- A buildup of aphid populations has been reported recently in corn in the 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.
- Widespread use of foliar fungicides in corn may be suppressing the natural fungal pathogens of these aphid species, resulting in a post-pollination population surge.
- Impacts to yield are not well-researched, but corn can typically withstand heavy aphid pressure with adequate moisture and good growing conditions.
Introduction
Heavy infestations of aphids are once again being reported in post-pollination corn in certain areas of Iowa and Illinois. Several years ago, the major concern was corn leaf aphid in the late vegetative and early reproductive stages through the end of pollination. Most land grant research was previously directed at corn leaf aphid control during this period of corn development. However, in recent years we have seen a mix of different species of aphid populations surge in corn during August after the pollination period. Why is this happening and what can or should be done about this pest during the late growing season?
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 stresses like drought, stalk rot, root rot, foliar disease, nutrient deficiency, and rootworm injury can actually 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. Both direct feeding activity and indirect damage have the potential to lower yield, although the extent of yield loss is not well understood today.
Aphids have a number of natural enemies, including lady beetles, aphid lions, syrphid fly larvae, parasitic wasps and even fungal pathogens. One theory explaining the late-season aphid explosion is the widespread routine use of fungicide and insecticide in the early reproductive stage (VT-R1). When natural enemies are suppressed, aphid populations are allowed to thrive, which is one of the reasons we continue to advocate integrated pest management tools like scouting and economic thresholds for making economically sound pesticide decisions. A prophylactic use of fungicide in the absence of disease can actually create a new problem that would not have otherwise existed.
Management Guidelines
No thresholds have been established for aphids in corn during the post-pollination period, so we have to 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 is essential with 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 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.
Applying these criteria, insecticides are rarely justified for late season aphid control. If you pull the trigger on an application, we recommend an on-farm discovery strip comparison and would be interested to hear your results.