Red Wood Ant (Formica rufa): Facts, Mounds and Colonies

What does a red wood ant look like?

Red wood ant workers are large for European ants and look two-toned. The head and thorax, the middle section that carries the legs, are reddish orange, usually with dark patches on top of the head and the front of the thorax. The rear section, called the gaster, is dark brown to black. The jaws are strong and the antennae are elbowed, like those of all ants.

Several closely related wood ants in Europe look almost identical. Formica rufa is the type species of the Formica rufa group, which also includes the hairy wood ant (Formica lugubris), the northern wood ant and Formica polyctena. Telling them apart often requires a microscope to compare the hairs on the head and body. The red wood ant is hairier than Formica polyctena but less hairy than the hairy wood ant.

How big is a red wood ant?

Workers vary in size within the same colony, a feature called polymorphism. Most are 4.5–9 mm (0.18–0.35 in) long. Larger workers tend to forage farther from the nest, while smaller ones often work inside.

Even the largest workers weigh only a few milligrams. Like other ants, they can carry loads several times their own weight, a topic explored in how strong are ants?

Habitat and range

The red wood ant lives in coniferous and mixed forests across Europe and into Asia, from Britain and northern Spain to central Scandinavia and Turkey, and east to around Lake Baikal in Siberia. In Britain it is found mainly in England and Wales; in Scotland its place is largely taken by other wood ants.

It needs warmth as well as trees. Nests are usually built in sunny clearings, along forest edges, rides and paths, or in open woodland where sunlight reaches the ground. Trees provide aphids for food and needles and twigs for building, while sunlight warms the mound.

The mound: a thatched, heated nest

The red wood ant’s nest is one of the most striking sights in a European forest. The ants pile up pine and spruce needles, small twigs, leaf fragments and bits of bark into a dome that is commonly about 1 m (3 ft) high and 1–2 m (3–7 ft) wide. Large old mounds can be larger, and the nest also extends well below ground.

The mound works like a solar collector and a thatched roof. Its sloping surface catches morning sun, while the thatch sheds rain. Workers open and close entrances to control airflow and move the brood (eggs, larvae and pupae) to the warmest chambers. In spring, workers gather in clusters on the sunny surface, warm up, then carry heat into the nest. The decomposing material inside also gives off some heat.

Wood ants also collect lumps of hardened resin from conifers and add them to the nest. Combined with formic acid, the resin helps keep down bacteria and fungi in the crowded nest.

Colonies and queens

A red wood ant colony can contain roughly 100,000 to 400,000 workers. Unlike many ants, colonies often have many queens, sometimes 100 or more, a system called polygyny. New queens may be adopted back into their home colony after mating, so a colony can survive for decades.

Large colonies often spread across several connected mounds linked by trails, forming networks across a forest. New nests can form by budding, when workers and queens walk off to found a daughter nest nearby.

A single newly mated queen cannot start a colony alone. Instead, she enters the nest of a smaller black ant, usually Formica fusca or a close relative, and takes over. The host workers raise her eggs until her own daughters replace them. This is called temporary social parasitism. How long queens and workers live is covered in how long do ants live?

Life cycle and seasons

Like all ants, the red wood ant develops by complete metamorphosis: egg, larva, pupa and adult. Workers care for the brood throughout, feeding the legless larvae with regurgitated liquid food and chewed-up prey. Fully grown larvae spin silk cocoons and pupate inside them. The pale, oval cocoons are often mistaken for “ant eggs”, but they are pupae.

Winged queens and males are raised in spring and early summer. They leave the nest on mating flights, typically on warm days in late spring, although in many colonies mating also takes place on or near the mound itself. Males die soon after mating, while the young queens either return to a colony or seek a host nest to take over.

Wood ants are active from early spring to autumn. As temperatures fall, the colony retreats deep into the lower chambers of the nest, below the frost line, and becomes dormant. In early spring, the first workers emerge to sunbathe on the mound surface, and the queens start laying again once the nest has warmed.

Diet: aphid farmers and hunters

Most of a red wood ant colony’s food is honeydew, the sugary liquid that aphids excrete as they feed on tree sap. Workers climb trees, stroke aphids with their antennae to collect the droplets, and defend the aphids from ladybugs and other predators. In return, the aphids get protection.

Wood ants are also fierce predators and scavengers. Foragers catch caterpillars, beetles, flies, sawfly larvae, spiders and other invertebrates, and carry dead insects home. Trails may stretch about 100 m (330 ft) from the mound. A big colony removes huge numbers of tree-eating insects, which is why some foresters regard wood ants as natural allies.

Behavior and defense

Red wood ants are aggressive and territorial. They patrol their trails, fight neighboring colonies of their own and other species, and in spring may hold battles along territory boundaries.

Like other ants of the subfamily Formicinae, the red wood ant has no working sting. Instead, it bites and then bends its abdomen forward to spray formic acid from a gland at the tip. A disturbed mound can release a sharp, vinegar-like smell. The acid was first isolated in 1671 by the English naturalist John Ray from distilled wood ants, and formic acid takes its name from formica, the Latin word for ant.

Workers communicate mainly with chemical signals, such as trail and alarm pheromones, but wood ants also have good eyesight for an ant and use landmarks and the sun to navigate. Learn more in how do ants communicate?

Predators and threats

Green woodpeckers and black woodpeckers dig into wood ant mounds, especially in winter, and badgers and wild boar also raid nests. Capercaillie and other forest grouse chicks eat many ants. Some birds, such as jays, practice “anting”, letting ants crawl over their feathers, possibly to use the formic acid against parasites.

The main threats to wood ants come from changes to forests. Clear-felling removes the trees that support aphids, while dense, dark plantations and forests left to close over shade out the sunny clearings that nests need. Habitat fragmentation can isolate colonies, and nests are sometimes damaged by people.

Conservation status

The red wood ant is listed as Near Threatened on the IUCN Red List. This listing is based on a 1996 assessment and is considered out of date, but it reflects declines in parts of Europe. Wood ants are protected by law in several countries, including Germany, and conservation programs in Britain and elsewhere manage woodland to keep open, sunny glades for nests.

Ants as a whole remain extraordinarily abundant, as explained in how many ants are there in the world?

Red wood ants and people

Wood ants are important in forest ecology. They prey on pest insects, spread the seeds of plants such as violets and cow-wheat, mix and enrich the soil, and provide food for birds and mammals. Forest managers in parts of Central Europe have even moved colonies to help control tree pests.

Visitors to European forests can watch wood ants safely from a short distance. The busy trails, the clusters of sunbathing workers in spring and the steady stream of ants carrying needles, prey and honeydew make a mound one of the best places to see a whole insect society at work.

In Bulgaria and Turkey, a few live wood ants have traditionally been dropped into warm milk to start yogurt; studies suggest acid and microbes from the ants help the milk ferment. Unlike its tropical relative the leafcutter ant, the red wood ant does not grow fungus, but it is just as much a keystone of its ecosystem.