This week, I've had several calls and emails about aphids feeding on corn. They aren't a persistent pest, but can occasionally be found in Iowa corn. I wrote up a short ICM News article in 2011. Although an economic threshold isn't well defined for aphids beyond pollination, there are a few to consider before treating.
1. At least two species can be feeding on the same plant, including corn leaf aphid and bird cherry oat aphid. They can be feeding on the brace roots, tassel and everywhere in between.
Bird cherry oat aphid. Photo by David Cappaert.
2. Corn can tolerate 500+ aphids per plant, especially with sufficient moisture.
3. Aphids are phloem-feeding insects that excrete sugary-rich honeydew. The honeydew can promote a black sooty mold that interferes with photosynthesis.
Black sooty mold (top) from soybean aphid honeydew. Photo by Brian McCornack.
Consider using a foliar insecticide if the aphids are congregated on the ear leaf and above and sooty mold is present. These leaves are the most important for grain filling in August and September. I strongly encourage you to leave 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. If you decide to treat for aphids in corn, I would like to hear about the yield comparisons. Your pooled data will help me formulate treatment guidelines for the future.