Issue No. 17, Article 3/July 20, 2007
Corn Leaf Aphids: Most Fields Beyond Treatment Consideration
This past week, we received a few reports of corn leaf aphid infestations in central Illinois (McLean County). Densities are light to moderate, generally well below economic levels in most fields. The plentiful soil moisture in many areas of the state will help reduce the number of insecticide treatments that may be necessary. Corn leaf aphids are primarily an economic threat in hot and dry summers in which drought conditions develop. 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 impressive. 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.
Corn leaf aphids and cast skins.
Producers monitoring fields for this pest should look for bluish green, soft-bodied insects the size of a pinhead. Their legs and the short cornicles ("tail pipes") 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. Unfortunately, specific guidelines are not available for determining how many predators and diseased aphids represent the point at which a producer should forego an insecticide treatment. Parasitized corn leaf aphids will be smaller, brownish, and generally stuck to plant tissue. Diseased aphids will be 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.
Severely infested corn plant with corn leaf aphids.
If 50% 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. We are well beyond these development stages throughout most of Illinois. 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). Based on these treatment guidelines, we should see relatively few fields this season treated for corn leaf aphids.
For information on insecticides labeled for use against corn leaf aphids, please consult chapter 1 of the 2007 Illinois Agricultural Pest Management Handbook.--Mike Gray