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Corn Leaf Aphid Colonies Are Increasing in Size

July 19, 2002
A well-known pest during hot, dry seasons, the corn leaf aphid is making its presence known in some areas of the state. Population densities are high in some fields in central Illinois, and the potential exists for these pests to interfere with pollination. Corn leaf aphids are adding insult to injury in areas where growers have been protecting corn pollination from the depravations of Japanese beetles and western corn rootworm adults.

As most of you know, corn leaf aphids do no overwinter in Illinois. Winged aphids migrate north in early spring and initiate colonies of wingless aphids in the whorls of corn plants. If predators (e.g., lady beetles, lacewing larvae, syrphid fly maggots), parasitoids, and pathogens do not suppress corn leaf aphid populations, the colonies increase in size in the whorl. When tassels emerge, the colonies of aphids are more evident. As colonies continue to grow, corn leaf aphids can cover tassels, leaves, stalks, and ears. The aphids excrete honeydew, a sticky substance that often coats plant parts and promotes the growth of sooty mold. When tassels and silks are covered with aphids and honeydew, pollination is affected.

Let's review what we know about corn leaf aphids.

What to look for. Adult corn leaf aphids (winged and wingless) are about 1/16 inch in length and are dark blue-green. The cornicles ("tailpipes"), 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.

Corn leaf aphids and cast skins on a corn leaf.

Close-up of a corn leaf aphid colony on corn leaf.

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

Will corn leaf aphids affect yield? When corn plants have adequate moisture, corn leaf aphids cause little damage. However, feeding by aphids during drought conditions exaggerates symptoms of drought stress. Feeding and excretions by corn leaf aphids may result in yellowing or reddening of leaves, stickiness of leaves and other plant parts, tassels that shed pollen poorly, tassels with dead sections, and barrenness or poor grain fill. If infestations of corn leaf aphids become severe, their feeding can cause death of plant tissue, even after pollination has been completed. However, this occurs only if aphid colonies continue to increase after pollination, particularly if the impact of natural enemies is not sufficient.

What are the treatment guidelines? Like other species of aphids, corn leaf aphids have tremendous reproductive potential--they 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.

What insecticides are suggested for control of corn leaf aphids? If an insecticide treatment seems justified, consider *Capture 2EC at 2.1 to 6.4 ounces per acre, dimethoate (see product label for rate of application), *Lorsban 4E at 1 to 2 pints per acre, or *Penncap-M at 2 to 3 pints per acre. Products preceded with an asterisk are restricted for use by certified applicators. Please follow all label directions and pay attention to the precautions.--Kevin Steffey

Author: Kevin Steffey

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

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