When ants 'move house'

Above: Lasius flavus workers with a cocoon in a disturbed nest. Picture courtesy of the author.

What happens when an ant colony needs to relocate? Dr Elizabeth Evesham CBiol FRSB explains the fascinating insights gained from over four decades studying ant nests near her home in Dorset

28th July 2021

Of all the various insects, worms, spiders, beetles and other organisms busy beneath my feet as I walk in the British countryside, the creature that fascinates me most is the ant. Walking in my home county of Dorset has led to a life-long obsession with the ecology and behaviour of a particular species of ant, Myrmica rubra, and why and how they relocate their nests.

This small, red-brown, shiny ant is found all over Europe and common enough that you may see them in your garden. In the Dorset meadows I frequent, Myrmica rubra likes to set up home under the large, flat stones that have fallen from derelict drystone walls. Some stones stand proud of the surface and are easy to spot, while others are nestled among long grass or nettles.

The casual turning over of such stones disturbs the creatures underneath and exposes them to predators. But if done very carefully and quickly, it allows me to map the composition, density and geography of nests over a given area before placing the stone back. And by repeating the process annually, I have accumulated many years’ worth of data on how ant nests change over time.

Myrmica rubra, also known as the European fire ant or common red ant.

A Myrmica rubra nest comprises individuals at different stages of their life cycle. If you look closely, you’ll see different coloured workers from pale to dark red – the darker the shade, the older the ant. Pale orange workers are the youngest and their job is to nurse the white grubs of various sizes, known as the brood. As they get older, the ants turn darker and change their role – graduating to become brave forages that move out of the nest to collect food. The ants that appear larger than the workers are queens. Larvae which have the potential to become queens are also much larger than those that will develop into workers or males.

Myrmica rubra are polygynous, meaning they have more than one queen in each colony.

A Myrmica rubra nest under a stone. Photo courtesy of the author.

It isn’t commonly known that Myrmica rubra workers lay eggs, too. These tiny, white eggs are unfertilised and develop into males or are used as a food source. Queens lay eggs that are destined to become either males, queens or workers. Fertilised eggs that are fed mainly on a sugary diet became workers, while those fed on a high protein diet become queens. The queen’s unfertilised eggs develop into males like those laid by the workers.

If you see winged ants in the nest during the warmer, summer months these will be unfertilised queens. You may also see ants that are blackish in colour with a thin body, which are the males. On warm, humid and still evenings, the winged ants leave the nest, an event known as their nuptial flight. Flocks of birds circling around at this time of year is a tell-tale sign that this easy food source has emerged en-masse from the nests below.

Dr Evesham studies a site where recent excavation works have resulted in increased vegetation and potential nest sites.

Back in 1977, my ant mentor Mike Brian noted that the area contained another species of ant as well as Myrmica rubra: Lasius flavus, commonly known as the little meadow ant. This little yellow ant usually builds mounds on hillsides, which become covered in vegetation. Both species still exist here today, but my work has shown that their distribution has changed from random to a more aggregated fashion[1, 2] – and over the years, L. flavus nests have become significantly closer to M. rubra nests. Some even occupy the same nest[3].

This is surprising for two species not only belonging to different genera, but L. flavus in monogynous (just a single queen per nest) and produces cocoons, while M. rubra is polygynous and produces pupae. These ants do, however, have different food sources and it is possible that they can cohabit the same area without conflict[2, 3]. Environmental pressures seem to have caused them to live under one roof. After all, why waste energy finding a suitable area and building another nest, when your existing home has lost its roof, when you can share a home with your neighbour?

A nest in which both M. rubra and L. flavus ants live side by side. Photo courtesy of the author.

Moving day

The movement and relocation of nest following disturbance is difficult to monitor in the field, but a combination of observations over the years and laboratory studies using whole nests have helped us understand how M. rubra colonies move to a new position. The time it takes to move to a new site varies, depending on the composition of the colony and distance to a new site, but generally the ants can pack up and move house within an hour.

First, foragers leave and scout out a new area, return to the colony and communicate the new location to others using antennal contact and odours exuded from their cuticles. Simultaneously, nurses pick up the brood in preparation for movement. The colony, including the queens, tend to cluster together until the workers around the edges enter the cluster, break it up and initiate movement. I have observed, on several occasions, workers pick up queens and move them if they have not moved themselves.

Movement is not a smooth transition, but occurs in pulses of erratic, jerky activity, approximately every 10 minutes, until most of the colony members have relocated. I have also observed[4] similar activity using video recordings. This perhaps helps the colony avoid continuous exposure, predation by birds and disturbance by grazing cattle and humans.

After moving, the entire excavation of the new nest, with tunnels and chambers, takes just 24 hours, with very little further change to the nest occurring after the first day[5]. What is even more remarkable is that M. rubra seem to remember the shape of their nests and often build their new one with similar dimensions to their old[5]. True architects!

Location, location, location

The successful growth of a colony is dependent on a number of factors. Food, climate and competition are probably the three biggest influences. However, land management and other human activities can also have a dramatic impact. In the ant colonies I have been studying, changes in farmland management, trampling by cattle and increased number of tourists, brought about by popular attractions such as water sports and a fossil museum in Kimmeridge village, will have influenced changes in population density of colony members and nest location on the hillsides.

During my studies I have seen grazing cattle kicking the stones on the hillside as they move about, causing them to roll over and exposing the top of the ant nests. There is a public footpath running through the site and it is likely that the vibration caused by ramblers and tourists as well as the cattle has changed the behaviours of the ants lying beneath.

