Northern Plains Integrated Pest Management GuideIPM

Authors: Buyung Hadi, Robert Wright, Janet Knodel, Phillip Glogoza, Mark Boetel, Phillip Sloderbeck, J.P. Michaud, Holly Davis and R. Jeff Whitworth

Identification

The wingless form of corn leaf aphid (Rhopalosiphum maidis (Fitch)) is oval, about 1/16 inch (0.2 mm), blue green in color with black antennae, legs and tailpipes. The nymphs have the same appearance as the wingless adults with smaller size and underdeveloped antennae and tailpipes. The winged form of the aphid is about the same size as the wingless ones, with dark green to black body and black tailpipes.

Corn leaf aphid colony

A close up on a corn leaf aphid nymph

Life Cycle and Seasonal History

The corn leaf aphid survives the winter in the southern states and migrates northward in the summer assisted by southerly winds. On corn in the Northern great plains, the aphids reproduce parthenogenetically; that is the female aphids do not need to mate with male aphids and are able to 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 moist corn whorls until tassel emergence, after which they move downward to the stalks and leaves. Formation of the winged forms of corn leaf aphid is usually observed around tassel emergence and senescence.

Plant Injury and Damage

The corn leaf aphids possess piercing and sucking mouth parts and injure the plants by removing plant sap from the phloem. High populations of corn leaf aphids causes leaf mottling and discoloration; reddening of corn leaves is another sign of feeding. Corn leaf aphid-related injuries are exacerbated under drought stress. The aphid also produce a sticky sugary secretion, called honeydew, that coat the leaf surface. Opportunistic fungi, commonly known as sooty mold fungi, colonize the honeydew producing a black layer of fungal colonies on corn leaf surface and reducing the photosynthetically active portion of the leaves. Corn leaf aphid is also reported to occur on sorghum, wheat, and barley.

Corn leaf aphid acts as a vector of Maize dwarf mosaic virus (MDMV). The symptoms of MDMV infection varies with timing; . early infection results in elongate chlorotic spots on young leaves. These spots later merge into chlorotic streaks along the leaves. These streaks form mosaic or mottled patterns and the leaves may turn mostly yellow later in the season. After periods of cool (60°F or 15.5°C) night temperatures, infected leaves may turn reddish. Plants infected with MDMV are prone to root rot. The aphid transmits MDMV non-persistently, that is the aphid requires a relatively short period of feeding to acquire the virus (a few minutes) and remains infective for 30-45 minutes.. In the field, MDMV can survive the winter in Johnsongrass.

Corn leaf aphid infestation

Chlorotic leaf streaking, a symptom of Maize dwarf mosaic virus infection

Leaves infected by Maize dwarf mosaic virus may turn reddish after periods of cool (60°F or 15.55°C) night temperatures

Management Approaches

Corn leaf aphid is deemed an occasional pest on corn. Late planted corn fields experiencing drought stress face the highest risk of damage from corn leaf aphid.

Scouting and Threshold

The critical period for scouting is around tassel emergence. Economic thresholds are variable with the state (see online resource section below), but generally treatment may be warranted if the plants are under drought stress and more than 50% of corn plants have more than 100 aphids per plant during tassel emergence. Treatment more than 48 hours after tassel emergence is of little benefit. Insecticidal control may not be necessary; if numerous predators (e.g. lady beetles and green lacewings) and high levels of light brown, mummified aphids (caused by parasitic wasps) are spotted in the field during this time.

Cultural Methods

In fields with history of MDMV problem, destruction of Johnsongrass before planting corn may reduce the initial source of inoculum for a given year.

Biological Control

Lady beetles, lacewings, syrphid fly larvae, and parasitic wasps play a major role in regulating aphid colonies on corn. When high levels of natural enemies are present in the field, the aphid population will likely be kept under economic threshold and no insecticidal treatment is necessary.