Nature 2019 October

Ants In Your Pants

I guess many of us have, at some time or other and found out too late that we are sitting on an ants’ nest…

Within the UK there are around 60 species of ant. Their social nature has provided plenty of opportunities to draw contrasts, both positive and negative, with human civilisations. In my experience one of the most fascinating groups are the wood ants that create nests of neatly incised, thatched mounds of twigs, pine needles, moss and grass. Large colonies can comprise over 250,000 individuals, around the population of Swansea. Wood ants are described as a ‘keystone species’ as they play a critical role in sustaining the local ecosystem. They have a prime influence over the survival and success of other plants and animals. Firstly, tree growth is affected by aphids living up to 30 metres in the tree canopy, which the ants ‘milk’ for the honeydew they produce, which amounts to 90% of their diet. As with all good farmers they protect their livestock from predators, hunting for other invertebrates that may prey on aphids but also cause damage to trees, such as caterpillars, beetles and other ants. Because wood ant colonies provide this natural means of pest control in plantations, they are encouraged by those who manage the woodland. Secondly, as the detritus from food waste and dead ants in the nest breaks down, nutrients are released to fertilise the soil. Next, ants are a valuable source of protein for foxes, badgers and birds, such as the green woodpecker. Finally, as part of their foraging activities, ants distribute plant seeds.

If you have ever disturbed an ants’ nest you will not have long to wait for the acerbic smell of formic acid to be released when the ants wave their ‘gasters’ in the air. They can project it twelve times their own body length. Some birds, for example, the robin, jay, blackbird and green woodpecker are four of over 200 species of birds reported to either ‘bathe’ on an ants’ nest or carefully pick up an ant in its beak and as formic acid is ejected, direct the abdomen towards their wing or body feathers. Either way, it is presumed that the formic acid aerosol provides the birds with a method of removing or repelling parasites such as lice and mites.

The wondrous treasures of wood ant nests have to offer the enquiring mind don’t stop with chemical warfare. The wood ant mounds provide a habitat for unique species of invertebrates not found elsewhere.

The first, surprisingly, is another ant which goes by the name of the guest ant! Much smaller that the wood ant it excavates its own nest inside the wood ant mound and when the wood ants move to a new site it ‘packs its bags’ and follows along. If the health of the wood ant community deteriorates then the guest ant packs its bags and leaves along with its eggs and brood. Second, a leaf beetle known by its Latin name, Clytra, rests on a branch above the wood ant mound and drops its eggs onto the nest. The larvae of the beetle crawl over the surface foraging for scraps left by the ant. Next you may be familiar with the cockchafer, or ‘May Bug’ which bumble into outside lights at night in late Spring. The larvae of a relative, the rose chafer, are often found in wood ant nests, where they may remain for up to three years. The adult has striking metallic green wing cases, and feeds on garden flowers. Like the guest ant earlier, there is a singular woodlouse, with no eyes, which lives in complete darkness in the rubbish dumps of the ant nest, feeding off droppings and waste detritus dumped there by the ants. In all the examples of co-habitation above there is a peaceful accommodation by the ants of these interlopers.

But there are examples of insects that invade and are undetected by the normally diligent soldier ants that guard the internal passages. Rove beetles are aggressive carnivores and this species devour on the ant brood larvae. To avoid detection the beetles secrete a sweet chemical disguising the beetles which ants also cannot resist. By distracting the ants the beetles can feed unmolested.

We humans might argue that we are the most powerful organisms on the planet. True we are the only species capable of causing our own extinction, and are working damn hard to achieve this as well as extinguishing many other species in the process. However, this perceived human superiority is far from unassailable. There may be over 7.7 billion (10 with 9 zeros) humans on the planet but on the other hand there are 10 quintillion (10 with 15 zeros) insects. Looking at it another way, for every human in the UK there are on average 17 million flies. So insects, in comparison, could be considered indestructible. In many cases there are close historical associations between insects and humans which, from time to time, have posed a serious threat to our species’ survival. Worldwide, insects are the most common vector, i.e. the carrier or unwitting host of; single-cell micro-organisms, bacteria and sometimes viruses. Such organisms carried by insects are the main cause of deaths in epidemic proportions. Plagues (bubonic, septicemic or pneumonic) have been responsible for most of the regional and worldwide epidemics that have wiped out millions, probably billions, of humans. The Black Death which swept from China across Europe arriving in England in 1348 eventually accounted for between 40-60% of the English population, some 3.5 to 4 million people. Fleas feed off the blood of their hosts and over millennia different species have formed bespoke relationships with a particular animal host, including several with humans. Today, we generally take strenuous steps to keep infestation by fleas at bay but in earlier times the association of fleas with humans might have been at least tolerated if not always welcomed. Today we know the bacteria that lurks inside the flea rightly gets the blame for the cause of the epidemics and colossal numbers of deaths. However, without the involvement of the flea in this case, and not any old humble rat flea, but the oriental rat flea, bacteria would not have been able to wreak havoc on the scale it did. Back in the 14th century the cause of the plague would have remained a mystery.

It’s not just the flea that enjoys its association with humans other pests such as; lice, ticks, mites, cockroaches and mosquitoes and many others all have adapted to live and benefit from a close association with us. Meanwhile, we have also formed equally close relationships with animals that provide both us and them with mutual benefits, like cats, dogs and horses.

In common with the wood ant, we humans also evolved to live in complex, social communities and so it is perhaps not a surprise that similarly both ants and us have, for good or evil, highly developed and sophisticated inter-relationships with other animals. Something to think about when you next sit on an ants nest and find yourself unintentionally interacting closely with them!