Mutualistic relationship beneficial for aphids and ants on giant hogweed (Heracleum mantegazzianum)

Giant hogweed (Heracleum mantegazzianum), a weed originating from the Caucasus and invasive in Western Europe, is frequently observed with myrmecophilic aphids, ants, and non-myrmecophilic aphids. The number of individuals of the two non-myrmecophilic, leaf-sucking aphid species, Paramyzus heraclei and Cavariella theobaldi, are negatively correlated with the growth of giant hogweed in its native habitat. A cavity at the stem basis of giant hogweed, the domatium, lodges colonies of the obligate myrmecophilic, stem-sucking aphid Anuraphis subterranea. We found a positive correlation between relative plant growth, ant activity, and the number of myrmecophilic aphids. Because of the domatium size, A. subterranea populations are limited in growth and consequently the damage they inflict is limited. In contrast to the few other systems where three-partner mutualistic relationships are described, these partners appear to be more adapted to each other. This is the first report of an ant domatium from the temperate zone, and it is moreover the first experimental result presenting a system, from which a secondary domatium is able to evolve, because it is involving initial relations with aphids.