Corn aphids are usually no longer a significant problem in Illinois corn crops by early August. That’s not the case this season. Ken Ferrie, Farm Journal Field Agronomist, says the pest is still the biggest yield threat he is seeing in cornfields throughout the central part of the state.
“I don’t remember a year when we had to fight aphids for so long,” says Ferrie, who is based just south of Bloomington, Ill. “We started spraying some of these fields three leaves before tassel to make sure we could get them pollinated, and many fields were sprayed to keep the top leaves from getting waxed up. This week, we’re still spraying, especially the later-planted corn,” he adds.
Usually when aphid pressure gets to a certain population level, it will become diseased and self-implode. For some unexplainable reason, Ferrie says that’s not happening this season. Instead, the high aphid populations are contributing to poorly filled ears on waxed-up corn plants.
“Many of the waxed up tops are on what we would say are very uniform stalk diameters, or uniform plants that should have produced a normal ear, but now we’ve lost a third to half of that ear,” Ferrie says. “It’s definitely a hybrid-sensitive situation. In some cases, where split planters were used, you can see 24 rows of corn that are loaded with aphids, while the next 24 are somewhat clean.”
Identify The Main Culprits
Farmers have been calling Ferrie to express concern about aphid pressure around corn ears and those even lower in the plants, especially after heavy rain events. While those aphids are pulling sap from the corn and creating a bit of crop stress, good moisture levels have prevented serious yield docks from occurring.
“It’s the aphids that are at the top of the plant that we worry about,” Ferrie says. “If you have aphids at the bottom of plants, keep scouting those fields every three days to watch for whether colonies form at the top of plants.”
Aphids are known as tasters, meaning they have flavor preferences. If they liked the flavor of a specific hybrid earlier this season, that hybrid might now be subject to a new aphid infestation, given the pests’ determination to hang around. Those infestations will show up as aphid colonies at the top of the plants.
This week, Ferrie says some central Illinois farmers have wanted to spray hybrids that have already been impacted significantly by aphids.
“Where kernels have been lost in the ear tips, where they are blowing off, we can’t get that yield potential back by spraying,” he reports. “We can only slow down the growth of other colonies that might start at the top.”
Actions To Take Now
Ferrie advises getting ear counts and estimating realistic yield potential before deciding your next action. Here are the four steps he recommends:
1. Count all the plants for your population.
2. Then do your ear count on the good ears that don’t have waxing at the top of plants, and calculate the potential on those ears.
3. Pull the remaining ears – the ones that are waxed up – off and get an accurate yield estimate of what those ears are going to make. Add that number to the potential you got from the good ears.
4. Divide by 65 or 70 for those corn ears not affected by aphids. Where you have had aphid feeding, Ferrie says to divide by 90.
Significant Yield Losses Have Occurred
In those fields where aphids have fed extensively, Ferrie says is seeing anywhere from a 10- to 35-bushel yield loss, depending on the hybrid. Not all hybrids have been equally affected by aphid feeding.
Fields planted to L2 hybrids – which flex in length – seem to have been hit the hardest, he says. This flex happens because of stress occurring between R1 and R3. These hybrids will have aborted kernels on the ear tip and will have visible cob sticking out, resulting in an “L2” rating. As an example, if a hybrid pollinates to 45 long and aborts back to 30 long, Ferrie calls that an L2 hybrid, meaning it flexed after tasseling.
He encourages farmers impacted by excessive aphid feeding to be in regular contact with their retailers or applicators to schedule insecticide applications.
Get In Touch With Retailers
“Some of our applicators are out about four days, so it’s good to know what you might be working with,” he says. “If your crop’s pollinated and has aphid pressure, I’d pull the trigger and put and insecticide with your fungicide, even if you’re not at brown silk.”
“Usually I’d like to see the field with a wave of brown silk, but in this case, on this later-planted corn if the aphids are out there, as soon as you get the tassels out and get pollinated, let’s go ahead and clean them up. We don’t want these aphids to do any more damage than we have to.
“If you sprayed a fungicide without an insecticide, odds are higher than usual that you’re going to have to go back and get these aphids, because the fungus that usually wipes those aphids out hasn’t showed up yet for some reason.”
Listen to this week’s Boots In The Field podcast here:
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