Aphids in corn (post-pollination)
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When you re-enter corn fields to scout corn rootworm beetle populations and begin to estimate yield potential, you’ll often find some unwelcome aphid visitors.
Heavy infestations on ears and adjacent leaves can grab your attention and make you wonder if you should spray. Before you make that decision, it’s important to review what’s known about aphids in corn.
Aphids on corn are the same ones that colonize cereal crops and other grasses.
Small, dark-olive bird-cherry oat aphids (Figure 1) are usually found lower in the canopy, particularly when populations first establish on corn. These move upward on the plant as populations increase.
English grain aphids (Figure 2) are found higher in the canopy. Sometimes, colonies of these larger, light-green aphids with long black cornicles are mixed with those of bird-cherry oat aphids.
Some years, the blue-green corn leaf aphid can also be found in the upper part of the canopy, but this species seems less common in southwest Minnesota than it once was. Even more rarely we may see some greenbug colonies on corn.
Why infestations are worse in some years or fields
The aphids that are found in corn don’t overwinter here; they migrate each spring, primarily colonizing cereals and cool-season grasses and, to a lesser extent, corn. Mass migration from maturing small grains and cool-season grasses may also initiate or supplement later-season infestations.
Bird-cherry oat and English grain aphids are often more abundant where corn is moisture-stressed. Other factors, such as soil types, stalk and or root rots or even corn rootworm injury, may accentuate stress under otherwise suitable rainfall levels. These stresses can improve the quality of plant sap from an aphid’s perspective.
Those of you who look at hybrid comparisons or walk numerous fields have probably noticed there are differences in hybrid susceptibility, with some more heavily infested than others.
The increasing use of pesticides in corn may indirectly contribute to aphid buildup. Sometimes, we see more aphids where fungicides or insecticides were previously applied. Fungicides can prevent infection of insects by beneficial fungi, while insecticides remove beneficial insects, such as predators and parasitic wasps.
How aphids injure corn
Regardless of contributing factors, aphid species can become very abundant, covering plants with aphids and honeydew.
Aphids are sucking insects that tap the phloem, the plant plumbing system transporting sugars and other important compounds. Aphids deprive the plant of these building blocks and, in sufficient density, may affect leaf function, plant growth and yield.
In extreme cases, aphids are associated with dying leaves or, rarely, the death of entire plants. Excess water and surplus sugars that aphids can’t use are excreted as honeydew, dropping down to coat lower leaf surfaces. Sooty molds can colonize these sugary leaf surfaces, further reducing leaf photosynthesis.
Populations often rapidly build and peak in R3 or milk stage corn. In response to plant development, aphid distribution changes at this time. Corn aphids put themselves in the best position to intercept the higher-quality sap moving out of upper leaves and lower stalk into the developing ear.
Aphids can, and often do, leave corn as it begins to mature to dough stage (Figure 3). If subsequent rainfall washes off the sooty mold, honeydew and cast skins, the only evidence of the infestation may be small discolored areas on leaf sheaths and shanks.
When to spray aphid populations
Published thresholds for aphids on corn are based on whorl and pre-pollination corn growth stage. Concerns about honeydew on tassels and ears interfering with pollination drove development of early thresholds.
Historically, little attention has been paid to the late-milk stage bird-cherry oat and English grain aphid populations we’ve been finding in Minnesota the past few years. If you hear a threshold, just realize it's an opinion and not research-based. The question is whose opinion? What is its basis?
Corn vs. soybeans
It’s difficult to compare aphids and their damage on corn versus those on soybeans, small grains and other crops. We’re not aware of significant virus diseases of corn that would be transmitted by aphids, particularly after pollination.
Corn biomass is much larger than a soybean plant and the amount and water and nutrient contents of the two crops’ sap are quite different.
What to focus on
The main focus should be on population development in the ear zone and above, because photosynthesis from these leaves significantly contributes to yield. A crystal ball providing a glimpse into how high population levels will build and how long they persist on the plant would be helpful to spray decisions.
Chances for a payback for insecticide applications in blister to dough stage corn are less likely than you might think based on aphid numbers you’re seeing at that moment. Many ag professionals who’ve sprayed in previous years indicate many didn’t see an economic return. Complicating factors include the following:
Aphids’ persistence. Aphid populations respond to changes in nutrient quality of corn sap, which reflects moisture stress, stalk rot, corn rootworm feeding, etc. Post-pollination corn aphid populations are incredibly fickle and capable of leaving en masse if they find host conditions becoming unsuitable. The challenge is knowing if aphids would’ve stayed around long enough that removing them creates an economic return.
Other stressors may limit yield response to spray. Very high aphid populations may, in part, be a symptom rather than cause of crop stress. Whether they voluntarily leave or are removed by a spray, the other stressors won’t leave and may limit the plant’s yield response to aphid removal..
Spray coverage and persistence. Sprays depend on coverage for aphid control. When aphids move from the lower canopy into the ear zone and above, this increases the potential for exposure. Taller-than-typical corn fields increase coverage challenges. In addition, aphid escapes lower in the canopy can quickly re-infest treated foliage as spray residual declines. If you’re contemplating a spray, make sure there’s adequate water volume for good coverage.
Collateral damage. Insecticides can make the situation worse by killing beneficial insects (because there’ll be none to control re-colonizing aphids) or flaring spider mite populations.
Potential for moisture stress. Tall plants with high kernel counts are more likely to undergo stress if there’s not enough rainfall.
If you spray
Good datasets on late-season infestations are rare, but most growers and advisors we’ve visited with in past years haven’t seen a yield response for treating corn aphids. However, if you feel an uncontrollable urge to kill some corn aphids or just want to experiment, there are several labeled products.
Use check strips
Take the opportunity to learn something from the situation, such as by trying to leave replicated check strips. Leave strips wide enough to account for spray drift and leave multiple check strips to account for spatial differences in corn yield potential and aphid populations.
Situations where check strips would be most helpful:
Corn hasn’t started to dent.
The field is generally infested with a high percentage of plants with aphids. It’s hard to draw conclusions from fields infested only in the borders of or pockets within the field.
If aphid populations are very heavy above the ear leaf, and there’s potential for photosynthesis-blocking sooty mold development on honeydew-covered upper leaves.
Predators and wasp parasitoids (lady beetles, Syrphid flies, parasitized mummies, etc.) and signs of fungal disease aren’t abundant. Once established, predators and disease can remove large numbers of aphids without your help and may trigger aphids to move.
Winged aphids or nymphs with wing pads aren’t easily found. If present, the population may be ready to leave the field.
The plants aren’t already covered with sooty mold or showing signs of stress. Any yield loss has already happened.
There are no guarantees on economic return on late-season aphid sprays in corn. Most often, it’s because heavy infestations are discovered late and the aphids were ready to leave anyway, or damage had occurred.
Our inclination is to leave these late-season populations alone. Walk away from dented corn where a large portion of yield is already fixed – the aphids usually do.
Reviewed in 2018