By Scout Nelson
Aphid populations are starting to rise in southeastern South Dakota corn fields this season. Two common species are often found in corn: the corn leaf aphid and the bird cherry oat aphid. So far in 2025, the bird cherry oat aphid is the species most often seen.
These aphids are dark green with a rusty orange patch at the end of their abdomens. They can infest many parts of the corn plant. At first, they often appear on the stalk near the ears. As numbers grow, they spread across the stalks, leaves, and ears, and can even hide under husks.
Currently, there are no official treatment thresholds for aphids in corn. While aphids can build large populations, their presence does not always mean yield loss. In South Dakota, infestations often follow small grain harvest but usually do not reach damaging levels.
Experts recommend scouting to understand population size. Dr. Erin Hodgson of Iowa State University advises farmers to sample 30 plants for every 50 acres to determine average density. Some factors to consider before deciding on insecticide use include:
- Are infestations mainly along field edges?
- Are 80% or more of plants infested?
- Are populations growing over time?
- How long have aphids been in the field?
- Is honeydew or sooty mold present, which can affect photosynthesis and grain quality?
- Are both winged and wingless aphids present?
- Is the crop under drought stress?
- What is the crop growth stage, and is harvest near?
- Are natural enemies like lady beetles present?
- Are weather conditions humid, encouraging natural fungi that reduce aphid numbers?
Farmers are reminded not to expect long-term protection from residual insecticides. Careful scouting and evaluation of field conditions help determine if management is necessary.
Photo Credit: gettyimages-v_zaitsev
Categories: South Dakota, Crops, Corn