As mentioned in a previous post, many phloem-feeding hemipterans, such as aphids, take in excess sugar, which they excrete as a substance called honeydew. This sugary substance attracts other sugar feeding insects, which has led to some interesting interactions. Among the most notable is the development of a mutualistic, or symbiotic, relationship between some ants and aphids. Ants may ‘tend’ aphids, drinking the honeydew and protecting the aphids from predators in return. This provides the additional benefit to the aphid of preventing the build up of their sugary excrement which could encourage the growth of fungi and mold.
Ant-hemipteran interactions can be observed almost anywhere if one takes the time to observe closely. I found the carpenter ants above tending aphids on the bushes right outside my front door.
Of course, as in any biological relationship, cheaters exist. Some ants tend aphids without providing any protection against predators. Some aphids manufacture less nutritious honeydew. On the other hand, some ants take their care of aphids to extremes, even building structures to protect the aphids against winter cold.