Currently, there are no treatment thresholds for aphids in corn past tasseling. But regular sampling will help you make educated decisions about a foliar application at this time. Here are some considerations to make before applying an insecticide for aphids in corn:

  • Are 80% of the plants infested with aphids?
  • Do most of the ears have aphids? What about the ear leaf and above?
  • How long has the field been infested and is the density increasing? Sample field-wide (30 plants for every 50 acres) to determine the average density.
  • Do you see honeydew or sooty mold on the stalk, leaves or ear?
  • Are you seeing winged aphids or nymphs with wing pads? This may be a sign of migration out of the field.
  • Is the field under drought stress? Dry weather will amplify potential feeding damage to corn.
  • Do you see any bloated, off-color aphids under humid conditions? Natural fungi can quickly wipe out aphids in field crops.
  • What is the corn growth stage? Fields reaching hard dent may be past the point of a justified insecticide.
  • Some insecticides have a 60-day preharvest interval. Check the label and calendar.
  • Get good coverage of the application; ideally droplets should make contact with the aphids for a quick knockdown. Don't expect residual to protect the corn from fluid feeders.

Hodgson strongly encourages leaving an untreated check strip or two in fields that you spray. Try to leave a strip that is a fair comparison to the majority of the field – not just the stunted corn around the field edge. If you decide to treat for aphids in corn, Hodgson would like to hear about the yield comparisons. Pooled data will help formulate treatment guidelines for the future. Erin Hodgson can be contacted by email at ewh@iastate.edu or phone at 515-294-2847.