Defensive mutualisms are common in nature and are readily found in ant-plant associations. Typically these associations involve the plant offering the ants food directly (carbohydrate exudates) in exchange for protective services. These relationships are constantly evolving and should be thought of as "mutual exploitation" rather than casual friendly relations. Plants will skip out on offering valued resources if there was no benefit and vice-versa.
Sometimes, other insects (Hemipterans) harm the plant by sucking plant phloem but excrete honeydew which attract ants. Renowned biologist Janzen (1979) proposed that plants that haven't yet evolved carb. exudates directly to offer ants, will tolerate these harmful bugs as they attract ants that scare away much more damaging insect herbivores. Janzen proposed the maintenance of the herds of scale insects and other honeydew exuding homoptera is more or less a fixed cost for the plant, the equivalent of maintaining secondary defensive compounds and alkaloids (think of poison oak). The ants protect the trees from herbivores, vines, and mammalian browsers. A removal experiment supported as much. Below is a video of a native ant Tapinoma sessile interacting with aphids. Visible in the movie are aphid "mummies" or aphids that have been destroyed by parasitoids (insects that live inside the host and eventually kill the host - like in that movie Alien). The parasitoid was most likely a wasp that had oviposited its egg into the living aphid. The mature wasp emerged and left behind the hollow mummy.
