Corn Aphids Observed in Iowa

By Erin Hodgson

Since 2010, aphids have been colonizing corn later in the summer and can build up to surprising levels in Iowa. They can be found at the base of the stalk, around the ear and sometimes above the ear leaf. Aphids have been sighted in corn again this summer.

Aphids have been confirmed in corn fields for about two weeks, particularly in north central and northeastern Iowa. In some fields, the infestations are only along the edge rows, but in others, aphids may extend into the field interior. Spot checking fields for aphid activity is recommended as the growing season progresses.

Aphids in corn can build up large colonies, sometimes exceeding 2,000 per plant. 

At least four species can colonize corn in Iowa. However, I generally see two after pollination: corn leaf aphid and bird cherry-oat aphid. They are closely related and look very similar in size and color. This week, while visiting Iowa State University’s Northern Research and Demonstration Farm near Kanawha, I saw a combination of corn leaf aphid and English grain aphid (Photo 1). These aphids were mixed together and colonized the stalk, undersides of leaves and ear husk. Honeydew was visible on some plants but sooty mold was absent. I estimated 200-500 aphids per plant in the drought-stressed corn demonstration plot.

Species identification is not critical for management at this point (i.e., an aphid is an aphid). All aphids have piercing sucking mouthparts and feed on the sap from the plant phloem. They excrete sugar-rich honeydew that can cover the aboveground portion of plants. The honeydew can promote a sooty mold that interferes with plant photosynthesis. We know soybean plants covered with mold and aphids can have serious yield loss, but we don't know the extent of yield reduction caused by aphids in corn.

Corn aphids infesting the ear and above the ear leaf

Currently, there are no treatment thresholds for aphids in corn. Regular sampling will help make educated decisions about a foliar application. Sample field-wide (30 plants for every 50 acres) to determine the average aphid density. Drought-stressed fields will amplify potential feeding injury to corn.

Here are some considerations to make before applying an insecticide for aphids in corn:

1. Are 80% of the plants infested with aphids or are they aggregated around the field perimeter?
2. How long has the field been infested and is the density increasing?
3. Are aphids colonizing the ears, or the ear leaf and above?
4. Do you see honeydew and/or sooty mold on the stalk, leaves, or ear? Mold can interfere with photosynthesis and the grain-filling process. Moldy ears could also reduce grain quality and make harvest difficult.
5. Are you seeing winged aphids or nymphs with wing pads? This may be a sign of migration out of the field.
6. Are you seeing puffy, tan or black mummies (Photo 2)? Parasitoid wasps can attack nymphs and adults and reduce the population.
7. Do you see any bloated, fuzzy aphids? Natural fungi can quickly wipe out aphids in field crops under humid conditions.
8. What is the corn growth stage? Fields reaching hard dent may be past the point of a justified insecticide.
9. What is the expected harvest date? Some insecticides have a 60-day pre-harvest interval. Check the label and calendar.
10. Are you able to use high volume and pressure for application to reach the aphids? Ideally small droplets should make contact with the aphids for a quick knockdown. Don't expect residual to protect the corn from fluid-feeding insects.

I strongly encourage you to leave an untreated check strip or two in any fields that are sprayed. Try to leave a strip that is a fair comparison to the majority of the field - not just along the field edge. 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.