August 25, 2011

Soybean aphids are still present, but we are nearing the period when populations decline and soybeans are less likely to incur economic yield loss. Declining night temperatures and shorter days initiate migration of soybean aphids back to buckthorn, their overwintering host.

Aphid populations in Nebraska typically crash the first week of September, if they have not already declined in late August. However, with insects there are always exceptions. A couple years ago we observed aphids on late planted or late maturing soybeans well into September. As soybeans enter R6 (full seed), yield loss associated with aphid injury significantly declines. We don't have good research-based thresholds for R6 beans, but we believe the threshold would be about 400-500 aphids per plant in early R6. Once the beans are nearing mid-R6, it is doubtful an insecticide treatment would be of any benefit.

So, what does this mean for the soybean farmer right now? If the population is rapidly rising, you exceed the 250 aphids per plant threshold, and you are still in R5, an insecticide application would likely be of benefit. If you are at 250 aphids per plant in early R6 or the population is slowly rising or relatively static, hold off but check again in a few days. If your beans are in early R6, use a treatment threshold of 400-500 aphids per plant. Once your soybeans are well into R6, you are past the “soybean aphid season.”

Tom Hunt
Extension Entomologist, Haskell Ag Lab