Corn Leaf Aphid on Field Corn
Rhopalosiphum maidis (Fitch)
The corn leaf aphid is found throughout the United States and southern Canada. This pest is a native species. Though it is abundant from June to November, all phases of its cycle are not fully understood. It is a minor pest in field corn grain production, but is sometimes of major importance in seed production.
Corn leaf aphids feed on sorghum, corn, small grains, and other grasses. Some year's populations are numerous on small grain seedlings in October and cause reduced yields. They also are known vectors of barley yellow dwarf virus.
Corn Leaf Aphid Colony
© D. Shetlar Dennis Calvin - Penn State Univ.
From early July until fall, colonies of corn leaf aphids can be found on or near tassels or whorl leaves in most corn fields. Some fields may have up to 50 per cent plant infestation level, but this is extremely rare. Normal ranges are from zero to possibly 2 or 3 percent. There are no data to verify that corn leaf aphids cause barren stalks. However, it is believed that 30 to 40 percent of heavily infested stalks may be barren.
Periods of dry weather seem to favor increases in aphid numbers and resulting plant damage. Dry conditions add stress to corn plants and also prevent the development of the fungal pathogens that infect and kill the aphids.
The adult females are bluish grey, plump, soft-bodied, and no larger than a pin head. The two cornicles (projections arising from the top rear of the abdomen) are dark, relatively short, and surrounded by a dark basal area. The winged forms have two pairs of delicate transparent wings. The nymphs are similar in appearance to the wingless adult females except smaller in size.
Corn leaf aphids start appearing in Pennsylvania corn fields in early July. It is not known how or where they overwinter but some probably spend the cold weather in small grain fields. It is also probable that a large number migrate into fields where southerly winds prevail.
Egg laying females of this species have never been seen and males rarely are observed. The females, unlike most other insects, do not lay eggs, but produce other insects; do not lay eggs, but produce living young (nymphs). The time required for nymphs to grow to adult females is relatively short (7 to 14 days); there are 8 to 9 generations per season in Pennsylvania.
On corn, the aphids appear in colonies on the upper leaves and tassels. In the colonies, nymphs, adult wingless females, and winged females may be present at the same time. The winged females usually are not present unless the colony is fairly large and somewhat crowded. The aphids disappear from the plants when the leaves start drying or at the onset of cold weather. Their tiny, white, flakey cast skins may remain on the plants for several months.
Corn leaf aphids feed by sucking sap from the upper leaves and tassels. The infested tassels become covered with a sticky substance called honeydew that drips onto the leaves and silks. Pollination probably is affected by honeydew covered silks. Heavily infested leaves and tassels may wilt and turn brown. A few weeks after the initial infestation, plants will have a black, sooty appearance due to a sooty fungus that develops and thrives on the honeydew excreted by the aphids.
Although a few colonies of corn leaf aphids are present in just about every corn field in the state each year, their damage is generally of little economic importance except in seed corn production. Corn leaf aphid populations normally are controlled by environmental factors, several parasitic wasps, a fungal disease, lady beetles, syrphid fly larvae, and lacewing larvae and adults. Corn plants are most frequently injured by the aphids during the late whorl or pollen shed stage. If 50 percent or more of the plants are infested with colonies of over 75 aphids each during these stages, treatment is warranted.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Stanley Gesell, Extension Associate, 1983
Dennis Calvin, Professor
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