I’m getting a lot of questions regarding treatment timing for soybean aphid. Mainly, when is it too late to treat? In the last week of August 2017, most fields are reaching R5/R6 and so the question is an important one. In my soybean aphid efficacy evaluation, I have sprayed at different plant growth stages to evaluate yield responses. I have never been able to demonstrate a yield response when spraying after R5.5. However, I would have to ask a few more questions before responding to a farmer:
1) how long have the aphids been there?
2) what other pests are present?
3) what are the yield expectations?
4) what is the expected market value?
5) what would the control costs be?
It is interesting that some fields in northern Iowa are seeing a small population surge and I recommend scouting fields until aphids disappear. Also, take into consideration the PHI (pre-harvest interval) on some insecticide labels can be 45 or 60 days. This could interfere with harvest schedules on some farms.