When is it too late to spray for soybean aphid?

The last few weeks have been full of soybean aphid discussions. Populations have been going up, down and otherwise looked sort of bumpy this summer. Many fields exceeded the economic threshold of 250 per plant and were treated with foliar insecticides in August, particularly in northern Iowa. My lab had our big spray day last week, as aphid numbers approached the threshold. Of all the products we are using, we are seeing good efficacy, or knockdown, for soybean aphid. That is a good thing because aphids have historically become resistant to big classes of insecticides.

Some scouts and farmers haven’t seen soybean aphids until very recently – where plants are reaching full seed set (R6). I’m getting questions about spray timing and when is it too late to treat and expect a yield response. In other words, can a farmer expect to make a profitable decision from treating late in the season? From my lab’s research, I can say we would expect a yield response if aphids exceed the economic threshold before mid-seed set (R5.5). I have not seen a benefit for spraying aphids at full seed set (R6). But I have seen data from other states that showed a benefit to spraying at R6. The odds of seeing a benefit depend on how long the aphids have been there and other potential soybean pests.

Here is an example of a late-season aphid population in 2013, where foliar insecticides were applied at the threshold. There was not a significant yield response for applying an insecticide. 

Soybean aphid efficacy evaluation from 2013 from the Hodgson Lab.