Why they are both Beneficial & Pest
Ants have an important role in the environment. They are scavengers and collect debris which is then recycled back into the soil. Some plants attract ants specifically so they will protect the plant from insects which may harm it. Ants can also disperse seeds. As part of their scavenging, ants may gather small pests such as caterpillars and other larvae for food to take back to their nests. In the instance of protecting scale and aphids, however, they are very much a pest. Ants love the honey dew which aphids and scale secrete. They are fierce protectors of these insects and will fight off predators allowing the bugs to feed on the plants and continue to produce the honeydew.
Ants actually ‘farm’ the aphids and will carry them from plant to plant which can spread viral diseases. Some ants will take aphid eggs back to their nests for the winter and in spring when new aphids have hatched; they will carry them to plants and continue farming the new generation.
If ants are swarming over your plants there will probably be aphids or scale on the plant.
A by-product from ants protecting the aphids and scale is sooty mould. This a is a black mould that grows on the honeydew which has dripped onto the leaves and stems of plants.
Whilst the mould won’t directly harm the plant, it will interfere with the plants ability to photosynthesise.