
“Corn leaf aphids are making an unusual appearance this year in field corn, reaching high numbers. … This outbreak is not restricted to Ohio, but appears to extend throughout the Midwest and also into the coastal Southern U.S. and also southern Canada.”
That is how authors Kelley Tilmon, Amy Raudenbush and Andy Michel of Ohio State University led off the Aug. 6 edition of the OSU C.O.R.N. newsletter. And while they didn’t recommend spraying for aphids in commercial corn based on experience, some growers found infestations were heavy enough to justify spraying last summer.
“While not widespread, there were some geographies and specific hybrids where corn leaf aphid populations increased to very high levels in 2024,” says Jeff Nagel, an agronomist with Keystone Cooperative, Lafayette, Ind. “Apparently, some combination of nutritional and/or sugar content of certain hybrids were conducive to rapid reproduction leading to unusually high populations.
“The high populations were observed post pollination. Entomologists said natural predators and fungi would provide rapid natural control. This did happen in some cases while other populations persisted. When combines rolled, yield was often negatively impacted.”
Looking toward 2025
Nick Seiter, Extension field crop entomologist in Illinois, watched aphid populations increase in corn last summer and observed similar outcomes to what Nagel reported. In this exclusive interview, Seiter provides insight into how to think about aphids in corn in 2025.
Why did we see more aphids in 2024? Will this likely repeat in ’25? Those are wonderful questions, but we don’t have reliable answers. In the past, corn leaf aphid outbreaks that lasted multiple years occurred in the Midwest, but not with a satisfactory explanation as to why. It is so rare that the most recent I’m aware of was in the 1950s. Production systems have changed in countless ways. So, it is difficult to know if we should expect anything similar in 2025.
When and how can farmers scout for aphids? Scouting should begin in earnest about two weeks prior to pollination in the late whorl stages. Examine the upper whorls for aphids. Corn leaf aphids are somewhat unique in that they tend to congregate on stems and within whorls. Most other species of aphids gather on the undersides of leaves.
How do you know when to spray? Our action threshold recommendation is to make an application if about 50% of plants have colony formation before and around pollination time. This would be about 50 to 100 aphids per plant.
Did aphids actually cut some commercial corn yield in ’24? How? Yes, in some fields. If aphid numbers during pollination and early kernel formation are high enough, they can result in blank or nearly blank ears. This didn’t occur everywhere and was usually patchy if it did. But clearly, some fields showed a substantial yield loss.
What are the biggest takeaways from 2024? Aphids don’t overwinter in the Midwest and must ride up on wind currents. Both magnitude and timing right at pollination made 2024 unique for aphids. Going forward, scout just before and at tasseling. Typically, aphid numbers build up and collapse rapidly. However, if numbers indicate that you need to spray an insecticide, focus on getting good coverage.
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