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Description
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.
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Figure
1. Corn Leaf Aphid
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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. |
Damage
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.
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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
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Figure
2. Corn Infested with Corn Leaf Aphids
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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.
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Click
on image for larger version
Figure
3. Winged Corn Leaf Aphid
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Control
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. |
Authors:
Susan T. Ratcliffe (sratclif@uiuc.edu)
Michael E. Gray (m-gray4@uiuc.edu)
Kevin L. Steffey (ksteffey@uiuc.edu)
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