Britain has a new farmed animal, which is kept in barns, milked and moved between high and low pastures – but not by humans.

The pale giant oak aphid, Stomaphis wojciechowskii, has lived undiscovered for thousands of years on English oak trees, where it has been looked after by brown ants.

In return for plentiful supplies of honeydew – the sugary water excreted by the aphid – the ants herd the aphids, keeping them safe in “barns” they build on tree trunks from mosses, lichens and the exoskeletons of beetles.

“It’s beautifully complex,” said Matt Shardlow, the chief executive of Buglife. “It is farming – they are milking the animals, moving them from high to low pastures and building shelters for them when there’s not enough protection.”

The giant aphid – its lance-like mouth is up to twice as long as its 5-7mm body – has remained unnoticed for so long because it is mostly nocturnal and so well protected by the brown ant, Lasius brunneus.

If the aphids are disturbed at one of their tree-trunk shelters, the ants immediately evacuate their “flock”, carrying the smallest individuals in their jaws and hustling the larger aphids down to the ants’ underground shelters.

The ants keep the aphids underground during severe weather. In summer, when the sap rises, the ants march the aphids up the tree trunk to ensure they are well-fed and can provide the ants with sweet honeydew.

Britain has two other species of giant aphid but the pale giant oak aphid was not expected to be found here after it was identified as a new species in central and eastern Europe by mitochondrial DNA analysis in 2012.

But photographer and naturalist Julian Hodgson spotted the unusual creature accompanied by several brown ant bodyguards while searching for barkflies in Monks Wood national nature reserve, an ancient woodland near Huntingdon, Cambridgeshire. Specimens were identified by entomologists in Poland and at the Natural History Museum.

Stomaphis wojciechowskii has since been found at five other sites in north-west Cambridgeshire, including Woodwalton Fen and Holme Fen nature reserves.

Further analysis suggests the giant aphid is not a new arrival but diverged from its central European relatives 30,000 years ago.

In the paper reporting its discovery in the British Journal of Entomology and Natural History, Hodgson and his co-authors said it was “short-sighted” that no aphid species had been given formal conservation status or protection in Britain.

Shardlow said further work needed to be done to establish the giant aphid’s distribution but it was likely to be rare because the brown ant upon which it is completely dependent is “nationally notable”.

“These types of giant aphid are all very special and are rarely seen,” said Shardlow. “Their life history is inspiring and we do need more people to study wild aphids so that we can understand which species are threatened and endangered.

“Just because something is an aphid doesn’t mean it can’t be a conservation priority.”