Corn Leaf Aphid
Rhopalosiphum maidis (Fitch), Aphididae, HEMIPTERA

DESCRIPTION

Adult - The oval, wingless adult, about 2.0 mm long, is usually pale bluish-green with black antennae, legs, and cornicles, and a dark area around the base of the cornicles. The head is marked with two longitudinal dark bands, and the abdomen with a row of black spots on each side. The body often seems to have a powdery coating. The winged form is about the same size.

Egg - Unknown.

Nymph - Similar to the wingless adult, the nymph is smaller and has no wings.

BIOLOGY

Distribution - The range of the corn leaf aphid extends throughout the tropical and temperate regions of the world. In the continental United States, it occurs in all areas except the Rocky Mountain region.

Host Plants - Rarely a problem in the northern states, the corn leaf aphid shows a preference for barley, sorghum, and corn, in that order. It also infests millet, broomcorn, sugarcane, Sudan grass, and many other wild and cultivated grasses.

Damage - Feeding by colonies of these aphids causes mottling and discoloration of the leaves. Heavily infested leaves turn red or yellow, shrivel, and die. The important damage usually occurs during and after flowering. At this time the aphid population peaks and feeds on corn tassels and silks. In sorghum, the developing heads of grain become infested. Areas fed upon become covered with sweet, sticky honeydew secretions. Black mold grows on the honeydew and may result in poor corn pollination, interference with photosynthesis and, in severe cases, reduced grain development. Entomologists have speculated that the honeydew attracts corn earworm moths and therefore, induces heavy earworm egg deposition.

Life History - Little is known about the biology of this pest in North Carolina. Since the relationship between corn and this aphid is not well understood, it has been difficult to estimate damage and to determine thresholds. This aphid generally is not considered a serious threat.

Corn leaf aphid adults overwinter each year in southern states, including North Carolina. On warm winter days, the females actively continue to feed and reproduce on winter grain crops or other grasses. The first spring adults are winged females which fly in search of suitable host plants, sometimes migrating far northward. Shortly thereafter, they give birth to live nymphs which usually develop into wingless females. Under favorable conditions, more winged females develop and migrate. Males are rarely found but females continue to reproduce without mating. No egg stage is known. Reproduction slows down in winter and summer and is most rapid during cool weather. Therefore, corn leaf aphids tend to be a problem on winter grains in spring and on late-planted corn in fall. The number of generations per year varies from 9 in Illinois to 50 in southern Texas.

CONTROL

Early planting and other cultural practices which hasten maturity will help prevent corn leaf aphid infestations which are usually restricted to late-planted corn and sorghum. This insect rarely requires control in North Carolina on sorghum or corn. In the northern states control is usually unnecessary because populations build up so late in the season. During summers in the southern and southwestern states, however, the corn leaf aphid is controlled both by extreme heat and by the insect's natural enemies.

Research in Kansas indicates that corn leaf aphid infestations of 800 to 1500 aphids per sorghum plant do not substantially reduce yield unless the aphids feed in the whorl. Much lower populations feeding on silks, tassels, and developing grain, however, may inflict significant injury. Should damaging populations develop, consult the current North Carolina Agricultural Chemicals Manual.