Beneficial parasites can be purchased and introduced to the garden. Aphids are actively spread from plant to plant by ants. Ants actually cultivate aphids and milk tha aphids for their secreted honeydew. This behavior has facinated scientists for centuries. Charles Darwin reportedly tried to duplicate the ants milking motions with a small brush, but was unsuccessful.
Aphids can reproduce by asexual production.
A female can produce as many as 100 offspring without mating, and those offspring are all female. If you ever see and aphid in your garden drop everything and handle the problem immediately. In my own experience, when aphids are left alone in a hydroponic garden, it will take about five days before they are completely out of control, and the whole garden will have to be destroyed. Aphids are attracted to nitrogen rich and young plant parts. Their growth can be controlled to some extent by reducing nitrogen in the hydroponic nutrient.