I've noticed it has been looking rather sorry for itself lately, listing towards the fence with its leaves curled up. When I went to straighten it up with a couple of six foot bamboo poles yesterday I noticed that it was crawling with ants and that the leaves were covered with aphids.
By the miracle of wikigooglisation I discovered that this is ant mutualism:
Some species of ants "farm" aphids, protecting them on the plants they eat, eating the honeydew that the aphids release from the terminations of their alimentary canals. This is a "mutualistic relationship".
These "dairying ants" "milk" the aphids by stroking them with their antennae.
Some farming ant species gather and store the aphid eggs in their nests over the winter. In the spring, the ants carry the newly hatched aphids back to the plants. Some species of dairying ants (such as the European yellow meadow ant, Lasius flavus) manage large "herds" of aphids that feed on roots of plants in the ant colony. Queens that are leaving to start a new colony take an aphid egg to found a new herd of underground aphids in the new colony. These farming ants protect the aphids by fighting off aphid predators.
I actually watched an ant chasing off a fly off a leaf this morning; a fascinating miracle of nature. I still need to massacre the SOBs though. Apparently washing with soapy water may do the trick, so I'll try that first rather than going straight to DEFCON 1.