ll living things have life cycles,
and often these life cycles are wonderfully strange and unexpected. A species' life cycle
is the series of predictable major events characteristic of a particular species, from
its first moment of coming into being, until its death
. When considering the life
cycle of an aphid, for instance, we can't avoid focusing on its curious reproduction
strategies and the kinds of host plants it lives on at different stages of its life.
picture at the right shows a bunch of Turnip Aphids, Lipaphis erysimi, on the
bottom of a turnip leaf. Amazingly, this leaf was plucked from my garden and scanned on a
cold January morning just two days after the local temperature had dropped to 14° F
(-10°C). Obviously these critters can endure some cold weather! The largest aphid in the
picture is only about 1/16th of an inch long (2 mm).
The drawing at the left gives a better idea of what a wingless aphid
looks like. Aphids spend most of their lives with their straw-like beaks stuck into leaves
and stems, sucking out sweet plant juices.
As is so often the case with any type of insect, there are many kinds of aphid. Not
only are aphids spread through several genera, there are even entire families of
them. There are Woolly and Gall-making Aphids in the family Eriosomatidae, Pine
and Spruce Aphids in the family Chermidae, Phylloxeran Aphids in the family Phylloxeridae,
and our Turnip Aphids are "typical" aphids in the Aphid Family, the Aphididae,
which includes several thousands of species.
Each of the many aphid species has its own life cycle, but there are some features
uniting nearly all of them. One feature most species share is that they are incredibly
prolific. Wingless adult female aphids can produce 50 to 100 offspring. A newly born aphid
becomes a reproducing adult within about a week and then can produce up to 5 offspring per
day for up to 30 days! The French naturalist Reaumur during the late eighteenth century
calculated that if all the descendants of a single aphid survived during the summer and
were arranged into a French military formation, four abreast, their line would extend for
27,950 miles, which exceeds the circumference of the earth at the equator!
An even more amazing feature of life cycles of most
aphid species is that reproduction during at least part of its life cycle can be
accomplished without the help of male aphids! Babies have no fathers.Young are born
from females without the benefit of sexual reproduction, in a process known
technically as parthenogenesis. When mother aphids reproduce
parthenogenetically, instead of laying eggs they give birth directly to smaller editions
of themselves, which is what the lowest aphid mother is doing in the picture at the right.
As the baby aphid was being born it wiggled its legs vigorously. Parthenogenesis
occurs in a number of lower animal species.
AN AVERAGE APHID LIFE CYCLE
In spring an egg hatches, producing a wingless
female aphid who soon begins parthenogenetically producing new wingless females.
Generation after generation of wingless females survive one another until hot weather
comes or maybe the plant on which they are living dies and then suddenly some of the
females grow wings and fly off. At the left you see what a winged aphid may look like,
though they come in many colors and shapes.
This new generation of female winged aphid very well may at this time find a plant host
of a completely different species from that on which their spring generations
developed. For instance, Green Peach Aphids overwinter as eggs on peach and related trees
but in spring they move to various weeds and agricultural crops, and then still later they
move onto potato crops, only in the fall returning to peach and related trees.
Typically late in the year when it's time to move back to the plant species on which
the aphid overwinters, finally some aphids develop into males as well as females. Sexual
reproduction then takes place and when the mated females return to the winter plant-host
they lay fertilized eggs. Then next spring the females hatch from the eggs and the cycle
begins again, with no males in sight.
Our Turnip Aphids differ from this scenario a little. Since they live in a part of the
country where winters are not so severe, so that an overwintering "egg stage" is
not really needed, reproduction throughout the year is often entirely or nearly
Though aphids look so plump and dumpy that they could never fly far, in fact they can
travel hundreds of miles with the assistance of low-level jet winds.