less than the R6 (full seed) stage. This threshold has data from numerous trials over numerous years and numerous environments behind it. This threshold is conservative and works even when crop prices are high and treatment costs are low.  It protects beneficial organisms and insecticide effectiveness to boot.

    Some collect the information to apply this threshold by using scouting techniques that require several hours in each field.  Inefficient scouting and poor math (counting) technique can lead to the adoption of one or more of the following thresholds.

  2. The "I am going on vacation and need to get this over with" threshold is widely used this time of year. It is closely related to the "I am sick and tired of scouting" threshold.  The "going on vacation threshold"  provides excellent economic results when you schedule your vacation before or after your aphid populations average 250 aphids/plant.

  3. The "Adding insecticide to your last herbicide pass just in case" is a unique subset of the "I am tired of scouting threshold".  It is most often practiced by growers or their advisors that like to re-treat soybeans fields for aphid re-infestations and spider mites. Those experienced in using this threshold are able to make all fields aphid problem fields and assign all application costs to the herbicide application. A complicated economic analysis then allows subsequent ground or aerial application costs to disappear along with yield losses from re-infested and un-scouted fields.

  4. The "Spray when my neighbor sprays" threshold is used by some and with varying levels of success. This threshold works perfectly as long as you and your neighbor have identical varieties, planting date, crop management practices and soybean aphid populations. To work most effectively, it requires that you neighbor knows what he is doing, has scouted your field and does not use threshold 2. Some of you do have bona fide genius neighbors!

  5. The "I just do what my crop advisor tells me to do" is also a valid threshold.   It is most effective if you advisor scouts your field and uses threshold 1.  Advisors that practice threshold 2 or 3 often provide fields free of soybean aphid yield loss but treat a lot of fields - sometimes more than once. Trust your advisor or get one you can trust to provide advice in your economic interest.

There are several other thresholds that can be found on the internet, TV, radio and watering holes. Those that I have come across are closely related to one of the four above.

Try to enjoy yourself out there.  The 2014 aphid season will soon be history.

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Late season aphid information by Captain Obvious

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.
  • Calibrate your eye by closely counting a few leaflets of varying aphid densities. Learn what 10, 20, 100, etc. aphids look like on different leaf sizes. 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.
  • 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 250. There is a difference between 50 and 250 though.
  • Don't waste time. Get through fields quickly but don't scout just the field edge. If you don't find any aphids on the field borders you are less likely to find economic infestations in the field interior. Of course, you can spend as much time in as you want.
  • Some folks pull enough plants trying to make a decision to cause yield loss.
  • Some fields are obvious problems or have few aphids.  Don't keep pulling and counting aphids.  Make a decision and move on.
  • Don't assume things are OK.  Deep throat has reported some early, "insurance" applications of "long"-residual insecticides now need re-treatment. Imagine that!  Change your insecticide class if spraying a field  a second time.
  • What is happening in one field may not be the same as what is happening in another. No one can see aphids must less assess a field through the phone.
  • Use common sense - don't spray everything because one field is at economic threshold. Conversely, don't call in a plane and skip one field because it is not quite at threshold. Growers who spray their own fields have more flexibility.
  • Watch for spider mite infestations in droughty areas.
  • Some are observing many fields where only field edges or small hotspots in the field are infested to any level. Spraying borders for aphids does not often work well because of undetected in field "hot spots".  If you do treat borders, be ready to treat the field interior later.
  • Population increases have slowed in many areas. This is due in part to cooler temperatures increasing the time needed for a generation to develop. Early R5 stage soybeans tend to have slower aphid population growth than earlier or later stages.
  • Winged aphids are abundant now. Late planted fields are attractive to immigrating winged aphids. Heck, even I can spot the light green replanted soybeans from a distance!
  • Watch late planted or full maturity beans closely as the season progresses. This season will be chaotic because of our late, wet spring.
  • Look for nights in the low 40s to begin the sexual cycle for aphids to move back to buckthorn. This can happen from now until soybean leaf drop.
  • Walk away from fields where lower leaves are naturally senescing or you start seeing yellow pods. Beware of Pre-Harvest Intervals (PHI) for insecticides. Soybean maturity and insecticide PHIs will start to pull soybeans off the scouting schedule soon.

We cobbled together a short scouting video on late season aphid scouting. It's temporary home is http://youtu.be/VidX3KZEPnI.  Would be better with a couple teleprompters and a better speaker.