Red wood ants in Switzerland

Red wood ants live in colonies in nests, or 'anthills' (fig. 1). In Switzerland, there are six species of such ants, five of which live in forests.

• The common red wood ant (Formica rufa) has protruding hair on its back but not on the back of its head. It tends to be found at the edge of fairly low-lying deciduous and coniferous forests. Most nests of this species are monogynous, i.e. they only have one queen. However, there are also polygynous populations, having several queens. Monogynous Formica rufa populations are made up of several hundred thousand individuals, while polygynous populations are larger.
• Like the common red wood ant, the small red wood ant (Formica polyctena) is found in deciduous and coniferous forests. However, unlike the other red wood ants, it hardly has any hair on its back. This species, whose nests are almost always polygynous, forms the largest populations of any wood ants. A large nest may grow up to two metres high and contain over a million worker ants and several thousand queens. They often create many twig nests, meaning that coherent colonies of the same population are formed.
• The two types of mountain wood ants (Formica lugubris and Formica paralugubris) can only be reliably distinguished by genetic analysis. In both species, populations number up to 100,000 workers, usually with several or numerous queens. A colony is usually established by forming a sub-nest. Formica paralugubris can form huge supercolonies with over 1,000 nests.
• In Switzerland, the Scottish or northern wood ant (Formica aquilonia) is only found in the coniferous forests of the Engadine valley, where it is the commonest mountain wood ant. It has shorter hair on its back than the other two mountain wood-ant species. This polygynous species also forms colonies with multiple nests.
• The black-backed meadow ant (Formica pratensis) builds its nests mainly in meadows and at the edges of forests, paths and roads. Its populations may be monogynous or polygynous.

The biology of the various wood-ant species, especially the common and small red wood ant, has been extensively investigated. The queen lays eggs in the nest, and the larvae hatch two weeks later (fig. 2). Over another period of around two weeks, the larvae develop inside the nest through four larval stages and then pupate. After a further two weeks, the adult ants emerge.

Like honeybees, wood ants live in strictly organised colonies made up of different castes, each of them having clearly defined tasks. Every population is centred around one or more queens (fig. 3); indeed, there may be up to several thousand queens, depending on the species.

Insects make up about one third of wood ants' diet, while honeydew accounts for around two thirds. They also eat some plant matter. However, these proportions vary a lot depending on the supply of prey. Insect prey – mainly caterpillars, flies and hemipterans – serves as a source of protein when rearing offspring and for the queens, helping them with egg production. The ants will also feed on larger carrion.

Honeydew provides the main source of energy for the workers. The ants 'milk' aphids (in particular Lachnidae), stroking their abdomens with their antennae to make them excrete more honeydew (fig. 6). The aphids benefit from this by being cleansed of the sticky honeydew, and the wood ants also ward off their predators. A large nest containing a million wood ants needs around 30 kg of insects (equivalent to some 10 million prey) and approximately 500 kg of honeydew per year.

Despite the formic acid they use as a weapon, wood ants have many predators. The most significant of these – at least among arthropods – are other ants. Populations of different species of red wood ants or even the same species have virtually the same requirements. This means that they occupy the same ecological niche and so there is fierce competition for territory or for productive aphid colonies. Many mites, spiders, hoverflies and rove and other beetles are among wood ants' natural predators. Most of these predators catch individual ants rather than endangering a whole population.

Some vertebrates are also major predators of wood ants. In particular, grey-headed, green and black woodpeckers as well as grouse mainly eat ants and may damage ants' nests (fig. 7). Wild boar and badgers may also pose a threat to a colony if they rummage through nests in search of beetle larvae. A damaged nest allows rain to get in, which can significantly weaken the ant population.

When establishing a nest, wood ants favour sunny spots at the edge of forests or by paths and clearings in coniferous, deciduous or mixed forests. They often build the nest over an old tree trunk; it consists of a dome-like mound (also referred to as an 'anthill') above ground and the subterranean nest. The latter may be up to two metres wide and deep. Inside the subterranean nest are the chambers and passages where the brood is kept and moved around. Various materials such as conifer needles, bud scales, twig particles and also locally available foreign material such as pebbles are used to build the mound. If resin particles are available, these are incorporated into the nest. They improve its stability and at the same time have something of an antibacterial effect.

In a functioning ants' nest, the temperature is actively regulated. From around March to October, the ants keep the temperature constant within a relatively narrow range between 25 and 30°C. One vital heat source is solar radiation. In spring there is also active heat transfer, with the first rays of sun giving the ants the chance to warm themselves on the mound's surface (fig. 8). They then retreat inside the nest, where it is still cold, and dissipate the heat around them. To prevent the nest from overheating in summer, the ants construct ventilation shafts from the mound to the inside of the nest, which can be opened or closed as appropriate.

With the onset of cooler temperatures and food becoming more scarce, the population starts preparing for winter with the ants building up their physical reserves. From October, the ants start to prepare the nest for the winter months, sealing the top of the nest with fine particles. The wood-ant population spends the winter with adult workers and queens in torpor in subterranean chambers of the nest, where they are protected against frost.

• Wood ants' subterranean nesting activity results in physical, chemical and biological improvements to the soil. The soil is loosened, mixed with organic matter and enriched with nutrients. Its pH increases by one to two units and its crumb structure improves. More oxygen penetrates into the soil, and rainwater infiltration is facilitated.
• Wood ants also help to disperse the seeds of many European herbaceous and woody plants. This form of seed dispersal is known as myrmecochory. The seeds of specialised plants such as dead-nettles, snowdrops, corydalises and violets have 'elaiosomes', i.e. fleshy structures that are attached to seeds and whose taste is highly appealing to ants. Ants drag the seeds to the nest, biting off the actual seeds (for which they have no use) along the way or once they are back in the nest, leaving only the nutritious elaiosome. In this way, seeds are deposited in new places and can germinate there.
• As mentioned above, wood ants serve as prey for insects, birds and vertebrates. In addition, woodpeckers and jays disturb anthills in order to be sprayed with formic acid or rub their plumage with ants to protect themselves from parasites. Wood ants stimulate aphid colonies to produce more honeydew by stroking them with their antennae (fig. 6), which means that honeybees can collect more of this substance and turn it into honey, thereby also benefitting us humans.
• Wood ants' primary importance, though, is as hunters of insects such as flies and caterpillars (fig. 9). As such, they play a key role in controlling the numbers of potential pests. Millions of prey are fed to the brood and the queens each year. The proportion of insects in wood ants' diet can be well over 90%.

To be able to better estimate the current distribution and abundance of these important insects throughout Switzerland, the team working on the fourth National Forest Inventory (LFI4, 2009–2017) surveyed and measured the anthills of wood ants in 6,357 sample plots forming a systematic grid across Switzerland. They took wood-ant samples from each anthill they found. The species was then determined in a laboratory. The resulting data provide the first systematic overview of the occurrence of red wood ants in Switzerland. However, it must be emphasised that these surveys do not represent a full inventory. The initial results of this survey can be found in an article entitled First Switzerland-wide wood-ant survey (in German).

Based on these data, on cantonal ant inventories and on occasional sightings, WSL scientists modelled the potential distribution of the red wood ant (fig. 10, Formica rufa) and of the four other main species of this group present in Switzerland (Formica polyctena, Formica paralugubris, Formica lugubris and Formica aquilonia). The resulting distribution maps are available on the IFN website. They show where environmental conditions are suitable for wood ants, distinguishing areas where each species is found with great certainty, where an occurrence is possible but not certain, and where the species is unlikely to be present.