The possibility of finding corn leaf aphids in cornfields is looming on the horizon. They are blue-green aphids, 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 migratory insects that quietly make their way into Illinois cornfields in the spring.
Corn Leaf Aphids.
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. Corn leaf aphids may produce as many as 40 to 50 generations each year; populations are higher in dry years. As corn begins to senesce later in the season, winged aphids are produced and migrate to other suitable hosts.
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.
Corn leaf aphids on tassel.
To monitor aphid populations, examine 100 plants (five 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), *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 pay attention to precautions.
As we proceed through July, the potential exists for these insects to interfere with pollination; don't forget about the injury they may cause.--Kelly Cook