Description
The pea aphid is a pea-green, soft-bodied insect about one-sixth
inch long. The adult may be winged or wingless, while the smaller
nymphs, which resemble the adults, are wingless. The nymphs
shed their shins several times in the normal growth process;
these cast-off skins are white and quite noticeable on plants
and on the ground. Aphids produce a sugary, sticky material
called "honey dew," which is visible on plants in
moderate to severe infestations. Alfalfa, clover, and garden
peas are the plants on which the aphids usually feed. |
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on image for larger version
Figure 1. Pea Apids on Alfalfa
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Figure 2. Pea Aphid Adults,
Nymphs, and Mummy
(parasitized) on Pea Plant
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Life
Cycle
Pea aphids spend the winter in both the adult and egg stages.
Although in mild winters the adults may survive in the crown
of the plant or in debris on the ground, during a severe winter
most of the adults die and only the eggs survive. The overwintering
eggs are located in the stems of alfalfa and clover. During
the summer all individuals are females, the males appearing
only in the fall. Each female gives birth to 50 to 100 living
young at the rate of 6 or 7 a day. There are 7 to 20 or more
generations a year. Most of the aphids are wingless, but some
winged forms are usually present. A large number of winged forms
may suddenly appear as a result of overcrowding or climatic
conditions. In this form they fly to nearby fields. |
Damage
Alfalfa and red clover that is severely infested may be stunted
and occasionally may die. The tops of the plants are generally damaged
first. If the damage continues, the leaves may drop from severely
infested plants. Thus both quality and quantity of the hay crop
are affected. Pea aphids can almost always be found in alfalfa and
clover fields each year in the spring. Occasionally, however, severe
infestations do develop. If the infestation occurs early and plant
growth is slow, the plants will be severely stunted and may die.
This aphid is also suspected of transmitting certain virus diseases
of alfalfa.
Control
A fungus that infects the aphids kills them and thus prevents
widespread outbreaks. When the humidity is high and temperatures
are moderate, diseased aphids shrivel and turn.brovn. This disease
may develop into epidemic proportions and almost eliminate the
pest in a short time; if temperatures are low, however, the
disease does not develop. Several kinds of lady beetles and
certain other insects prey on these aphids and consume them
in quantity. There is at least one wasp that parasitizes aphids.
At first the aphid body appears swollen; then the wasp grub
tnside cuts a hole in the back of the aphid to escape. When
the weather does not favor these natural enemies, chemical control
may become necessary. But no control measures should be used
if lady beetles are abundant. |
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on image for larger version
Figure 3. Diseased Pea Aphid on Alfalfa
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Authors:
Susan T. Ratcliffe (sratclif@uiuc.edu)
Michael E. Gray (m-gray4@uiuc.edu)
Kevin L. Steffey (ksteffey@uiuc.edu)
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