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Southern wood ants (Formica rufa)

Where: Most often in coniferous woodland
When: All year round, although the insects are not usually active in the midst of winter
How many: Very numerous
A southern wood ants' nest - a huge, neat conical mound
A southern wood ants' nest - a huge,
neat conical mound

When walking through coniferous woodland in the New Forest, it’s likely that somewhere along the way will be an enormous nest belonging to southern wood ants; particularly in places where the trees are not too close-planted, and sunny glades brighten the otherwise dark, brooding conifer trunks and dense, green foliage.

Although they blend superbly with their surroundings, southern wood ant nests are quite conspicuous, which is hardly surprising considering the above ground portion is a bulky, dome-shaped structure up to 90 centimetres (36 inches) high and 180 centimetres (72 inches) in diameter. And whilst not entirely restricted to areas of conifers, few southern wood ant nests are found in wholly broad-leaved woodlands.

Constructed of twigs, leaf stalks and pine needles, southern wood ant nests are carefully piled high to provide a warm, humid, weather-proof home. They are often placed around a central, often rotted, tree stump, the decayed root system of which provides ready-made underground passageways for the ants.

Locations that offer direct sunlight, such as the edges of woodland rides, are preferred, although the sun is not the only source of warmth, for these enormous structures of organic material act like huge compost heaps, efficiently generating heat of their own.

Potentially sheltering tens of thousands of southern wood ants, interior chambers and galleries, many underground, provide living quarters for the ants, and also space within which eggs will be laid, larvae develop, pupae form and new adults emerge.

Southern wood ants - a flurry of activity
Southern wood ants - a flurry of activity

All are the centre of extremely complex social structures, places where southern wood ants lives are played out, largely out of sight of prying human eyes. Indeed, recommending social ants as an example of individuals working for the greater good, The Book of Proverbs quotes King Solomon thus: ‘Go to the ant, though sluggard; consider her ways, and be wise….’ And that’s absolutely right.

Three southern wood ant castes are present within each nest: queens, of which there may be more than one; males and workers.

Boldly marked in red and black, all are sizable insects, the largest of Britain’s ants - southern wood ant queens measure up to 12 mm (1/2“) in length, whilst workers and males are slightly smaller.

The queens live for up to 15 years, workers sometimes for more than 1 year and males no time at all – they perish shortly after mating! All have closely defined roles - southern wood ant queens produce eggs, winged males fertilise the queens, and wingless workers do exactly what their name suggests: work.

In late-winter or early spring, the southern wood ant's annual cycle begins - workers emerge from hibernation and become active, whilst queens lay a first batch of eggs that hatch into winged males.

The winged males mate with queens during spring-time nuptial flights, after which the queens shed their wings – one mating is all that is ever needed - and, thus flightless, retire to the nest or move away to start a new colony elsewhere. Further batches of eggs are laid, are immediately carried off by worker ants, and placed in underground chambers to hatch into further wingless workers or new queens.

Southern wood ant workers, the most numerous occupants, are part-developed females incapable of proper reproduction whose job it is to collect food, build, clean, maintain and guard the nest, tend the young and feed the queen. They do occasionally lay unfertilized eggs, however, but these are used for food to sustain other members of the colony.

These workers are incredibly active creatures, particularly when the nest is disturbed, frantically milling about, carrying larvae and pupae to safety. At other times, though, vast numbers can be seen moving about the woodland floor in well regimented columns, searching for nesting material or food, or fighting over territorial boundaries. Dense, dark clusters of insects march in procession, radiating out from the nest, snaking along the ground, and at times climbing trunks to hunt on the bark or in the canopy.

Programmed by nature as habitual builders, workers regularly add material to the nest, and reposition existing material, ensuring that damp material from the bottom of the structure or the shaded side, is circulated to warmer, dryer parts above.

And if interactions between themselves were not enough, these complex creatures enjoy a mutually beneficial relationship with a number of aphid species. When gently stroked, the aphids release droplets of honeydew, a food rich in sugars, acids, salts and vitamins, and in return, the ants provide the aphids with a degree of protection from predators.

Live prey or scavenged items are also taken. Vibrations, scent, chemical signals detected by the ant’s incredibly sensitive antennae, and somewhat limited visual senses help the workers locate prey, which, when alive, is stalked and attacked with pincer-like mandibles. Then, should further weaponry be needed, each ant can call upon its very own store of formic acid.

Once subdued or killed, the prey item - caterpillar, other insect, or whatever else has been caught - is carried back in triumph to the nest, sometimes by multiple workers, each willing and able to share the heavier loads.

Southern wood ants: two workers toil together as they carry a twig back to the nest
Southern wood ants: two workers
toil together as they carry a
twig back to the nest

But southern wood ants’ nests are a tempting target for predators of their own. Green woodpeckers, for example, feed on ants, and have a long, specially designed tongue that helps gather up the hapless creatures. Badgers, too, especially when other food is scarce, feast on eggs, larvae and pupae, and can cause considerable damage to a southern wood ants' nest. Ant defence relies on a combination of biting, and squirting formic acid at intruders, but it’s doubtful that either of these would deter badgers or woodpeckers.

Indeed, some birds deliberately annoy the ants, and then daub the formic acid on themselves as a pesticide that guards against tics, mites and other unpleasant, plumage occupants – the process is known as anting.

With the approach of winter, surviving ants – queens and some of the workers – close down the nest’s ventilation hatches to protect the interior from the worst excesses of the weather.

Thus secured ready for hibernation, nests and colonies are able to withstand all but the coldest, roughest conditions. Indeed, it’s often on the southern wood ants’ nest that snow and ice first melts, yielding to the heat generated from within the mound.


References:

The New Forest – A Natural History: Colin R. Tubbs
The Living Countryside - Wood Ants: Forest Colonists (216)
The Countryside Detective: Editor, Brenda Houghton
Insects of Britain and Europe: J. Zahradnik and M. Chvala
Ant Hill Wood: http://www.anthill.org.uk/woodlandants.htm

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