All living things have life cycles, and often these life cycles are wonderfully strange and unexpected. A species' life cycle is simply the series of predictable major events or manners of being characteristic of that 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 during different stages of its life.

The 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.

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 is accomplished without the help of male aphids! In the picture above you can see that some aphids are much smaller than the others. Those small ones have no fathers.They were born from the larger 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. Parthenogenesis occurs in a number of lower animal species.

An "average" aphid life cycle would be something like this:

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 right 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 have 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 to 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 entirely parthenogenetic.

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.