Soybean Aphid Update

By Bruce Potter

Although aphid populations are increasing in some areas of western and southern Minnesota, many areas still have low populations.

Even in those locations that have seen economic threshold soybean aphid populations, aphid populations vary among fields. Planting date, soybean growth stage, rainfall/soil moisture and soil fertility management practices all affect soybean aphids.

Assume many of these aphid populations are resistant to pyrethroid insecticides (e.g. warrior, asana, bifenthrin).

Many fields have uniform, but low soybean aphid populations. Speed scouting will often over-predict the need to treat aphids in these fields. Make sure populations are increasing and you are averaging 250 aphids/plant.

Some fields are seeing large numbers of winged aphids leaving and arriving.
Some soybeans are now at the R5.0 to R5.5 stage. As new vegetative growth ceases, many aphids leave the plants. The remaining aphids will be found lower in the canopy.

Look for smaller aphids and nymphs lower in the canopy. During late R5 and early R6 these "dwarves" can reproduce rapidly and eventually produce more typical nymphs and adults.

Late-planted fields and those with longer maturity varieties will be preferentially colonized by winged aphids now.

Use the ET

Use the 250 aphid/plant economic threshold through the R5 stage. Aphids have less time to reduce yield as the growing season starts to wind down and yield loss from economic threshold populations becomes less certain after the R4 stage. While yield loss can occur into the R6 (full seed) stage, it takes 4.500 or more cumulative aphid-days to cause detectable yield loss, even on earlier reproductive stages. If curious, you can play with this spreadsheet to estimate when aphids will reach the treatment threshold or start to cause yield loss: Aphid-day calculator DEMO

Management practice can impact soybean aphids. I received an anecdotal report from an area that has seen more consistent rainfall this year that fungicide treated fields tend to have higher aphid populations than those without fungicide. 

Fields treated earlier with a just-in-case insecticide should be rechecked. Because predators were removed by insecticide, fields where insecticides were added to a late herbicide application are at risk of recolonization and subsequent population explosions. Some of these "insurance" applications used pyrethroid insecticides.  Again, assume many aphid populations are resistant to pyrethroids.

Aphids could begin to move to buckthorn as soon as late August. Photoperiod, cool temperatures and senescing soybeans provide cues for aphids.