Parasitic Ant Forces Workers to Kill Their Queen

Scientists have discovered an ant species which uses a technique so devious to start a colony that it could change our understanding of animal social systems.

Beginning a colony can be perilous for a queen ant, as there is a high chance of being eaten by other animals while she is alone and therefore unprotected. Having yet to found a colony and raise her own workers, the lone queen must travel above ground to find a suitable nesting location.

Because of these risks, some ants species have evolved workarounds where, instead of doing the work themselves, they benefit from another ant species' labour .

A new study has found that newborn queens of Lasius orientalis (found across Eurasia) and L. umbratus (also found across Eurasia and possibly the US) have developed a strategy that has not been observed until now. They attack the queens of other species, L. flavus and L. japonicus, by using the queen's own workers to kill her.

To do this, the parasitic queen uses a chemical, which has yet to be identified, although based on what we know about ants it is probably a formic acid-based chemical. Formic acid is a defensive chemical created within the bodies of some groups of ants that don't have a sting (it is found in nettles).

The formic acid can bind with other chemicals produced in various glands within the ant, allowing for a wide range of uses - one of these being as an alert pheromone .

The parasitic queen ant sneaks into a colony of L. flavus or L. japonicus (the hosts) and sprays the queen with this chemical. This changes the scent of the sprayed queen, tricking her workers into believing that she is a threat and causing them to attack and kill her.

This gives room for the parasitic queen ant to take her place as the new queen of the colony, without risking injury by attacking directly. She will then use the old queen's workers to rear her own young until they eventually take over the colony, replacing the host species with her own.

In other insects, workers can gain direct benefit from matricide. For example in bumblebees , only the queen is allowed to reproduce and worker reproduction is suppressed by aggressive encounters (including egg-eating) and pheromones from the queen along with a select number of specialised workers.

When the colony is large enough and there is sufficient food, the queen stops producing female workers and instead switches to producing male drones and new queens (gynes) to begin the next generation.

At this time the workers are also able to produce male eggs as these can be laid without the need to mate. The workers and queen compete to have their own eggs survive, eating each others' eggs. To increase the chances of their own (male) eggs surviving and going on to reproduce, a group of workers will sometimes kill their queen.

This matricide, although incredibly complex in its causes, and still not fully understood , clearly benefits the bumblebee workers. In contrast, the matricide caused by the parasitic queen ant provides no benefit to the original colony or its workers.

This form of matricide opens up new possibilities for scientists studying the use of chemical communications within "eusocial" (highly organised) insects. This chemical trickery may be more common than scientists think. There's still much we don't understand about how ants communicate and the role of different chemicals within this.

Many animals, including humans, benefit from living in social groups, and these social systems can vary widely across animal groups. Matriarchy is a social system in which one female is in charge. It is unusual but appears across animal groups from mammals to insects, and it may have different causes.

Female orcas and elephants live longer than males, accumulating knowledge and passing on behaviour and traditions to younger group members, increasing their chances of survival. The oldest female dominates the close-knit family group, as she can guide them to food when it is scarce.

In spotted hyenas, the young males are forced to leave their own clans and must join other matriarchal clans as the lowest-ranking individuals in a hierarchy dominated by females.

Naked mole rats only have one reproductive female at a time, the queen. As with bees and ants, the presence of the queen suppresses the other females' hormones, which prevents them from breeding.

The role of these females centres on the care of young, alongside other specific roles such as tunnellers and food gatherers. These naked mole rat groups are not without competition, however, with evidence from captive populations that female workers may kill and replace the queen.

Most eusocial insect groups are matriarchal, although termites have both a king and queen . Nearly all ants are eusocial. The main castes are the egg-laying queen, short-lived male drones and sterile female workers.

The workers serve the colony by foraging for food, transporting resources and caring for the eggs of the next generation. Being unable to reproduce, a worker's only route to passing on their genes is through the survival of the colony. This promotes a social structure with high cooperation and low competition, different to the within-colony competition that can happen in naked mole rats.

Normally, when queen ants infiltrate other colonies, they use chemicals to disguise themselves as part of their host colony (chemical mimicry) or to remain undetected (chemical camouflage). Some queen ants sneak into other species' colonies and trick them into rearing her workers, who then go and raid other species' colonies , bringing back pupae which become slave workers.

Other queens, for example Monomorium inquilinum, become permanent social parasites without killing the original queen. These invaders (who use chemicals to camouflage themselves in the colony) depend on the host workers for food, and trick them into raising the invaders' eggs, which are often reproductive young.

Finally, some ants such as Acromyrmex insinuator are temporary social parasites, entering a host colony, killing the queen and taking over the colony until their own workers replace those of the host.

As the new study shows, we may only be starting to understand the complexities of matriarchal animal societies.

Matthew Sparks receives funding from a PhD studentship through an EPSRC UKRI Doctoral Training Partnership between Swansea University and Rentokil Initial under the name 'Characterisation and manipulation of urban light environments for fly control'.

Wendy Harris does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.