It has been a while, but as in years past, we have received reports of cornfields in northwestern counties being infested with aphids. Two common cereal aphids (frequent pests of wheat), bird cherry-oat and English grain aphid, are often found colonizing plants, usually quite randomly in a field.
However, unlike previous “outbreaks,” the pictures sent this year seem to implicate corn leaf aphid, Rhopalosiphium maidis, as the culprit. The infested plant patterns are hybrid specific, making scouting and assessments more challenging in fields with multiple hybrids.
It takes a very large aphid infestation to affect corn plants to be point of impacting yield, but it can happen. Before and during pollination is the most critical time corn leaf aphid may cause economic problems, especially when their excrement (“honey dew”) potentially interferes with pollination. After pollination, some plants may already be discolored from the aphids removing plant juices, and neighboring plant leaves look nasty with black sooty molds growing on the honeydew, our recommendation is to let things be at that point. Most of the time the appearance is far worse than any actual damage and treatments now are too late to make any difference. The only exception is if the plants are under moisture stress, and plentiful rainfalls from Hurricane Beryl a couple of weeks ago makes that unlikely. Also, aphid predators, parasites and fungal pathogens are always found in these large aphid colonies, doing their part to keep aphids in check.
Even if one decides to treat for corn leaf aphid, there are some important considerations. Aerial application is going to be difficult to schedule anytime soon, as fungicide treatments are keeping the planes/helicopters quite busy. More importantly, insecticides to control corn leaf aphid are limited, as synthetic pyrethroids aren’t very efficacious against this pest, yet they are detrimental to the natural enemies (predators and parasites) that control them. With spraying, there is a definite risk of making the situation worse. There is a reason that the words “suppression only” are found on the insecticide labels for this pest – “control” is elusive.
It is unclear why some cornfields are heavily infested and most are not. It is likely a combination of growth stage, plant architecture, hybrid susceptibility with geography playing the largest role. Most corn leaf aphids arrive into the Midwest carried upon storms from the south. It is possible that the remnants of Hurricane Beryl brought more winged adults than normal from the Gulf Coast. Although in past years we suspected widespread use fungicide/insecticide tank mixes during R2 aerial applications were “flaring” aphid populations, this year is different. Most fields with current infestations have not been sprayed yet. Any insights from your observations would be appreciated!
Happy scouting!
Corn leaf aphid colonization on the ear, note the “honey dew” mess (Credit: Marty Park)
Corn leaf aphid infested tassel with leaf discoloration (Credit: Marty Park)
Corn leaf aphid infested plant with leaf discoloration (Credit: Marty Park)
