From KQED's Deep Look at the secret life of aphids:
"Aphids are just experts at reproducing," said Ian Grettenberger an entomologist at the University of California, Davis. "Given the right conditions, they can multiply really quickly and get out of control."
For most of the year female aphids giving live birth to clones that are themselves already pregnant. That means that most of the time, all of the aphids you see in your garden are pregnant females.
When their population gets big enough and the plant they're on starts to wane, aphids start giving birth to a new type of winged clones called alates. Amazingly, the alates are genetically identical to their wingless mothers. The alates take to the air to find new plants to colonize.