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Keep On Alert for Aphids
In the last few weeks there has been an increase in aphids in corn. Aphids have been everywhere on the plant including the leaves, leaf sheaths, and husks. The picture above shows an aphid population on a corn leaf.
In most years aphids are not an issue and they are usually more of a problem on sweet corn and corn that is in tassel. Natural enemies such as ladybird beetles and their larvae along with lacewings, syrphid fly larvae, wasps and fungi often keep aphids in check. In the fields I have checked, however, these natural enemies are in low numbers compared with aphids.
Aphids can cause damage in a few different ways:
Their piercing-sucking mouthparts feed on the phloem of the plant. This is especially a problem in drought stressed conditions (which luckily we have not had in most areas).
Aphids excrete a honeydew (a sticky substance) that covers the plant (it almost looks like a glaze as the picture shows). There is a fungus that feeds on this substance called a sooty mold that is black in color (you can just start to see it on the lower left side of the picture) and can reduce the photosynthesis rate. It is the same sooty mold that you see when aphids feed on soybeans.
There is not a lot of recommendations for control of late season aphid infestations. One recommendation (from the Handbook of Corn Insects) says that “moderate infestations (50-400 corn leaf aphids per plant) on 50 pertcent of the plants may warrant control” if the corn is drought stressed. We don’t have drought stressed corn, but we do have higher infestation levels in some fields. Others have said the aphids will start to die off as the corn matures. I have seen corn that is just starting to dent, so we have at least four more weeks for the corn to mature and it is possible that heavy infestations can reduce kernel depth or stalk strength. There is not a clear cut answer.
If you scout your fields and decide to spray an insecticide (especially on the late May planted corn), please leave a check strip so we know for the future whether spraying is worth it.