Wood ants play a very important role in the Caledonian Forest ecosystem and are excellent indicators of undisturbed pine woodlands.
(Formica aquilonia, Formica lugubris, Formica exsecta, Formica sanguinea)
Wood ants of the genus Formica are widely distributed in Europe and Asia, with both Formica aquilonia and F.lugubris occurring from Scandinavia in the north to Bulgaria and Italy in the south, and from the UK eastwards through France and Germany to Russia, while they are also found in the coastal areas of the Sea of Okhotsk in eastern Siberia. F. exsecta is localised but widely distributed in Europe. F. sanguineaoccurs from central Europe northwards, as well as in Russia, and in the east, in northern Japan, northeast China and the Korean peninsula. They live mainly in coniferous forests but they do also occur in some deciduous woodlands.
Formica aquilonia occurs in the pinewoods of the Caledonian Forest throughout the Highlands. It has also been recorded in the birchwoods of Inverpolly National Nature Reserve and on the Isle of Skye off the west coast. F.lugubris has a similar distribution, although it is not known on the west coast. Formica exsecta is one of the rarest ant species and is known in Scotland only from the pinewoods of the Caledonian Forest around Loch Morlich in the Cairngorm area. The only other British population occurs in Devon. Formica sanguinea is also rare in the Highlands and is entirely dependent on old, native pinewoods. At present it is known from three locations in the northern Highlands - Glen Affric, Abernethy and Migdale Woods.
Both Formica aquilonia and F. lugubris are included on the International Union for the Conservation of Nature's (IUCN) Red List of threatened species, where they are classified as Lower Risk. F.exsecta is included on the UK Red List of threatened species, where it is classified as Category 1, Endangered - the most critical conservation status in the country. However, internationally, this species is not considered to be threatened.
Because of concerns about their future here, F.aquilonia, F.lugubris and F.exsecta are the subject of Species Action Plans, as part of the UK's strategy for protecting biological diversity.
All the wood ants have a distinctive red and black covering. With practice the four species can be distinguished by their head features. They are the largest of Britain's ants, with the queens measuring about 12 mm in length. The other two castes found in the wood ant colony, workers and males, are slightly smaller. The queens may live for fifteen years or more, the workers about one year and the males are very short lived, dying after their mating flight in the spring.
Wood ants are social insects and live in colonies which can number up to half a million individuals, though in Scotland colonies rarely number more than 100, 000. Most of the individuals are worker ants. The workers are all female and as they are not fully developed, do not reproduce, though they do occasionally lay eggs which are used as food. As the name implies, they do all the work in the colony, maintaining the nest and tending the queen and her brood. The queen is the only ant that lays eggs that are reared to adulthood. The males do not work, and their sole purpose is to mate with the queen.
Wood ant nests are a common sight in some parts of the Caledonian Forest. They consist of a characteristic dome-shaped pile of pine needles that can be over one metre high and two metres in diameter. Formica lugubris and F. aquilonia build large mounds, though the nest of the former tends to be larger, flatter and more in the open. F. exsecta nests are smaller, typically 25 cm at the base.The nest provides protection from predators and the elements, as well as a steady environment in which to rear eggs and pupae. The nest temperature and humidity varies very little in comparison to the surrounding soil and the ants maintain this in several clever ways. The needles are not placed randomly but are very carefully maintained by the workers to make the dome like a thatched roof, very efficient at shedding water. If examined closely it can be seen that the dome shape is usually not symmetrical; the southern side of the nest is flatter to present a greater surface area to the midday sun and so absorb the maximum amount of solar energy. Simple actions of opening and closing entrance holes control heat, but more sophisticated methods are used by the workers acting as storage heaters. They will 'sunbathe' and then go to the nest brood chambers to release the excess heat. Often the nest is built over a decaying tree stump which, along with other decaying organic matter, provides heat. Damp nest material is carried to the surface from the lowest part of the nest to dry out.
The slavemaker ant (Formica sanguinea) earns the name partly from the behaviour of the queen. Once fertilised, she is known to enter a nest of another wood ant species, kill the queen and appropriate the workers who tend her and her brood. Eventually the original wood ant species will die out in the nest as their original queen is no longer there to lay the eggs to replace them. F.sanguinea workers are also slavemakers, with a ferocious reputation. In the summer months they will surround and raid nearby nests of other ants species, pillaging the larvae and pupae, taking them back to their own nest to raise them to be auxiliary slave workers.
