In the past, corn leaf aphid could be a problem during corn tasseling. This species aggregated around the ear and silks, and sometimes their honeydew production interfered with pollination. But natural enemies and the environment rarely let them persist past July. Therefore, economic thresholds for corn leaf aphid are targeted around VT-R1 and mostly for drought-stressed cornfields. Since 2010, aphids have been colonizing corn later in the summer and are building up to striking levels. They can be found at the base of the stalk, around the ear, and sometimes building up colonies above the ear leaf.
Starting in early August, I've been seeing corn aphid populations again. Iowa State University Extension and Outreach Field Agronomist Brian Lang has also visited fields that have exceeded 2,000 aphids per plant. The areas having moderate to severe problems now include the northeast and northwest corners of Iowa. Some of these heavily-infested fields have already been sprayed with an insecticide earlier this year. From my observations this week, I noticed aggregated colonies at the end rows, but some areas have aphids throughout the field.
One important observation I've noticed is that cornfields can have two aphid species - corn leaf aphid and bird cherry oat aphid. They are closely related and look very similar in size and color. You can see more than one species in a field and even on a single plant. The bird cherry oat aphid has an orange-red saddle between the cornicles. Other aphid species can also be found, including greenbug and English grain aphid, but are not as common in corn this year. Species identification isn't that 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. You probably remember seeing grey-looking soybean leaves from soybean aphid in 2003. 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.
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. Sample field-wide (30 plants for every 50 acres) to determine the average density. 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. Are aphids colonizing the ears, or the ear leaf and above? This would be more important than those aphids colonizing below the ear.
3. How long has the field been infested and is the density increasing?
4. Do you see honeydew and/or sooty mold on the stalk, leaves, or ear? Mold can interfere with photosynthesis and interfere with 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. Is the field under drought stress? Dry weather will make amplify potential feeding damage to corn.
7. Do you see any bloated, off-color aphids under humid conditions? Natural fungi can quickly wipe out aphids in field crops.
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 of an insecticide 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 feeders.
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 - 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.
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