No. 16 Article 3/July 9, 2004

Corn Leaf Aphids--An Occasional Pest of Corn

Though the corn leaf aphid is not an insect we have been readily hearing about, it is an occasional summer pest. Corn leaf aphids have blue-green bodies, with black legs and cornicles (or "tailpipes"). As the season progresses, adults become dark green to black in color and may be winged or wingless. Corn leaf aphids are a concern during late-whorl and tassel stages of corn. They are primarily a problem when populations are large and moisture is limited. Moisture doesn't seem to be a problem as of late, but as we move into midsummer, you can never tell what the weather will do.

Corn leaf aphids.

Corn leaf aphids are migratory insects that quietly make their way into Illinois cornfields in the spring. These aphids start colonies in the whorls of corn plants. They remain in corn whorls until tassel emergence. As tassels emerge, aphids move down the plant, forming colonies on stalks and leaves, and behind ears. These colonies are often kept in check by natural enemies such as lady beetles, lacewing larvae, syrphid fly maggots, and other insect predators. However, during hot, dry summers or times when these predators are scarce, populations can increase dramatically. As corn begins to senesce later in the season, winged aphids are produced and migrate to other suitable hosts.

Corn leaf aphids on corn.

During the growing season, both adults and nymphs will feed in the whorl, causing injury by removing water and nutrients. Damage is most severe between the late-whorl and pollination stages. Corn plants heavily infested by corn leaf aphids may wilt, curl, and show yellow patches; tassels and silks may become covered with honeydew and turn sooty black as mold grows on them. Heavily infested plants may experience poor grain fill or barrenness. If corn plants have adequate moisture, corn leaf aphids cause little damage. Stress caused by high densities of aphids is more severe when it occurs at a time of moisture deficiency for plants, and injury may exaggerate symptoms of drought stress. If aphid colonies continue to increase after pollination and natural enemies are scarce, feeding by corn leaf aphids may cause the death of plant tissue.

To monitor aphid populations, examine 100 plants (5 sets of 20) during the whorl stage (3 weeks before tasseling). A treatment may be warranted if 50% of plants have light to moderate infestations (50 to 400 aphids per plant) and plants are under drought stress. If soil moisture is adequate, treatment may be warranted if there are more than 400 aphids per plant.

If such a treatment is needed, the following insecticides are labeled for use: *Capture 2EC at 2.1 to 6.4 ounces, dimethoate (see product label, do not apply to corn during pollen-shed period), *Discipline at 2.1 to 6.4 ounces, *Lorsban 4E at 1 to 2 pints, and *Penncap-M at 2 to 3 pints. Products preceded by an asterisk are restricted for use by certified applicators. Please follow all label directions and precautions.--Kelly Cook

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