No. 15/July 3, 1997
Corn Leaf Aphids in July: A Common Occurrence in Illinois
As scouting efforts for the first generation of European corn borer begin to "wind down," don't be surprised to find other insect occupants in whorl leaves. Corn leaf aphids migrate into Illinois on storm fronts in late June through early July. Some corn leaf aphids may be able to overwinter successfully on grain crops in southern areas of the Corn Belt. The reproductive powers of corn leaf aphids are remarkable. Approximately nine generations of this insect are estimated to occur each year in much of the Midwest. Producers generally begin to notice the aphids in clusters that appear on leaves and in the whorls of corn plants. As the number of aphids increases on a given plant, winged females become more apparent and eventually fly from infested plants to less crowded plants to begin new colonies. Corn leaf aphids secrete a sticky substance referred to as "honeydew." As aphid densities increase, leaf surfaces and tassels often become black and sooty as mold grows on the honeydew.
As producers monitor their fields for this pest, they should look for bluish green, soft-bodied insects the size of a pinhead (Figure 2).
Figure 2. Corn leaf aphid, winged and wingless forms.
Their legs and short cornicles ("tailpipes") near the rear of the abdomen are completely black. When scouting fields for corn leaf aphids, examine at least five sets of 20 plants per field. The number of predators (lacewings and lady beetles) should also be determined (Figure 3).
Figure 3. Common predators of corn leaf aphids.
Lady beetle larva.
Unfortunately, specific guidelines are not available for determining how many predators and diseased aphids represent that point at which a producer should forego using an insecticide. Parasitized corn leaf aphids are smaller, brownish, and generally stuck to plant tissue. Diseased aphids are shriveled and possibly moldy. Because corn leaf aphids must shed their skins when they molt, white to grayish cast skins are often found. Don't confuse cast skins with diseased or parasitized aphids.
Corn leaf aphids (adults) and cast skins.
If 50 percent of the plants during the late-whorl to early tassel stages have light to moderate infestations (50 to 400 aphids per plant) and plants are under drought stress, a treatment may be warranted. If the pollination process is well over halfway complete and the plants are under no moisture stress, corn leaf aphids pose less of an economic threat. Corn that is under serious moisture stress after pollination may still suffer yield losses if plants are heavily infested (upper leaves and tassel completely coated).
According to the Seed Corn Pest Management Manual for the Midwest (published by Purdue University), fields should be monitored when they reach the whorl stage of development. It is during the whorl stage that yield losses are most likely to occur if aphid densities are significant. In addition, information contained within this manual suggests that "corn leaf aphid control is most effective two to three weeks prior to tasseling, after which it is rarely advisable."
Insecticides suggested for corn leaf aphid control include Dimethoate 400 at 2/3 to 1 pint product per acre, Lorsban 4E at 1 to 2 pints product per acre, malathion 57% EC at 1-1/2 pints product per acre, and *Penncap-M at 2 to 3 pints product per acre. Penncap-M is a restricted-use insecticide and may be applied by certified applicators only. If corn leaf aphids are the only target insect and residual activity is not a main concern, malathion 57% EC should provide satisfactory control.
Mike Gray, Extension Entomologist, (217)333-6652