Corn Leaf Aphid
Rhopalosiphum maidis (Fitch)
Appearance and Life History
The corn leaf aphid sucks sap from plants and deposits a sticky substance called "honeydew." The honeydew can become moldy, giving the tops of the plants a black, sooty appearance. Corn leaf aphids not only infest cultivated grasses, such as corn, but many grassy-type weeds as well.
The corn leaf aphid is a blue-green or gray, soft-bodied, spherical insect about the size of a pinhead [1/16 inch (1.6 mm) in length]. It has approximately 9 generations per year. Female corn leaf aphids do not lay eggs, as do most other insects, but give birth to living young. These young, called nymphs, resemble the adults except that they are smaller and are sexually immature. Adults and nymphs can often 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.
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. Aphids also secrete a sticky, sugary substance, known as honeydew (excrement).
Corn leaf aphids do not generally appear until mid-June or early July. Since they are not cold hardy, they migrate each year into the mid and upper Midwest from southern areas of the USA. It is feasible that some overwinter in the lower Midwest.
Heavily infested corn leaves may wilt, curl, and show yellow patches of discoloration. When tassels and silks are covered with honeydew, the pollination process may be disrupted. Also, excessive aphid feeding within the whorl prior to tassel emergence appears to be directly related to incomplete kernel development and/or barren ears.
Aphids cause the greatest damage while feeding within the whorl, where their presence is not usually apparent. If samples are not taken until the aphids are visible on the exposed surface of plants (usually after tassels have emerged) the greatest damage will already have occurred. Initiate sampling for corn leaf aphids approximately 3 weeks prior to tasseling.
Prior to Tasseling - In each of 5 areas of a field, randomly select 4 plants. Do not select 4 consecutive plants, but walk 10 to 15 paces between the plants to be examined. For each plant to be sampled, carefully pull the whorl upward and out of the plant. Slowly unroll each whorl, counting and recording the number of aphids found. After inspecting the 20 whorls, total the number of aphids found and determine the average number of aphids per plant.
During Pollen Shed - Examine 20 consecutive plants randomly selected in each of 5 areas of the field for aphids and honeydew on the tassels. Calculate the percentage of plants with aphids and honeydew. Also, determine the status of pollen shed and ear pollination.
Corn Insect Control Recommendations: E-series 219-W (PDF)
Prior to Tasseling - Corn leaf aphid control is most effective 2 to 3 weeks prior to tasseling. It is rarely advisable after this period.
During Pollen Shed - Although control is not normally required once the tassels have emerged, on occasion aphids may interfere with pollination and treatment may be warranted. If greater than 50% of the tassels are covered with aphids and their honeydew prior to 50% completion of pollination and the plants are under stress, treatment may be needed if the amount of pollen being shed is insufficient for good pollination. Remember, there is normally an overabundance of pollen produced in a field.
If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.