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More Aphids, but on Another Crop--Corn

July 13, 2001
On July 6, John Coers with Royster Clark reported a fairly heavy infestation of corn leaf aphids in a cornfield in Mason County. Although the field was only partially tasseled out, many of the tassels were heavily infested with the blue-green aphids that typically pose a threat to corn yields only when moisture is lacking. Well, you know that story.

With all the focus on soybean aphids in soybeans right now, it's easy to forget that the corn leaf aphid has been with us for years and can cause some significant injury during the late whorl and pollinating stages of growth. When environmental conditions are very hot and dry, corn leaf aphids can cause significant yield losses even after pollination is completed. I remember way back to 1980 (yes, some of my memory is still intact) when we had a huge outbreak of corn leaf aphids in central and east-central Illinois. Don Kuhlman (retired extension entomologist) visited some cornfields in which the tops of the plants had been killed outright. Numbers of aphids in these fields were extremely large (aphids covering every square inch of plant from the tassel to the ear), and the lack of soil moisture was critical.

Corn leaf aphids and cast skins on a corn leaf.

Colony of corn leaf aphids on a tassel and upper leaves.

During the critical stage of pollination and shortly thereafter, monitor for corn leaf aphids in cornfields, especially in areas that are short on moisture. Adult corn leaf aphids (winged and wingless) are about 1/16 inch in length and are dark blue-green. The cornicles, bases of cornicles, and legs are black. As the aphids age, the adults become almost completely dark green to black. You will find corn leaf aphids in different-sized colonies that started in the whorls and continue to increase in numbers on the upper parts of the corn plants, if environmental conditions are favorable for the aphids. If corn plants have adequate moisture, corn leaf aphids cause little injury. However, feeding by aphids during drought conditions exaggerates symptoms of drought stress. Symptoms of the aphids' feeding include yellowing or reddening of whorl leaves; sticky, gummed-up leaves and tassels (the consequence of honeydew excreted by aphids); tassels with dead sections; and barrenness or poor grain fill.

Like other species of aphids, corn leaf aphids have tremendous reproductive potentialthey can complete 40 to 50 generations per year. So watch for the buildup of aphids in corn wherever a lack of moisture prevails. During the early tassel stage, if 50% of the plants have light-to-moderate infestations (50 to 400 aphids per plant) and plants are under drought stress, treatment may be warranted. After pollination, continue to watch the colonies of aphids to determine if their numbers are increasing or decreasing. And always look for beneficial insects, such as lady beetles, that can suppress aphid populations.

If an insecticide treatment seems justified, consider *Capture 2EC at 2.1 to 6.4 oz per acre, dimethoate (see product label for rate of application), *Lorsban 4E at 1 to 2 pt per acre, or *Penncap-M at 2 to 3 pt per acre. Products preceded with an asterisk are restricted for use by certified applicators.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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