Soybean Aphids – What Is the Threshold Again?

It has been a few years for some since they have had to worry about soybean aphids, let alone make a spray decision.  So I want to cover off the key points you need to know through some of the repeat questions that have been coming in.  Anyone with more questions after reading this, please ask in the comment sections so we can all continue the discussion there.

Q1.  With soybeans at $15, does that threshold change?  Answer:  No. The economic threshold of “250 aphids per plant and increasing on 80% of the plants in the field during R1-R5”, already takes into consideration a wide range of soybean prices.  The actual economic injury level (where the cost of control is equal to the damage they are causing) is around 660 aphids per plant.  So you have a large buffer area between the threshold and the economic injury level to determine if a spray is needed.

Q2.  Isn’t it getting late to spray for aphids?  Answer:  No. Soybeans are susceptible to aphid injury from R1 (first bloom) all the way until your crop reaches R6 (Full Seed – when the pods at the top of the plant have seeds filling the cavity of the pod).  Once it is in the R6 stage, you need a lot more aphids (over 1000/plant) to see any yield response.  And you need to be cautious as you approach R6 as you are also close to approaching the pre-harvest interval aka days to harvest limit for the products registered   But much of the soybean crop still has a ways to go before reaching R6 so please keep scouting.

Q3.  I have a field that has been fluctuating from 100 to 400/plant and back down over the last few weeks.  Do I need to spray?  Answer:  Most likely not -but keep watching. The fact that the aphid populations keep fluctuating in this fields is a good thing.  It indicates that the natural enemies are helping.  Plus the cooler night temps are not as ideal for the aphids so their development is going to slow down a bit, allowing for the natural enemies to keep up.  If the crop is not stressed (i.e.  it has been getting a decent amount of rain and is lush) you can hold off to closer to the ECONOMIC INJURY LEVEL before you need to spray.  So only if you see these fields hitting 500 or more and are staying at that level might it still be worth spraying.  Again, make sure the crop hasn’t reached R6 yet.  Any fields that are really stressed from lack of moisture (and I know there are some out there) need to stay closer to the 250 threshold and increasing  (but not below that number).  If populations keep fluctuating from 100 to 300 and back down, even these fields may not need to be sprayed.  As long as populations fluctuate, it is a sign that the aphids aren’t getting the upper hand.

Q4.  I am seeing 150 aphids per plant.  Should I get out and spray now and get ahead of them?  Answer:  Absolutely NOT. The threshold of 250 per plant and increasing is there for a reason.  You are not losing yield equivalent to the cost of the spray until closer to the 660 aphids per plant.  At 250 aphids per plant and below, there is a lot of opportunity for natural enemies to keep up with the aphid populations and provide you with free biological control.  If you spray too early, you kill these natural enemies off and give the aphids the freedom they need to build up more rapidly.  You will never get 99% control of the aphids but you are likely to get that good of control of the natural enemies.  Even if 10% of the aphids survived a spray, they will easily build up again in the absence of their enemies.

Q5.  I saw a Speed Scouting Aphid App recently launched out of the US.  Is it something we should use here?  Answer:  Unfortunately No. Nothing against my US counterparts who developed this concept.  But when we tested it here in Ontario in many field scenarios, when the aphid populations were building up rapidly, it did not tell us to spray soon enough and when the populations were building up very slowly, it often told us to spray when we didn’t need to.  So I don’t recommend its use in Ontario.  What I will recommend is the aphid app that we are developing here in Ontario (between UofG and OMAFRA).  I am not being biased here.  It is called the Aphid Advisor.  We are in the final testing phase this week.  Currently it is only for blackberries but if it is successful, we will be working on getting it for the other smartphones in use.  It does still require you to do the full plant counts on 10 random plants in the field but it also has you punch in the number of natural enemies hanging out on those plants.  Based on the forecast for your area (determined by the location you enter) and the ratio of aphids to natural enemies you are seeing, the app will tell you if you need to 1) Spray, 2) Wait and Watch or 3) Don’t spray.  It is based on the dynamic action threshold work that Rebecca Hallett et. al. did the last few years at U of G. We tested it in the field and it really does help to determine if the aphids are going to win the battle over what natural enemy population exists in that field.  It takes care of that grey area between the 250 to 500 aphids per plant range where you are uncertain as to whether you need to spray or not.  So keep an eye out for that launch expected in just a matter of days.  And we will be on display at the OMAFRA/OSCIA tent at the Outdoor Farm Show for all to try out.

If you want any more information on soybean aphids, here are a few helpful links:

Agronomy Guide for Field Crops – Soybean aphids:

Field Crop Protection Guide – Soybean aphids control options:

Soybean Aphid Scouting Cards:   English French  Soybean Aphid Scouting Card – FRENCH