Once a common feature of the Dorset landscape, limestone walls have become derelict or replaced by electric fences in recent years. The lack of stones has meant there are fewer opportunities to establish a colony underneath them. Until a few years ago, the use of herbicide to remove the nettle cover was a common farming practice, making more space for roaming cattle. The nettles were colonised by aphids which, in turn, provided a prime food source for ants.

By mapping the ant nests in a study site at Kimmeridge over many decades, my research has shown that such changes in farming practice can have a significant impact on colony growth [2,3]. Therefore, environmental pressures are likely to have contributed to a disturbance of a stable ecosystem and stimulated colony relocation.

Different ant stages in a lab nest of Myrmica rubra. Photo courtesy of the author.

To move or not to move

The energy flow within a nest is likely to change significantly if disturbed. Energy that would be directed towards feeding larvae and queens, to ensure continued egg laying, is redirected to foragers and nurses to help manage relocation. Fewer resources to feed the brood is likely to affect colony growth. But the presence of brood, whether it is just a few or many, is a key factor in whether a disturbance triggers relocation; nests without broods are far less likely to relocate. (This has also observed in biological control studies in the fire ant Solenopsis invicta). Queen presence also ensures effective relocation, although an increased number of queens (more than two), does not make it any hastier unless worker number is also increased.

Workers will often carry queens and even other workers in an effort to speed up the process of moving. But interestingly, if there are more workers than brood, the efficiency of the moving process can be reduced. The colony appears disorganised, with ants running all over the place. Similar behaviour was found in a study on food distribution[6] where workers got in each other’s way when trying to gain access to larvae. When worker number matches that of larvae, nest relocation occurs at a faster rate.

Therefore, effective and rapid relocation of a nest is dependent on the presence of brood and queens, with a delicate balance between the ratio of queens to workers and larva to workers.

A stressful move

Overall, these long-term studies have shown that direct and indirect disturbance caused by changes in farmland management and the landscape – as well as the physical disturbance by cattle and humans, has led to movement of nests and a reduction in the ant population density[1,6,3].

The relocation of M. rubra colonies requires energy, leaving less to feed larvae and egg-laying queens[2]. This means a reduction in colony growth, leading to an overall decrease in population density. The knock-on effect of this, and the imbalance in the ratio of colony members, such as fewer brood and queens and an ineffective worker force, is a less well-organised colony that is slower to relocate. This leaves the moving ants open to predation or further trampling by cattle and ramblers. Brood care becomes difficult to manage, particularly if food sources become scare and worker numbers low.

There is no doubt that moving house is a stressful experience, but can M. rubra adapt, over time? The answer is yes they can.

The changes in habitat I have witness over many years in Dorset seem to have offered opportunities for the establishment of new nests. Although the population density of ants was once up to ten times higher than it is now, the numbers of nests here has actually increased, albeit each with smaller population sizes of between two hundred to five hundred ants.

A newly laid pipeline, in a shallow trench, has brought stones to the surface which offer sites for new nests. This, together with a large amount of rainfall and warmer weather, perhaps due to climate change, has seen an increase in nettles and shrubs offering further habitats and microhabitats and a lush food supply for the ant populations. The delicate balance of queens to workers to larvae may take time to get right. Although larger colonies might be considered safer, they are less flexible and more difficult to relocate. Therefore, if there is a sudden change in the environment, budding off into smaller nests may be a better strategy for survival.

Norman et al.[7], stated that the plasticity of organisms enables them to respond to environmental changes and that this was fundamental to their survival and evolutionary success. In this study, it would seem that ant M. rubra has adapted by creating smaller colonies and in some cases, shared nests with other species.

Should these ants need to move again, it seems a smaller colony is much easier to move than a larger one. It would be interesting to see what further changes to our climate and landscape might have on the complex but highly sophisticated world of social insects.

Dr Elizabeth Evesham CBiol FRSB is an entomologist and biology teacher from Dorset.

References

1. Evesham, E.J.M. A Field-based study of the Biotic and Abiotic Factors Affecting the Distribution of ant Nests. Proceedings of XIII International Congress I.U.S.S.I. Adelaide, Australia (1998).
2. Evesham, E.J.M. The Effect of Biotic and Abiotic Factors on the Behaviour and Ecology of Myrmica rubra colonies. Proceedings of XVI International Congress I.U.S.S.I. Copenhagen, Denmark (2010).
3. Evesham, E.J.M. The impact of farmland management on the distribution of ant colonies. Proceedings of XVI International Congress I.U.S.S.I. Guaruja, Brazil (2018).
4. Evesham, E. J. M. A video-based study of the role of workers in the movement of queens in the polygynous ant Myrmica rubra L. Entomologia Experimentalis et Applicata 37, 143-148 (1985a).
5. Evesham, E.J.M. A study of nest excavation in the ant Myrmica rubra L. In Biology and Evolution of Social Insects, Leuven Uni. Press, Leuven, 61-67 (1992).
6. Evesham, E. J. M. The interaction of food distribution and caste composition of an ant colony (Myrmica rubra L.). Journal of Zoology London (A) 207, 241-250 (1985).
7. Norman, V.C., Pamminger, T & Hughes, W.O.H. The effects of disturbance threat on leaf-cutting ant colonies: a laboratory study. Insectes Sociaux 64 (1), 75-85 (2017).