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Why Are There So Many Aphids?

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Aphids reproduce rapidly, giving birth to live young.

Aphids reproduce rapidly, giving birth to live young. A group of aphids contains individuals at various stages of development.

Photo: Flickr user Newtonian (CC license)

Aphids thrive by the force of their numbers. When just about every insect predator looks at you as an appetizer, your only chance of survival is to outnumber them. If aphids are good at one thing, it's reproducing.

Consider this factoid from entomologist Stephen A. Marshall: in optimal environmental conditions and lacking any predators, parasites, or disease, a single aphid could produce 600 billion descendants in one season. Just how do these tiny sap suckers multiply so prolifically?

Parthenogenesis – Aphids Reproduce without Mating:

Parthenogenesis, or asexual reproduction, is the first key to an aphid's long family tree. With few exceptions, aphids in spring and summer are all females. The first wingless matriarchs hatch from eggs in early spring, equipped to reproduce without the need for male mates. Within a few weeks, these females produce more females, and soon after that, the third generation arrives. And so on, and so on, and so on. The aphid population expands exponentially without the benefit of a single male.

Viviparity – Aphids Give Birth to Live Young:

The life cycle goes much quicker if you skip a step. Aphid mothers are viviparous, meaning they give birth to live young, rather than laying eggs. Her offspring reach reproductive maturity much sooner since they don't have to sit around waiting to hatch.

Aptery – Aphids Go Wingless Until They Need to Fly:

Most or all of an aphid's life is spent feeding on a host plant. It doesn’t need to go very far, so walking suffices. Producing wings is a protein-intensive task, so aphids wisely conserve their resources and their energy and remain wingless. The aphids do quite well in their apterous state until food resources run low, or the host plant gets so crowded with aphids that the group must disperse.

When Conditions Deteriorate, Aphids Disperse:

High populations, which occur quickly in light of the aphids' prolific reproduction, lead to less than optimal conditions for survival. Once there's too many aphids on a host plant, the aphids begin competing with each other for food. Host plants covered in aphids are rapidly depleted of their sap, and the aphids must move on. Now's the time to grow some wings. Hormones trigger the production of winged aphids, which can then take flight and establish new populations. If everyone's got enough food and elbow room, more will survive.

Sexual Reproduction and Oviparity – Aphids Change Their Reproduction When Needed:

In some circumstances, aphids gain an advantage by switching to more traditional means of reproduction. All would be for naught if the aphids in cold climates just froze to death at year's end. As days become shorter and temperatures fall, aphids begin producing winged females and males. They find suitable mates, and the females lay eggs on perennial host plants. The eggs will carry on the family tree, producing next year's first batch of wingless females.

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