If the plant they are feeding on becomes too crowded, some of them can develop wings and fly off to another plant. Some aphids are plant-specific, preferring a certain plant over another. Roses are particularly susceptible to pink and green potato aphids, as are potatoes. Fruit trees and other vegetable crops bothered by the green peach or spinach aphid. Some live in the soil, sucking sap from the roots of the plants.
Ants like to follow aphids around because they secrete a sweetish substance known as honey dew.
Honey dew not only attracts ants but other diseases, too, like sooty mold. In the garden, a strong spray of water from a hose will help remove aphids from plants with stiff stems. They may not be able to get up off the wet soil. The water should be focused on the undersides of leaves where the aphids congregate. I place my houseplants in the sink and spray them every few days.