Wood ant workers spend much of their time collecting food either by hunting or scavenging dead insects. In the morning, columns of ants can be seen setting out on a half dozen or so regular trails that radiate from the nest. They are essentially predators who detect their prey by vibration and scent, though they can see up to about 10 cm. Small invertebrates are stealthily stalked and then suddenly pounced upon whereas larger victims will be subdued with the aid of nearby fellow workers. As well as pincer-like mandibles, wood ants have another formidable way of overcoming their victims. In the bulbous end section of their bodies (the gaster) they have glands that contain formic acid which can be sprayed accurately to a distance of 5 cm. This behaviour is used not only help to subdue prey, but also as a defensive measure when threatened. Each foraging ant brings back about one and a half times its own weight in food each day.
Wood ant tracks are not only on the ground, but are three dimensional, leading up trees into the canopy above. The ants not only hunt in the trees, they are known to farm as well. Wood ants have a symbiotic relationship with aphids (for example Symydobius oblongus). The aphids are `milked' by the ants who gently stroke them to get them to release droplets of honeydew, a food that is rich in sugars, acids, salts and vitamins. To the aphids this is their waste product as they have to suck a lot of tree sap to get the protein they require. In return for the honeydew the ants protect their precious sugar source from predators and competing sap sucking insects.
Within the nest, running into the ground, are a series of chambers and galleries which vary slightly in temperature and humidity. The workers constantly fuss over the eggs, larvae and pupae, moving them around the nest to keep them in the most favourable growing environment. Eggs hatch into helpless larvae that totally rely on liquid food provided by the workers. They moult several times as they outgrow their skin, eventually going into a period of pupation within a cocoon when they undergo complete metamorphosis, emerging as adults.
In the spring winged sexuals are raised - queens and males. On calm, balmy evenings in May these sexuals take to the air to mate. During this nuptial flight one mating provides enough sperm for the queen's lifetime. Hundreds of ants swarm at once but many perish, being eaten by birds and other predators. The fertilised queens of Formica aquilonia, F. lugubris and F. exsecta, usually return to their home nest or a neighbouring one and at this point shed their wings. A nest may contain many queens. A newly mated queen may stay in the nest where she was raised or establish a new colony nearby with some loyal workers. Often the daughter colony is very closely connected to the mother nest by trails both above ground and underground.
Wood ants have fascinating associations with other animals at various levels, from having guests, to parasites, to scavengers that live in the nest, ignored by the ants. For example, the beetle, Lomechusa strumosa, is a guest in the Formica sanguinea nest and is treated more or less as part of the colony, being given food and protection. The worm, Dendrodrilus rubidus, is found more readily in the nest of Formica aquilonia than in the surrounding soil, as conditions within the nest are very worm friendly, with an abundant food supply. As the worms help prevent the mounds from becoming overgrown with moulds and fungi, this is a mutually beneficial relationship for ants and worms. Outside the nest, relationships continue as exemplified by the farming of aphids described above. At the opposite extreme the capercaillie can completely exploit and destroy the nest if it is in need of a dust bath.
Wood ants play a very important role in the ecosystem of the Caledonian Forest. Where ants have been removed by forest practices, many herbivorous insects can reach numbers out of balance with the rest of the ecosystem and so become damaging to forest trees. The ants are known to predate upon caterpillars of species such as the sawfly (Neodiprion sertifer) and pine looper moth (Bupalus piniaria) that eat Scots pine needles. But the truth is, little is known about them and the extent of their importance. This however is being rectified with several organisations and universities undertaking extensive research to aid future conservation measures. Formica aqulionia, F. lugubris and F. exsecta are intolerant of modern forestry practices such as underplanting with non-native trees in native woods (creates too much shade), clear felling, insecticides, habitat fragmentation, overgrazing and disturbance by people poking at their nests. They are excellent indicators of undisturbed pine woodland and old birch woodland as they require woods with a diverse and open structure that provide a balance between surrounding trees and sunny glades.