Here's the eleventh in a series of articles that will help
you tell friend from foe in your garden.
Part XI: Aphid Biology
by Ron West
Most gardeners would be quick to name aphids as the insects
that give them the most trouble. Few other pests are as
widespread, persistent, and downright annoying as these
small, soft-bodied "plant lice."
Aphids constitute a large family (Aphididae) of the order
Homoptera, a group which also includes the cicadas,
leafhoppers, whiteflies, mealybugs, and scale insects. Like
their kin, aphids feed on plant juices through a tubular
proboscis, which is inserted, hypodermicstyle, into the
plant's tissues. In addition to causing much physical
damage, this method of feeding contributes to the
transmission of many plant diseases, especially when the
homopteran is winged and moves readily from plant to plant.
Aphids often blend in with the color of the host plant or
hide on the underside of leaves, so the damage they inflict
is usually the first clue to their presence. Curled,
mottled leaves are-a sign that aphids (or similar insects)
are at work. In time, infested plants become stunted and
sickly, and a great deal of chlorophyll may be lost. Young,
tender growth is often preferred by aphids, and the damage
done in such instances can be devastating.
The typical aphid hardly needs description. It has a
pear-shaped body one-tenth of an inch or less in length, is
winged or wingless, possesses weak legs, and can be any of
a number of colors. Although these sluggish insects may
seem quite defenseless, they do possess one weapon.
Protruding from the rear of the abdomen are two tubular
structures, the cornicles, which secrete a waxy substance
that deters at least some attackers.
When ants are observed moving up and down a plant or tree,
it's almost a sure bet that aphids are present. Many of
these pests excrete copious amounts of a sweet fluid called
honeydew, which ants relish and readily gather up. In fact,
an ant will even use its antennae to milk an aphid, rubbing
the homopteran's abdomen in order to stimulate the flow of
Indeed, many ants regularly milk their aphid herds and, in
exchange for the sweet treat, provide their six-legged
cattle with a certain degree of protection. The high sugar
content of honeydew also provides an ideal growth medium
for sooty mold, a black fungus that often develops on
heavily infested vegetation. While this fungus does not
attack the leaf, it eventually covers the surface and
hampers many of the plant's vital functions.
One key to the enormous success of aphids is their amazing
reproductive capacity, which is potentially higher than
that of any other insect. It's estimated that a single
aphid could have 5 billion descendants between
spring and fall! Each female produces 50 to 100 offspring,
and there may be over 40 generations annually in mild
climates (and even more in greenhouses). Of course, a wide
variety of predators, parasites, and pathogens keep the
aphid population in check, but not always well enough to
prevent damage to our plants.
I mentioned that a single aphid could have many
progeny. Now, before you conclude that I don't know much
about the birds and the bees, let me explain a fascinating
aspect of aphid reproduction. Female aphids can give birth
without mating-a fact that no doubt enhances their capacity
to produce offspring! Known to scientists as
parthenogenesis, this phenomenon results in a true virgin
birth and in some respects is a natural cloning process.
Parthenogenesis is also common in some scale insects and in
In addition, aphids can give birth to living young, as well
as laying eggs as most insects do. Actually, the process is
somewhat different from the live birth of mammals in that
the aphid merely incubates each egg within its body prior
to giving birth.
Aphids live in densely packed colonies, and the seasonal
life history of the society is often quite complex. While
there are many variations, it typically proceeds something
like this: In the spring, wingless female aphids hatch from
eggs that were laid the previous fall. These insects
reproduce asexually and give live birth for two or more
generations on a specific host plant. In time,
winged females appear, which migrate to a
secondary host plant species, continuing their atypical
reproductive habits. As summer comes to a close, succeeding
generations migrate back to the original host plant
species; only then do the males appear. Mating then takes
place, and the resulting eggs pass the winter on the plant,
thus completing the cycle.
There are many variations on this scenario, depending on
which species is involved and on the climate of the area.
In the mild southern regions, for example, males and eggs
may not occur at all, since the hardy egg stage is
Once we become familiar with an enemy, we are in a much
better position to battle it. Therefore, in the
next issue, we'll be taking a look at some of the
different types of aphids, and at various methods of
controlling these vampires of the vegetable garden.