Corn Leaf Aphids: An Insect Pest to Respect in Dry Seasons

July 23, 1999
Reports of corn leaf aphids became more common late last week. On July 20, we observed corn leaf aphids with little difficulty in our research plots located near Monmouth in Warren County. Corn leaf aphids migrate into Illinois on storm fronts in late June through early July. Some corn leaf aphids may overwinter successfully on grain crops in southern areas of the Corn Belt. The biotic potential of corn leaf aphids is staggering. Roughly nine generations of this insect pest may occur each year throughout much of the Midwest. Producers most often begin to notice aphids in small clusters that appear on leaves and in the whorls of corn plants. As the number of aphids increases, 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). Their legs and short cornicles ("tailpipes") near the rear of the abdomen are completely black. When scouting for corn leaf aphids, examine at least five sets of 20 plants per field. The number of predators (lacewings and lady beetles) also should be determined (Figure 3). Unfortunately, specific guidelines are not available for determining how many predators and diseased aphids represent that point at which a producer should forgo an insecticide treatment. 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 and cast skins.

Heavy corn leaf aphid infestations may "coat" entire leaves.

Lady beetle larva, an effective predator of corn leaf aphids.

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 will 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 with honeydew and aphids).

Corn leaf aphids can reduce pollination by coating tassels with honeydew.

Insecticides that are labeled for use against corn leaf aphids include dimethoate (see product label), Lorsban 4E (1 to 2 pints of product per acre), and Penncap-M* (2 to 3 pints of product per acre). (Penncap-M is a restricted-use product.) Note that dimethoate cannot be applied to corn during the pollen-shed period. So use of dimethoate will present a problem for producers who wish to use an insecticide to prevent excessive silk clipping by rootworm beetles and also achieve corn leaf aphid control.--Mike Gray

Author: Mike Gray