The current threshold of 250 aphids per plant and increasing should be followed to the end of the R5 stage of soybeans.
The R5 stage is known as the “Beginning Seed” stage, when the pods on the top 4 nodes of the plant have a tiny seed developing inside them. Once the soybeans start to enter into the R6 stage, the plants are starting to shut down and requires a lot more aphids on the plant before it would respond to a spray.
The R6 stage aka “Full Seed” has been reached when the upper 4 pods of the plants are full of plump green seeds. Past research has indicated that at least 1000 aphids per plant are required before there is a yield response. This would only apply for the very start of the R6 stage. Once the plants reach the middle to end of R6, the plants no longer respond as they are shutting down for harvest. Also you need to worry about the preharvest intervals of the foliar insecticides registered which are 21 days to harvest for Matador or Silencer and 30 days to harvest for Cygon or Lagon.
The majority of the soybeans we scouted this week were in the R4 stage. So unfortunately we have a few more weeks of scouting to do before we can walk away and be done with the aphids.