Aphids are tiny (about the size of a pinhead), common garden pests that feed on plant sap. They eat a wide variety of plants and attack all parts of a plant, from the roots to the flowers. Food crops, fruit trees, and ornamental plants can all fall victim to aphid infestation. In large numbers, aphids can become a danger to your garden or farm. They cause yellowing and curling of leaves, deformation of flowers and fruit, and wilting of entire plants. The younger the plant, the more vulnerable it is to irreversible aphid damage, though long-term root infestations can hurt full-grown trees. Aphids also transmit plant viruses, which are dangerous even to established plants.
Aphids can go from a small population to a full-blown infestation quickly because they reproduce rapidly. This is because the majority of their young are produced asexually. Aphids actually employ two different modes of reproduction: asexual and sexual. Because of this, they are considered facultative asexual reproducers rather than obligate asexual reproducers.
Facultative means that they can reproduce sexually as well as asexually while obligate would mean that they could only reproduce asexually. For example, humans and other mammals are obligate sexual reproducers. We cannot reproduce asexually.
One adult female aphid can produce as many as 80 new aphids in a single week via asexual reproduction. Here, we will discuss aphids’ life cycles, including when they reproduce asexually when they reproduce sexually, and why they even need males.
Aphid Life Cycle
When spring begins in temperate climates, aphid eggs start to hatch and the baby aphids emerge and begin feeding. All aphids that hatch from eggs are female. Juvenile aphids are called nymphs and they resemble even tinier adult aphids. As they grow, they molt (shed their skin), passing through around four different size stages before reaching adulthood. In warm weather, it takes around a week for an aphid to grow from nymph to adult, but it can be slower in cooler years.
The hatched aphids are called stem mothers and they produce the next generation of aphids. Aphids produce many generations in a season. Eventually, a stem mother’s host plant becomes overcrowded and some aphids must seek out a new home. Most aphids lack wings, but winged versions are born when there are too many aphids on a plant. It is these winged aphids that set out on their own to colonize a new plant. There, they continue to produce additional wingless nymphs until the new plant becomes overpopulated and winged young are needed. In the late summer, as temperatures begin to drop, the current generation of females finally produces male aphids. At the end of summer, a final generation of females lays eggs. These eggs provide a safe way for aphids to overwinter, ensuring a new generation the following spring. What do aphid eggs look like? They’re tiny, smooth, black ovals.
Some species and populations of aphids, specifically those that live in warm climates, don’t have an egg-laying phase. They simply continue to produce additional female nymphs throughout the year, never needing to overwinter.
Most aphid reproduction is asexual via a process called parthenogenesis. The aphid is one of around 2,000 species thought to perform this type of reproduction, along with some plants, other insects, birds, reptiles, and amphibians. In the process of parthenogenesis, a female produces a live clone of herself instead of laying a fertilized egg. This is why males are not needed until late summer. Females are perfectly capable of keeping the population going on their own until the weather turns cold.
Aphid sex chromosomes are similar to those of humans in that females are defined by having two X chromosomes (they are labeled XX). Unlike in humans, however, males only carry the single X chromosome. There are no Y chromosomes in aphids. Rather than being labeled XY, like human males, male aphids are labeled XO. Throughout the summer, females produce clone after clone, all with two X chromosomes. When a host plant has an overpopulation problem, the expression of other genes is modified to produce winged offspring. These winged aphids then produce wingless nymphs on their new host plant. Finally, as autumn approaches, females give birth to males simply by producing offspring that are missing one of their X chromosomes.
So, why do aphids even need males? Turns out, the males produced late in the summer are crucial for the survival of the population. They’re the reason that there are new aphids in the spring.
Adult aphids cannot survive the winter, but eggs can. Without males, females can’t lay eggs. Once males mature, they mate with mature females.
Male aphid sperm contains an X chromosome. When combined with a female’s X-containing egg, only another female can be produced. This is why only females hatch at the beginning of the spring. The fertilized females lay their eggs on their host plants before the adult aphids die. The eggs overwinter to hatch in the spring and begin the cycle anew.
Aphids have a fascinating life history that has made them an efficient and widely hated pest. They are able to do without males for most of their life cycle by producing multiple successive generations via asexual reproduction. Males are only required for sexual reproduction to make sure the next generation makes it through the winter. This small role is nevertheless critical in preserving the species.