Description
The mature aphid is about 1/16 inch long, and is either winged
or wingless. It is yellow and has several rows of black or brown
spots on its back and on the underside of its body. The young,
or nymphs, resemble the adults,but are smaller and have no wings.
When magnified, they appear spiny. Usually, spotted alfalfa
aphids are most abundant on the lower part of the plant. |
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on image for larger version
Figure 1. Spotted Alfalfa Aphid on Leaf
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A sugary
secretion of aphids, called honeydew, is often apparent in moderately
to heavily infested fields. A black, sooty mold may develop on the
honeydew. Apparently this aphid feeds almost exclusively on alfalfa
and does not attack any of our other forage crops. However, two
other species of aphids closely resemble it--the yellow clover aphid
on red clover and the sweet clover aphid on sweet clover. These
two do not become established on alfalfa. Therefore, an aphid fitting
this description on alfalfa is likely to be the spotted alfalfa
aphid, while that found on red, alsike, or sweet clover is not the
spotted alfalfa aphid.
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on image for larger version
Figure 2. Spotted Alfalfa Aphids on Alfalfa Stem
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Life
Cycle
Spotted alfalfa aphids winter primarily in the adult stage in
the crown of the alfalfa plant, feeding whenever the weather
permits. They begin to produce young early in the spring. Since
no male spotted alfalfa aphids have been observed in the United
States to date, every spotted alfalfa aphid is assumed to be
a female capable of giving birth to living young, at the rate
of 1 to 6 a day under optimum conditions. |
Each
adult may produce 25 to 100 young in her life span. Depending on
temperatures, it takes 1 to 4 weeks for a nymph to mature; thus,
there may be 15 to 30 generations each year. This pest was first
found in the United States in 1954, spreading since into 32 states.
It flies readily from field to field; it can migrate long distances
in strong winds. This migratory ability may be important to Illinois
alfalfa growers. The aphid apparently did not survive in Illinois
in the winter of 1956-1957; therefore, future infestations evidently
must come from migratory flights.
Damage
In the early stages of an infestation, the plants begin to wilt;
healthy green plants begin to yellow and turn brown. This damage
usually appears first in a small area that gradually enlarges,
causing many plants to die. Thus, old stands are seriously thinned
and the life of the crop is shortened. Death or serious damage
to established stands occurs most often when moisture is scarce.
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on image for larger version
Figure 3. Severe Damage in Alfalfa Field from Spotted
Alfalfa Aphid
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Moderate
to heavy infestations of the spotted aphid seriously reduce hay
quality and yields. New alfalfa seedings are killed rapidly. The
honeydew secreted by the aphids may become sothick that cutter bars
on mowers gum up and have to be washed. Balers may have to be washed
to keep them operating. In severe cases, tractor tires become covered
with the sticky material and must be washed. A sooty mold that develops
on the honeydew seriously affects the quality of the hay. One aphid
per seedling will produce enough young in 7 to 10 lays Lo kill the
tiny plant. An infestation of 20 aphids per stem in old stands of
alfalfa may develop into a very serious infestation within 1 to
2 weeks.
Control
Natural or biological control is quite important. Several species
of lady beetles and their young feed on aphids. It has been said
that a lady beetle is capable of eating a teaspoonful of aphids
in one day. The young of the green lacewing, or aphids lions, as
well as the larvae of flower flies, also feed extensively on aphids.
Several other insects also prey on these pests. Species of wasps,
specific parasites of the spotted alfalfa aphid, have been imported
from the insect's native home and are now becoming established in
the United States. Whether some of our native parasitic insects
will attack this pest remains to be seen. Naturally occurring fungus
diseases of aphids may help to control it. Winter weather, as it
affects the ability of the aphid to overwinter, is important in
determining how numerous this pest will be. Rainstorms during the
growing season may also have an effect. Humidity may be a factor
in Illinois, since the insect appears to thrive best under dry conditions
and moderate temperatures. Unfortunately, these natural factors
are often not able to cope with heavy infestations of this pest
before serious damage or loss of stand occurs. In these cases, chemical
control will be helpful. Since these aphids migrate readily, chemical
control operations must be a community endeavor. In applying chemical
control measures, however, always evaluate the populations of beneficial
insects. If they seem to be numerous, the use of chemicals may be
unwise. On the other hand, if such pests are scarce or absent, chemical
control may be advantageous.
Authors:
Susan T. Ratcliffe (sratclif@uiuc.edu)
Michael E. Gray (m-gray4@uiuc.edu)
Kevin L. Steffey (ksteffey@uiuc.edu)
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