If you find a group of aphids on a plant, you will often find an ant(s) that are “herding” them like cattle. The ants feed on the aphids’ honeydew (a sugar-rich sticky liquid, secreted by the aphids) and will tend and guard their herd of aphids against potential predators.   

Photo by: Bri Wentworth in Missoula, MT on 9/29/21