The corn leaf aphid (Rhopalosiphum maidis Fitch) can be problem for Florida sweet corn growers. These aphids usually feed within the whorl of corn, and are most apparent at or just before tassel push. This pest sucks sap from plants and excrete a sticky sugary substance called “honeydew.” The honeydew can become moldy, giving the tops of the plants a black, sooty appearance.
The corn leaf aphid is a blue-green or gray, soft-bodied, spherical insect about the size of a pinhead (roughly 1/16 inch in length). The end of the abdomen and bases of cornicles are not reddish as in birdcherry aphids.
Nymphs resemble adults, except they are smaller and are sexually immature. Adults and nymphs often can be found clustered within the whorls or upper parts of corn plants over isolated or wide areas of a field.
Most corn leaf aphids are wingless. However, as populations increase, some develop delicate, filmy wings. These wings enable them to fly to uninfested plants to start new colonies. Winged females have a black head and thorax and a green abdomen.
Like other insects, aphids shed their skin as they grow. These numerous white-to-gray discarded skins give the appearance of a white mold or ash on leaf surfaces.
Heavily infested corn leaves may wilt, curl, and show yellow patches of discoloration. When tassels and silks are covered with honeydew, the pollination may be disrupted. Excessive feeding results in incomplete kernel development and/or barren ears. Aphids cause the greatest damage while feeding within the whorl where their presence is not usually apparent.
Heavy accumulation of honeydew on husks also can affect the marketability of ears.
The insect also acts as a vector of Maize dwarf mosaic virus.
Survival and Spread
The corn leaf aphid overwinters in the southern states and migrates northward in the summer. Aphids reproduce parthenogenetically — that is the female aphids do not need to mate with male aphids and give birth to live nymphs. The generation time is very short and overlaps. This so-called telescoping of generations enables rapid multiplication of corn leaf aphids in a short period of time. Initially, colonies of corn leaf aphid reproduce rapidly in corn whorls until tassel emergence, after which they move downward to the stalks and leaves. Winged forms known as alates are typically observed around tassel emergence and senescence.
Natural enemies may control these aphids under low pesticide input situations such as silage corn. However, in sweet corn, they appear at a time when pesticide use increases for armyworm, greatly reducing the potential control by predators and parasitoids.
Chemical control of this aphid is more effective when they are exposed on the emerging tassel. Several insecticides are available for corn leaf aphid control in sweet corn. They are more susceptible than birdcherry oat aphids to pesticides frequently used for armyworm control.
Make sure to consult UF/IFAS recommendations for currently labeled insecticides for corn leaf aphid control in Florida sweet corn.