Corn growers are being urged to tighten their late-season management as aphids and nitrogen deficiency damage threaten to erode yield potential in fields that may still look good from the road.
Consider Treating Aphids At High Populations
Farm Journal Field Agronomist Ken Ferrie says aphids are no longer confined to the most susceptible hybrids in l Illinois and are now moving across more acres, with winged adults from heavily infested fields moving into the broader corn population.
“Up until now, we’ve been seeing and spraying them mainly on susceptible hybrids, but now they’re moving into other hybrids,” Ferrie reports.
Beyond Illinois, parts of Indiana and Ohio are also reporting a significant influx of aphid problems in corn this season.
So far, Iowa has not reported significant numbers, though Erin Hodgson, Iowa State University (ISU) Extension entomologist says
researchers there have tracked a clear behavioral shift in recent years: “multiple aphid species are now colonizing corn much later in the summer (late July through August) and occasionally exploding into populations exceeding 1,000–2,000 aphids per plant.”
In late-season Iowa corn, Hodgson says infestations are typically a mix of species:
- Corn Leaf Aphid: The traditional pest, typically blue-green to slate-gray.
- Bird Cherry-Oat Aphid: Olive-green with a distinct orange-red “saddle” mark on the lower abdomen.
- English Grain Aphid / Greenbug: Less common, but frequently found intermixed on leaves, husks, and lower stalks.
There is no rigid formal insect threshold post-tasseling, but ISU guidelines suggest corn growers consider treatment if aphid populations (any species) exceed 500-plus aphids per plant field-wide before hard dent.
Ohio State Extension suggests considering chemical intervention only under extreme combined conditions: when at least 50% of plants have tassels heavily covered with aphids and sticky honeydew AND the field is actively moisture-stressed and populations average 1,000+ aphids per plant across a majority of the field.
Farmers Left Insecticide Out Of Their Program
Ferrie warns growers not to assume tolerant hybrids or “clean-looking” fields are safe. He encourages growers to check every field where populations in parts of Illinois are building. One of the compounding issues, he notes, is that growers frequently left out their insecticide in their fungicide pass this season.
“I was in one field that had a fungicide application about two weeks ago with no insecticide with it, and those aphids blew up,” he says. “Out of 34,000 plants, we had 8,000 plants that were waxed up enough that they aborted the ear, and that’s going to hurt no matter what.”
That experience is shaping his recommendation going forward now: if a fungicide pass is on the calendar and aphids are present, don’t wait to address them.
“If we’re spraying a fungicide, we’re going to throw an insecticide in to clean up on these aphids,” Ferrie notes. “Guys are waiting maybe for brown silk to spray their fungicide. But if the aphids are there, move that fungicide application up. Take care of your fungicide and your aphids in one shot.”
Where northern corn rootworm beetles are building, he adds, that same pass can also help tamp down next year’s pressure.
Nitrogen Deficiency Brought On By A Wet June
As the corn crop moves into the last half of the season, Ferrie says one of the biggest yield threats now is nitrogen deficiency — especially in fields that suffered through a wet June and are “yellowing”. These fields indicate significant nitrogen loss has occurred.
With only a few weeks of grain fill left in many fields, there may be limited opportunity to fully correct N deficiencies, and Ferrie urges farmers to be proactive.
“We got 45 days, maybe 50, to go in a lot of these fields where we’re showing N deficiency at the top. That’s going to get brutal on yield. It’s going to take the top end of that yield, so we need to address it,” he says.
Water Damage, Weeds and Hidden Yield Loss
Beyond insects and nutrients, Ferrie is increasingly concerned about ponded areas that never fully recovered after early-season flooding. Those zones are often riddled with root diseases in both corn and soybeans, and they also pose a long-term weed problem.
In corn, he warns that in these damaged pockets, the canopy may not have fully closed —particularly in 30-inch rows — creating ideal conditions for aggressive weeds, especially tough broadleaves like waterhemp and Palmer amaranth (pigweed).
From the road, many fields might still look clean, but Ferrie stresses that the worst damage is often hidden inside the rows. “It’s a lot more deceiving from the road compared to the air,” he says.
Let Your Landlord And Banker Know What You Expect
Ferrie is encouraging farmers to bring technology and “boots-on-the-ground” scouting together in fields before harvest.
“This is a good time if you don’t have them already to get some aerial imagery, either by drone, plane or even satellite,” he says. “Once we know where the damaged acres are, then we need to get out there and visit that. One reason is to look at weed pressure… but the other part is to actually get in this damaged corn and assess what we think the actual yield loss is. You know, is it a zero loss or is it a 50% yield loss? Come up with some type of prediction, and then get the acres calculated on your map to get a realistic yield number.”
With many central Illinois fields sitting in the 15% to 30% water-damaged range, he says now is the time to communicate the implications.
“Prepare your landlords, your farm managers, your accountants to know what lies ahead. We don’t want anybody shocked because they drove by a field that looked perfect from the road, but didn’t realize 20% of the yield is gone,” Ferrie says.
“We’ve got about 40 to 60 days left to finish this crop. Let’s not drop the ball now. Don’t walk away from it. Stay strong to the finish,” he adds.
Hear Ferrie’s complete recommendation in his Boots In The Field podcast, available at the link below:
