Wood Ants – the world beneath your feet

The distinctive thatched domes of wood ant nests are a ubiquitous part of the National Nature Reserve (NNR) landscape, frequently ignored or the cause of mild distress if you unknowingly stand within a foraging route on a woodland track. They may be one of life’s ‘you don’t know what you’ve got till it’s gone’ as despite being locally common across England and Wales there are real concerns about contraction of red wood ant populations. So a timely look at the NNR ant population, as spring approaches and nest occupants rouse from their slumber, seems in order.

The distinctive thatched mounds of the Southern Wood ant can be seen alongside south facing tracks  

The Southern wood ant, Formica rufa, of local repute, is the largest of the 4 true UK red wood ant species, however you would need to travel north or to the Channel Isles to see the other 3 species. UK Wood Ants, the website of the National Wood Ant Steering Group, states they are mostly found in coniferous woodland with few nests seen in broad-leaved woodland, so Yarner Wood would seem to buck the trend with its proliferation of nests.  The Formica rufa species are all recognisable for the large mounds of vegetation they create for communal living – other related wood ants such as the rare Narrow-headed ant, Formica excecta found at Chudleigh Knighton Heath differ in nest type and are not so apparent to the casual observer.

The Southern wood ant Formica rufa, colloquially called horse ants. Biting ants may be referred to as ’emmets’ in the SW and ‘pish-minnies’ in Scotland.

The thatched nests are seen along south facing tracks orientated to gain the maximum benefit from solar energy to warm their nests. The thatch is made from locally collected organic material so East Dartmoor nests have an Atlantic woodland finish of oak/birch vegetation or pine needles within the conifer neighbourhoods of the Bovey Valley. Nests constructed of pine needles benefit from the anti-bacterial properties of pine resin which may advantage those colonies over our broad-leaved thatched residents. Constructed at the correct angle to act as a solar panel the temperature of the nest is well above the surrounding ambient temperature and engineered to be waterproof.

We only see a portion of the nest as the network of chambers and tunnels, like a medieval city, extends underground, invisible to the outside observer.  A window into the city can be seen on warm days when access tunnels are opened on the nest surface to control the internal temperature and humidity. A core temperature of 20 – 25° C can be maintained even under a snow covering and is crucial for the successful functioning of the nest colony. Ants may be seen in early spring basking on the nest surface to warm up before adventuring out to forage for food or returning deep into the nest taking their body heat with them. Colony success depends on workers moving developing broods to different chambers to ensure an optimum temperature is maintained.

The nest functions as a working metropolis with chambers allocated for different purposes and individual ants serving different purposes. A well-established nest can extend to 2 metres in diameter housing up to ¼ million ants with several queens – it is well worth listening to the sound of the city for a few minutes.  Multiple, overlapping generations of the long-lived queens (lifespan can be 15-20yrs) mean that a nest may remain active for decades.  Satellite nests are commonly seen to assist with foraging which may eventually become independent breeding colonies with resident queens.

Nest life follows a seasonal pattern, the queen(s) and a few workers overwinter, relying on their own body fats for sustenance, become active as the temperature increases in spring with the queens laying eggs and workers feeding the queen and young to build up the nest population. The life cycle is typical of invertebrates with eggs, then larvae, which go through several moults before metamorphosis within a cocoon to produce the adult form. Control of the different adult forms that make up the colony lies with the queen – unfertilised eggs produce males and fertilised eggs females. Those females destined to be queens receive extra food but how the colony decides when this is required seems unclear.

Workers will defend the nest aggressively against intruders, as well as strong biting mandibles they spray formic acid from their abdomen up to 12 times their length – the acid contact on human skin can be felt but leaves no lasting damage.  As with our own cities, the nest is home to many other species including approx. 100 specialist ant nest dwellers. Myrmecophiles are organisms that live in association with ants benefiting one or both species, in some instances the association is essential to the survival of one of the species. Some myrmecophiles have useful roles such as nest cleaning like Rose chafer larvae that feed on nest plant debris. Others predate the colony; some rove beetle species using chemical mimicry to move incognito around the nest feeding on the ant larvae. The ants themselves are part of the wider food web providing a food source for the green woodpecker. Some birds use the formic acid spray to repel mites / lice, bathing on the nest surface to agitate the ants.  

Wood ants can carry prey many times their weight and frequently cooperate in teams to increase their lifting capacity

Stand near any active nest and it is easy to see ant foraging routes, wood ants are predatory and obtain protein by catching small insects, primarily for their developing larvae. However, 90% of their diet is honeydew, the excess sugars excreted from aphids feeding on tree sap.  The ants climb into the canopy of suitable forage trees where they milk aphids, at the same time protecting the aphids from predators. A symbiotic relationship where food is obtained from another organism is known as trophobiosis – in this case the aphid is the trophobiont or provider of nourishment. With full stomachs the ants descend back to the nest where the queen and other workers will benefit from the regurgitated high energy drink.   A medium size nest can process 13-16 kg of honeydew a year.

Individual wood ants are diminutive in size but their density in a healthy habitat, 500 per square metre, play a significant role in the woodland ecosystem earning them ‘keystone’ species status – a species which plays a critical role in the structure of the ecological community. There is still much that remains unknown about the processes they drive above and below ground, though research has shown that their actions improve soil condition, recycling and decomposition rates. Wood ant insect predation regulates populations of other insect species up to 150m around each nest, with an average nest consuming 20,000 invertebrate prey annually possibly benefiting tree growth and reducing pests such as the pine looper moth.

A survey on the Southern Wood Ants nest population within Yarner Wood in the 1980s looking at their presence on two plots was repeated 30 years later. The follow-up survey makes some interesting observations and hypotheses. In the initial study (1981-83) large annual fluctuations of active nests were seen with suppressed nest numbers noted at the end of the monitoring period that appear to have persisted into the 2011 study. The limited data available means that it is unknown if this was due to normal cyclical variations in the population. It was hypothesised that the fluctuations may have been due to extensive larch felling at the time of the first study. Removal of mature foraging trees may have caused the active nest colonies to contract and then explode in number as a behavioural survival response before falling again as the population were forced to establish new territories outside the study plots, due to inadequate foraging resources. Whilst specific management is not undertaken for Formica rufa, management on woodland rides and edges for other invertebrates such as butterflies ensure suitable sunny nest sites are sustained in the long term.

For the frequent visitor to the NNR it is not apparent that wood ant populations have declined in range in recent years as presence is frequently under-recorded. The conservation charity Buglife aims to gain a better understanding of their status in Devon through a public participation wildlife survey, Nest Quest, looking at wood ant nest distribution in the county. Nest sightings are easily recorded – go to the Nest Quest page at www.buglife.org.uk/get-involved/surveys/nest-quest/ for further information.   From now onwards is the ideal time to note sightings as you can observe if the nests are active. So on your next local walk look out for some ‘Pish-minnie’ nests – as the Scots would say!

Blog by Linda Corkerton

Useful places for further information: https://www.woodants.org.uk

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