Dark guest ant  (Anergates atratulus)

The dark guest ant is classified as Vulnerable (VU) on the IUCN Red List (1).

A highly specialised parasite with a remarkable life history, the dark guest ant (Anergates atratulus) not only takes over the nest of another ant species, but also enslaves its workers (3) (4). Male dark guest ants are cream to yellow in colour and similar to pupae in appearance, being wingless and barely able to walk (4). Queen dark guest ants are tiny and winged (5) and normally have an abdomen greatly swollen with eggs when found in the host nest (4). Both the queen and the male dark guest ant are greatly reduced in size in comparison to non-parasitic ant species (3).

The scientific name of this species, Anergates, derives from the Greek words meaning ‘no’ and ‘workers’, referring to the fact that, unlike most other ant species, there are no worker ants (wingless, sterile females). Instead there are just males and gynes (females destined to become queens) (3) (4).

The dark guest ant is native to Western Europe but it has also been accidentally introduced into North America, where it is now found along the Atlantic seaboard. It has had a recent population expansion into central North America, including Colorado (6).

The dark guest ant is found in grassland, heathland, and rocky and coastal areas. It generally prefers habitats with sparse vegetation cover and short, dry, acidic turf (7).

Unlike most ant species, the dark guest ant is not able to produce worker ants, those individuals that are responsible for searching for food and caring for the brood. This means that the only way this particular ant can survive is by enslaving the worker ants of another species, Tetramorium ceaspitum (known as the ‘pavement ant’, due to it frequently nesting in pavement cracks) (3) (5).

The dark guest ant is an extreme example of an ‘inquiline’, that is, an animal that lives in the nest or burrow of another species. A queen dark guest ant, which has just mated, invades a Tetramorium ceaspitum colony and becomes the new queen (3) (4). It is not clear whether the female achieves this position by taking over an old, queenless colony, or by influencing the Tetramorium ceaspitum workers to kill or starve their own queen (8). The loss of the Tetramorium ceaspitum queen means that no more host workers are produced, and because the dark guest ant now relies on these to survive, the dark guest ant colony can only exist for as long as the worker colony does, typically for two to five years. The queen, which is now swollen with eggs, reproduces quickly in order to secure the next generation (8).

The Tetramorium ceaspitum workers now forage for food for the new queen and her brood as if they were their own. These workers are predators and scavengers of animal and plant material, in particular seeds (8).

As the male dark guest ants are wingless, pupa-like and unable to leave the nest to find unrelated females, they mate with their female siblings in the nest (5). After mating, the females fly out of the nest in search of a new Tetramorium ceaspitum nest to invade (9). 

Known threats to the dark guest ant include loss of habitat through agricultural and coastal development. Afforestation and changes in grazing practices which result in vegetation growth also threaten the dark guest ant because it thrives in areas with little vegetation (7).

There are currently no known direct conservation measures in place for the dark guest ant; however, it has been found in nature reserves and indirectly gains protection through this (7).

To learn more about the conservation of ants and other insects see:

• The Xerces Society:
  http://www.xerces.org/
• Buglife:
  http://www.buglife.org.uk/