Aphids and ants have a unique symbiotic relationship. As most know, this type of relationship in nature benefits both parties. For the ants, aphids excrete a sugar- rich fluid called honeydew which feeds them. Ants may even tickle the aphids (or milk them) with their antennae to promote excretion. For the aphids, ants provide protection against predators and also make sure they are well fed. Ants attack any threat to the aphid colony including the eggs or larvae of predators. They will also move aphids from one food source to another once a source becomes depleted. If you've ever witnessed aphid damage, you know this happens to food crops as well as other plants such as milkweed (above image) often.
To keep aphid numbers in check, predators such as assassin bugs and ladybugs (or ladybird beetles) are vitally important. These beneficial insects are effective predators of aphids and are a welcome visitor to any garden or crop. Above, a ladybug feeds on Aphis nerii or the oleander aphid on a common milkweed pod.