Aphids are showing up in Iowa cornfields this August

Aphids are showing up in Iowa cornfields this August

Here are 10 things to think about before applying insecticide to control aphids in corn.

Over the years, aphids have attacked soybean fields in Iowa and have been treated with an insecticide. What about aphids in corn? They haven’t been as much of a problem. However, this insect pest is being reported in more cornfields this summer. In mid-to-late August of 2016, corn leaf aphid has built up to larger levels in some fields.

SCOUT FOR APHIDS: Several species of aphids may be seen in cornfields throughout the growing season. Typically, these populations are managed by natural enemies, but occasionally they can reach very large densities that may warrant insecticide management.

“Historically in past years, corn leaf aphid could sometimes be a problem during corn tasseling in Iowa,” says Iowa State University Extension entomologist Erin Hodgson. “This species of aphid that attacks corn aggregated around the ear and silks, and sometimes the honeydew production by the aphids 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 the VT to R1 stage of corn growth, says Hodgson, and mostly for drought-stressed cornfields.

Recently, aphids have been attacking corn later in summer

Since 2010, however, aphids have been colonizing corn plants later in the summer and in more cases, 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.

In 2016, starting in early August, Hodgson has been seeing corn aphid populations again in some Iowa fields. Iowa State University Extension agronomist Brian Lang, based at Decorah in northeast Iowa, has also visited fields that have exceeded 2,000 aphids per plant. The areas in Iowa this summer that are having moderate to severe problems with corn aphid now include the northeast and northwest corners of the state. “Some of these heavily-infested fields have already been sprayed with an insecticide earlier this year,” says Hodgson. “From my observations this week, I noticed aggregated colonies at the end rows, but some areas have aphids throughout the field.”

Aphids are showing up in Iowa cornfields this August

CORN APHIDS: Aphids can build large colonies on plants in cornfields, with two or more species possible. Photo by Brian Lang, ISU

Cornfields can have two species of aphids attacking plants
“One important observation I've noticed is that cornfields can have two aphid species: corn leaf aphid and bird cherry oat aphid,” says Hodgson. “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 these are not as common in corn this year in Iowa. Species identification isn't that critical for management at this point in time, she notes. In other words, “an aphid is an aphid.”

Aphids are showing up in Iowa cornfields this August

ANOTHER KIND OF APHID: Bird cherry oat aphid is green with an orange saddle between the cornicles; it looks a little different than the corn leaf aphid. Photo by David Cappaert, www.ipmimages.org

We don’t know how much yield loss occurs from aphids in corn
All aphids have piercing-sucking mouthparts and feed on the sap from the plant phloem, says Hodgson. “They excrete sugar-rich honeydew that can cover the aboveground portion of plants,” she explains.  “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.”

Aphids are showing up in Iowa cornfields this August

APHIDS ATTACKING CORN: Corn aphids can colonize the ear and ear leaf. Photo by Brian Lang, ISU

When does it pay to spray corn with insecticide to control aphids?
Currently, there are no treatment thresholds for aphids in corn past tasseling, but regular sampling of fields will help you make educated decisions about a foliar application of insecticide at this time, says Hodgson. She says you should sample field-wide (30 plants for every 50 acres) to determine the average density.

Here are some considerations you need to think about, advises Hodgson, 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 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 preharvest 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,” says Hodgson. “Try to leave an untreated strip that is a fair comparison to the majority of the field, not just along the field edge. If you decide to treat with an insecticide 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.” Send her an email at [email protected].

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