Corn Leaf Aphid

Rhopalosiphum maidis

The corn leaf aphid is a small bluish-green or gray, soft-bodied, spherical insect about the size of a pinhead. The adult females do not lay eggs, as do most other insects, but give birth to living young. These young, called nymphs, resemble the adults except in size. The aphids appear in clusters in the curl of the leaves and upper part of the cornstalk and may completely cover a large area. They are also found in appreciable numbers down in the whorl and on the unemerged tassel. Most of the aphids in a cluster are wingless. However, when clusters become large, females with delicate, filmy wings appear. The wings enable them to fly to other uninfested plants and start a new colony.
Like other insects, aphids, shed their skins in the process of growing. These numerous white to gray skins give the appearance of a white mold or ash. They also secrete a sticky, sugary substance known as honeydew. Moderately to heavily infested plants are usually quite sticky from accumulations of the honeydew.

Corn Leaf Aphid
Figure 1. Corn Leaf Aphid

Life History
Leaf aphids do not usually appear in Illinois until late June or early July. It is not known whether they overwinter in Illinois or whether they migrate from the southern states each year. It is feasible to assume, however, that some of them ovexwinter on grain crops in the southern one-third of Illinois. True egg-laying females have never been observed. Males have seldom been observed. In general, the time required for one generation is short. Under Illinois conditions, there are nine generations each year. Most clusters of corn leaf aphids include nymphs, adult wingless females, and adult winged females at the same time.

The corn leaf aphid prefers sorghum as a host plant, but will feed on corn, barley, broom corn, millet, sudan grass, and many other plants belonging to the grass family. Heavily infested corn leaves may wilt, curl, and show yellow patches. Tassels and silks may be covered with honeydew, which may interfere with pollination. There is no conclusive evidence that leaf aphids cause barren stalks, but there is circumstantial evidence that barrenness occurs in about 40 percent of the heavily infested cornstalks.

Damage is most severe between the late-whorl and pollination stages. Aphids feeding at those times is suspected as the cause of stunting (shortening of the inter-nodes), shriveled and shrunken ears, and possibly barrenness. After pollination, the major damage caused by aphids is competition with the plants for available moisture.

Scouting Procedure
Corn leaf aphids are found in the whorl of younger plants and later on the tassel and upper leaves. To monitor aphid populations, examine 100 plants (5 sets of 20) for corn leaf aphids during the whorl stage (3 weeks prior to tassel emergence). Rate the infestation for each plant using the following system:

0 - No aphids
1 - 1 to 100 aphids per plant
2 - More than 100 aphids

Threshold Guide
Low numbers of aphids found prior to tassel emergence are just as imporant as larger numbers found later since aphid populations can increase rapidly. Often one or more adults will be found with many young aphids in the protected tassel. Aphid predators, parasites or diseases often alleviate any need for chemical control. Look carefully and note the number and kind of predators on those plants that have aphids. These often include lady beetles (adults and larvae), insidious plant bugs, aphidlions and lavewing adults and syrphid fly maggots. The aphid colonies may also contain discolored brown or gold aphids These are diseased or ones that have been parasitized. If 50 percent of the plants have more than 100 aphids per plant and plants are under drought stress, treatment may be justified. Control is also warranted if 3 percent or more of the plants have their tassels and upper leaves heavily infested, plants are under moisture stress, and the population is increasing. The presence of numerous predators and parasites suggests that natural controls may be reducing the number of aphids.

Winged Corn Leaf Aphid
Figure 3. Winged Corn
Leaf Aphid

Aphids are parasitized by some species of wasps and are also susceptible to a fungus disease. Brown swollen aphids, abnormally larger than other aphids in the colony, indicate parasitism. Brown aphids with the bodies collapsed and a moldy appearance indicate a fungus disease. Lady beetles, syrphid fly maggots, and green lacy-wing larvae, or aphid lions, are always abundant in a heavy aphid population. They and several other insects prey upon aphids and help to hold populations in check.