Attack of the killer worms

At first, no one seemed terribly troubled by the mysterious arrival in Britain of a carnivore flatworm from Down Under. Now the alien is taking over, and concern is growing, writes Simon Hadlington

How about this for a nice science fiction alien: slimy-bodied carnivore, comes out at night, takes shelter where it is dark and wet while it lies in wait for its next meal. When it detects a potential victim nearby, it sidles up to it and gently slides on to its body, enveloping it like a long, thin cape. It extends a rudimentary mouth and secretes a lethal cocktail of poison and powerful enzymes that dissolve the flesh of the prey. It sucks up the resulting soup.

How about this for a nice science fiction alien: slimy-bodied carnivore, comes out at night, takes shelter where it is dark and wet while it lies in wait for its next meal. When it detects a potential victim nearby, it sidles up to it and gently slides on to its body, enveloping it like a long, thin cape. It extends a rudimentary mouth and secretes a lethal cocktail of poison and powerful enzymes that dissolve the flesh of the prey. It sucks up the resulting soup.

To reproduce it forms an egg capsule inside its body. The capsule grows big – as much as a fifth of the entire body size. Then, rather than emerging from a purpose-made orifice, this capsule simply bursts out of the body – a kind of auto-Caesarean. The wound soon heals and the egg capsule releases six or seven squirming young. When food is scarce, rather than starve to death our alien decides that the parts of its body that are not strictly necessary for daily survival – its reproductive apparatus, for example – are redundant. These are re-absorbed into the body, which shrinks to a size that just about sustains itself, with a minimum of biological sophistication. It "de-grows". When food reappears, it re-grows and resumes its daily routine.

The thing about this alien is that it is not science fiction. It has set up home in Northern Ireland and parts of Scotland and, to a lesser extent, northern England. It is the New Zealand flatworm, Arthurdendyus triangulatus, and it feeds exclusively on our native earthworms. It's been over here for around 40 years now, and while initial fears that it might wipe out the earthworm population have not been borne out, scientists are nevertheless unsure about its potential as an ecological menace.

The flatworm was first officially recorded in the UK in Northern Ireland and Edinburgh in the 1960s, although there have been anecdotal reports that it was found in the Royal Botanic Gardens in Edinburgh in the 1950s. Dr Hugh Jones, a zoologist at the University of Manchester, has been studying A. triangulatus for several years. "It is typically seven or eight centimetres long, flattish in section, a brown colour and with pale, flecked margins," he says. "It tends to rest in damp, shady areas – under stones or logs. No one knows for sure how it got here, but the most likely explanation is that it was transported in potted plants for the horticultural trade."

For 20 years, little attention was paid to the invader. But gradually it was noticed that A. triangulatus was becoming increasingly common in Northern Ireland and across central Scotland. "Northern Ireland is completely infested, and there is a concentration in central and western Scotland, with some occurrence in the north-west of England," says Dr Jones. "The flatworm appears to like very wet areas, and for this reason it seems likely that its spread will be contained naturally. It cannot tolerate dryness, so is unlikely to migrate very far south or east."

So in parts of the country the flatworm seems here to stay. But does it matter? At the Scottish Crop Research Institute in Dundee, Dr Brian Boag thinks that it might. He and Dr Jones carried out a census of the earthworm population in two fields of grass in the west of Scotland infested with the flatworm. This was the first systematic study to establish the relationship between flatworms and earthworms.

Of the 15 species of earthworm identified, most appeared to be unaffected by the presence of the flatworm. However, two species of deep-burrowing earthworm were badly down in numbers. These are the worms that are important for improving the quality of the earth. Their burrows aerate and drain the soil, and by dragging down leaf mould from the surface they also help to improve the soil's fertility.

These fields were noticeable for their poor drainage – they were boggy and had rushes growing in them. Whether this can be laid at the door of the flatworm – because of the effect on the earthworms – is not clear. But perhaps significantly there was no evidence of moles in these fields. "Speaking to retired farmers it was clear that in the past there were problems with moles," says Dr Boag. "Now it is impossible to find them. It seems the moles have been driven out. If this was due to the flatworms it suggests there could be problems for other animals that feed on earthworms: shrews, hedgehogs, badgers, foxes and birds."

The flatworm population appears to be shifting into grassland. "For the first 10 or 15 years it was recorded only in garden centres and nurseries," says Dr Boag. "Then it spread to domestic gardens and finally on to pasture land. In Northern Ireland in the early 1990s a survey of some 70 farms showed that only 4 per cent were infested; more recently those farms were re-surveyed and the figure had risen to 70 per cent. On the other hand, here in Scotland in the early Nineties around a quarter of garden centres were infested, while the figure now is about 8 per cent," he says.

"The garden centres got their act together and have largely eliminated the flatworm through control measures. The chances of people picking it up from garden centres is much less likely than it was; the most likely form of spread now is from people giving plants from their own gardens to their family or friends or selling them at car boot sales."

Meanwhile, at the University of Edinburgh Dr Derek Cosens and Dr Peter Gibson have carried out a series of studies to uncover fund-amental aspects of the biology of the flatworm. Dr Cosens says, "In the early 1990s we were told that this alien invader was a potential pest that could decimate our earthworm populations. Our interest was to know if the flatworm was a genuine threat and, if so, whether there were any ways in which it might be controlled biologically."

One study set out to discover how far the flatworm could move under its own steam – rather than being transported in the root balls of plants. "We fed flatworms with earthworms dyed with a stain which caused the flatworms to become stained in turn," says Dr Cosens. "We then set the flatworms free and monitored their movements. The flatworms tended to roam from their shelter, a small pit beneath a flat stone, and return to it. They stay where there is a good supply of food. After 30 days the furthest distance that a stained individual was recaptured was only 15 metres. But, we only recovered a small proportion of the 100 flatworms released."

The study also produced an unexpected finding. "While we were looking for our dyed flatworms we found larvae of certain ground and rove beetles with their gut also coloured with the vital stain: they had been feeding on the flatworms. So we had stumbled on natural, indigenous predator-scavengers that feed on the flatworms. While this in no way represents biological control, it does mean that we can advise gardeners that if these beetle larvae are found in the soil to leave them be."

In the meantime Brian Boag is off to New Zealand to check on some of the other 150 species of flatworm over there. "We know that there are other species that eat European earthworms. I am going to carry out tests to find out what sort of conditions they live in so that we can keep an eye out for them in areas over here that are similar."

As for those who have made their home in the UK: "It is here and spreading insidiously. If Northern Ireland is anything to go by it will invade grassland in the west of Scotland and could spread to areas in northern England such as the Pennines and Lake District. But I do not think it will be a major agricultural pest for the majority of Britain."