The younger aphids are often on the undersides of new green growth. Any aphids found can be squashed between fingers, or infected plants can be removed. They are ladybugs, lacewings, and wasp parasites. 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.