Aphids are soft bodied, rather sedentary insects that feed on plant sap. At first glance, they appear to be relatively uncomplicated feeding machines, secreting honeydew that attracts ants which then protect them. Examination of their lifestyles, has only recently revealed complexities in their reproductive activities, interactions with ants, and symbiotic relationships with several types of organisms. Many of these complexities are unique to aphids and have boggled the minds of scientists as they have come to light. This article focuses on the reproductive mechanisms of aphids.
“Standard” Methods of Reproduction
Most insects lay eggs and leave them, allowing the offspring to fend for themselves. Aphids sometimes do this. The greatest percentage of insects have fathers, but male honeybees and most aphids do not.
Birds lay fertilized eggs in a nest, the mother incubates the eggs, and the parents feed the chicks. Although aphids sometimes lay eggs, the eggs are never placed in a nest, are never incubated, and the offspring are never fed by the parents.
Humans reproduce when a male inseminates a female, the sperm and egg fuse, and a new individual develops from the fertilized egg. The baby develops inside the mother and she nourishes it after birth with milk. Aphids use the same mechanisms to reproduce, but each mechanism is usually used for an offspring produced in very different manners.
Aphid Reproductive Styles
Aphid reproduction alternates between a sexual phase and an asexual phase. Most aphids are females produced asexually with males (also produced asexually) appearing in the fall to produce winter eggs. When aphids mate, fertilized eggs are produced and the offspring follow the same genetic mixing that other animals do.
Unlike the standard insect method, most aphids are produced from unfertilized eggs – a process called parthenogenesis, and are born alive after a period of development inside the mother's body. The easiest way to introduce the different modes of reproduction is to follow the aphid life history through the year. Although the following scenario is typical of aphids, some aphids use different reproductive cycles.
Spring Aphids
In the spring, fertilized eggs hatch into female aphids that reproduce asexually through the process of parthenogenesis. In less than a week, these females begin to produce live female offspring at the rate of about five per day for about a month. These offspring are pregnant at birth with the next generation of aphids already developing inside their bodies. At first, these aphids are wingless and stay on the plant on which they are produced. As the host plants mature, their food value diminishes and the aphids produce spring winged individuals. These winged aphids fly to nearby host plants and resume the process of producing unwinged females by parthenogenesis. Photos of all forms of aphids can be found by clicking here.
Ants are attracted to colonies of aphids because the aphids produce a sweet honeydew the ants bring back to their nests for storage. These ants often transport unwinged aphids to new host plants which benefits both species of insect. Thus, aphid colonies can disperse under their own power or through the intervention of ants.
Summer Aphids
Changes in plant community composition occurs as the weather becomes hot and dry in summer. The early season host plants wither and die. When the aphids detect these changes, they again produce winged females that fly to new hosts – often plants of entirely different species. When winged aphids settle on new plants, they return to producing unwinged females in the same manner as spring aphids.
Fall Aphids
When daylight shortens in the fall, aphids again produce both parthenogenetic unwinged and sexual winged forms. At this time, the winged forms are of both sexes. Unlike their sisters earlier in the year, autumn winged aphids do not reproduce by parthenogenesis. These aphids fly to the winter host plant, mate, lay fertilized eggs, and die.
Fertilized Eggs
When aphid eggs are fertilized, the female lays the eggs in clusters on the stems of their host plant, and the eggs stay there overwinter. These eggs always produce female offspring. As in most animals, aphid females have a pair of sex chromosomes and males have only one sex chromosome. Scientists call these “Z” chromosomes. It s only in the autumn that females produce eggs with only one Z chromosome, producing male offspring. Thus, a male aphid's sex chromosome complement is Z- and a female's complement is ZZ. Males produce two types of sperm in their gonads: sperm with a Z chromosome and sperm with no Z chromosome. Only the sperm with Z chromosomes survive. When these sperm, carrying Z chromosomes, fertilize eggs, also with Z chromosomes, all the eggs produce ZZ females.
Yearly Life Cycle
The seasonal cycles of aphids alternate between unwinged forms and winged forms, asexual and sexual forms. The first unwinged females of spring produce an asexual line of females that do not produce wings until their hosts become less nourishing. Then winged forms are produced that disperse to plants of other species and return to producing unwinged forms. In summer, winged forms are again produced, again allowing dispersal to new types of plants, with a return to unwinged forms. Finally, in autumn, winged sexual aphids are produced that lay eggs as the asexual summer colonies die off.
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