Beginning last week, Erin Hodgson, Iowa State University entomologist, has seen and heard about an aphid explosion – in corn. For the last three to four years, certain areas in Iowa have had serious problems with aphids infesting corn in August. The areas having problems right now include the northeast and northwest corners of Iowa and west central Iowa. Some of these heavily infested fields were sprayed with an insecticide earlier this year. From her observations this week aggregated colonies at the end rows were noticed.

In the past, corn leaf aphid could be a problem during tasseling. This species aggregated around the ear and silks, and sometimes their honeydew production interfered with pollination. But natural enemies and the environment rarely let them build up past July, so economic thresholds have only been determined for aphids around tasseling and mostly targeted to fields during drought-stressed summers.

Now, it seems aphids are colonizing corn later in the summer and are building up to striking levels. They can be found at the base of the stalk, around the ear and sometimes building up colonies above the ear leaf.

One important observation is that most fields have two aphids species: corn leaf aphid and bird cherry oat aphid. They are closely related and look very similar in size and color. The bird cherry oat aphid has an orange-red saddle between the cornicles. Other aphid species can also be found, including greenbug and English grain aphid, but are not as common in corn this year. Species identification isn't that critical for management at this point (an aphid is an aphid). You can see more than one species in a field and even on a single plant.

All aphids have a piercing sucking mouthpart and feed on the sap from the plant phloem. They excrete sugar-rich honeydew that can cover the above ground portion of plants. The honeydew can promote a sooty mold that can interfere with plant photosynthesis. You probably remember seeing gray-looking soybean leaves when soybean aphids first hit Iowa about 10 years ago. We know soybean plants covered with mold and aphids can have serious yield loss, but we don't know the extent of yield reduction caused by aphids in corn.

Treatment and thresholds

Currently, there are no treatment thresholds for aphids in corn past tasseling. But regular sampling will help you make educated decisions about a foliar application at this time. Here are some considerations to make before applying an insecticide for aphids in corn:

Hodgson strongly encourages leaving an untreated check strip or two in fields that you spray. Try to leave a strip that is a fair comparison to the majority of the field – not just the stunted corn around the field edge. If you decide to treat for aphids in corn, Hodgson would like to hear about the yield comparisons. Pooled data will help formulate treatment guidelines for the future. Erin Hodgson can be contacted by email at [email protected] or phone at 515-294-2847.