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By Jon Tollefson and Marlin Rice, Department of Entomology
For supposedly being a soybean aphid “off year”, the aphids have been doing surprisingly well during August. Typically the aphid numbers are declining at this time of year because the soybeans are approaching maturity and the temperatures are quite warm. However, this year is slightly different from recent years and you should not let down your guard.
There are a couple of reasons to stay on the alert for soybean aphids. One is the later planting dates for many soybeans. The other is the cooler than normal weather.
The soybean aphid is not expected to cause any additional yield loss after soybeans reach R5.5 stage. The R5.5 stage is between the R5 stage, when the seed is 1/8 inch long in the pod at one of four uppermost nodes on the main stem, and the R6 stage, when the pod contains a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem.
At the R5.5 stage, the canopy should be closed in the bean fields and there will not be any additional gain from spraying for aphid control. However, with the cool, wet spring and summer this year some fields were planted quite late and the maturity of the beans has lagged behind. As a result, you need to continue to scout soybeans for aphids in fields where the canopy has not closed across the rows.
The second reason scouting should continue is the cool weather. As long as the daytime temperatures stay in the 70s and low 80s, the aphids will continue to reproduce and feed. They will not begin to move to overwintering sites until there are shorter days and night-time temperatures drop into the 40s.
Therefore, continue to scout aphids in any soybean fields that have not reached maturity stage R5.5 and while the temperatures stay cool. Don’t forget to recheck fields that were sprayed two weeks or more ago. These sprays will have destroyed the predator populations and the chemical’s residual activity will have ended by now; aphid numbers may be rebounding in those fields.
Jon Tollefson is a professor of entomology with extension and research responsibilities. Marlin E. Rice is a professor of entomology with extension and research responsibilities.