Ants will protect and actually harvest aphids to serve their food supply. The ants eat the sticky, white, sweet substance secreted by the aphids and so the aphids are literally harvested by the ants and protected for the food they offer. Ants will collect aphids and put them under leaves for future food storage use.
One gardener I recently talked to found a clever way to prevent ants from taking their aphid harvest up into his cherry trees. He put a non-drying adhesive or sealing compoundaround the base of the trunks of his trees. The ants would get stuck in the compound and not go any further, however, ladybugs, who feed on aphids were not disturbed by the sticky barrier. The ants would drop their aphid cargo at the base of the trees and the ladybugs would eat them up.