Image 1. English grain aphids on corn stalk. (Source GROWMARK, Inc.).
- A buildup of aphid populations has been reported recently in corn in late vegetative and early reproductive stages.
- Aphids that have been showing up during the post-pollination period in recent years are a mix of species that include corn leaf aphid, bird cherry oat aphid, and English grain aphid.
- Impacts to yield are not well-researched, but corn can tolerate moderate aphid pressure with adequate moisture and good growing conditions.
Introduction
Infestations of aphids are once again being reported in pre-tassel and post-pollination corn in Illinois. Last year we saw high aphid numbers in Illinois and significant differences in aphid populations based on corn hybrids.
Description and Crop Damage
Aphids are piercing-sucking insects and feed on sugar-rich sap from the plant phloem. Late-season aphids often position on or around the ear and associated leaves to capitalize on the high-quality sap moving to the ear to support grain fill. Any in-season stress like drought, stalk rot, root rot, foliar disease, nutrient deficiency, and rootworm injury can increase the quality of plant sap, driving aphid populations even higher. Aphids also excrete excess sugar and moisture as ‘honeydew’, which can stimulate growth of sooty gray mold that covers the leaf and interferes with photosynthesis. The sap is low in protein, so aphids consume large amounts to get enough protein for growth and reproduction. Although direct feeding activity and indirect damage have the potential to lower yield, the sooty mold was likely the main culprit for yield loss last year after examining plants in the late reproductive stages.
Corn Hybrid Interactions
Corn plants exhibit an immune response to aphid feeding that can be both physical and chemical. Physical response to aphid feeding can induce the production of callose, which is a carbohydrate that can seal the opening of cells and creates a physical barrier to prevent feeding. The chemical response is the production of DIMBOA that deters feeding and increases aphid mortality. Corn hybrids vary in the amount of DIMBOA and callose that is produced.
Management Guidelines
Economic Threshold for late whorl to early tassel – University of Illinois
- Control may be warranted if 50% of plants have light to moderate infestations (50-400 aphids per plant) and plants are under drought stress. If there is adequate soil moisture, 400 aphids per plant per plant on 50% of plants is the threshold.
No thresholds have been established for aphids in corn during the post-pollination period, so we must leverage our existing knowledge of corn aphids, apply sound IPM principles, and use common sense to evaluate whether the problem has reached an economic level. Here are some common considerations when making a pesticide decision on late-season corn aphids:
- Infestation Level. If 80% or more of the plants are affected with 200 aphids or more, economic response to insecticide is more likely. Note that this threshold is an educated guess only and not based on research. Distributions are rarely uniform, so be sure to scout representative areas of the field.
- Infestation Type. If aphids are infesting the ear and/or ear leaf or upper canopy and honeydew or sooty mold is interfering with photosynthesis, treatment should be considered.
- Growth Stage. The quality of sap is declining rapidly by growth stage R5 (dent), which may cause aphids to leave en masse on their own. The likelihood of economic return on insecticide is low at this growth stage.
- Disease Risk. Unlike wheat and other crops, aphids are not currently known to vector any serious diseases in corn, which lowers the justification for insecticide.
- Stress. When other stressors are present, most notably drought, corn yield may benefit from aphid control.
- Natural Enemies. When populations of beneficial insects and parasites are high, insecticide can be counterproductive. Make note of beneficials during scouting.
- Coverage. Good coverage and contact control are essential for sap feeding insects. If aphids are present in the lower canopy, they can be more difficult to control and will likely re-infest the plant in the absence of natural enemies.
- Harvest Interval. Some insecticide labels have harvest intervals of up to 60 days. The risk is even greater with forages that may be harvested earlier in the season and fed to livestock. ALWAYS READ AND FOLLOW LABEL DIRECTIONS.
Image 2. Bird cherry oat aphids on corn stalk. (Source: GROWMARK, Inc.)
Related Categories: Corn, Insects, Sap feeding, Aphids, Scouting, Thresholds, Drought