To learn that mice, cats and frogs find foreign flatworms too distasteful to eat confirms a general disparagement of the creatures. One shrew, in a laboratory, did manage to tuck in, but shrews must keep eating or die.
Sticky, ribbon-flat, pointed at both ends, the dark-brown Arthurdendyus triangulatus from New Zealand and orangey Australoplana sanguinea from Australia are shapeshifting animals, best inspected on the internet.
They seem to attract much human curiosity. Three of the Australian flatworms were discovered this month in the vegetable garden of Friends of the Irish Environment (FIE) on the Beara Peninsula in west Cork.
A posting of this on the FIE’s Facebook page was viewed more than 17,000 times in four days. Perhaps there was relief from boredom with one plague in sharing the concern about another.
Apart from their over-smooth looks, the flatworms’ chief offence is their appetite for earthworms. My drawing is half of their favourite prey, the large and admirable Lumbricus terrestris earthworm, key architect of the structure and fertility of soil.
The flatworms’ predatory appetite sparked concern when, in the 1960s, the first New Zealand species was found in a Belfast garden, most probably arriving in the soil of imported plants. The adult animals can last a year without food and pot plants can also contain their egg capsules, resembling small blackcurrants.
The global plant trade has also spread this invader to Scotland, the Faroes and the Republic, but its density in Northern Ireland remains exceptional. Scottish records are mainly from city gardens and garden centres but its rapid expansion in the north of Ireland has taken it almost everywhere, across farmland, heather moor, woods and wetlands.
An assessment by Dr Roy Anderson of Queen’s University Belfast noted its ability to devastate garden populations of large earthworms. He found only “patchy” evidence of serious damage on agricultural land but he accepted that, with the loss of earthworm drainage and a surface thatch of uneaten leaves, the North’s problem with impacted soil could grow even worse.
Moreover, systematic surveys of grass fields found the flatworm in 70 per cent of them by 1998, often at the moister margins of pasture. Concern was especially for the toll on lumbricid earthworms, whose wide vertical burrows make it easy for flatworms to pursue, attack and absorb them.
The burrows, topped with fresh casts of earth, bring oxygen and water into the soil, adding greatly to its structure and drainage. They also allow worms to retrieve leaf litter from the surface, digesting and recycling their nutrients. This vital role, first examined by Darwin, is at the heart of flatworm research.
In four years of fieldwork, for example, Dr Archie Murchie and Dr Alan Gordon of the North's Agri-Food and Biosciences Institute identified and weighed more than 25,000 individual earthworms.
Their 2013 report concluded that a spread of less than one flatworm per square metre, matching “natural infestations” in grassland, would cut earthworm biomass by about one-fifth. This, they said, “poses a serious risk to L terrestris, with implications for soil functioning and indigenous earthworm-feeding wildlife”. Among the latter are the blackbirds, thrushes, seagulls, badgers, foxes and pine martens.
The flatworms’ spread in the North is in contrast to sparse occurrence in the Republic, mainly in the disturbed soil of gardens and garden centres. Dr Anderson had pointed to the animal’s intolerance for soil temperatures above 23 degrees, hence its preference for the cooler, moister landscapes of the North.
The Australian flatworm, however, is clearly at home under the rocks, pots, plastic sheets and buckets of the Republic's gardens and plant centres. A map of reports to the National Biodiversity Data Centre shows clusters in city gardens, but also in rural Cork and Waterford. This echoes its chief distribution in the southwest of England.
‘Left with nothing else’
Eye on Nature has already drawn many reports of Australian flatworms and the 17,000 visits to the FIE Facebook resulted in many more. A gardener on the Beara Peninsula wrote of being “left with nothing else”. The FIE has urged more citizen gardeners to search under anything lying on the ground and send their records to the data centre.
In the UK, flatworms in imported pot plants engage the vigorous campaigners of the NGO Buglife, which deplores the lack of biosecurity measures to exclude or check for the animals or their eggs. This may well become a cause for the Irish Wildlife Trust, but devising any practical measures will remain a challenging prospect.
On both islands, gardeners are urged to uncover, seek out or trap them (the FIE offers ways and means). They should also encourage large, predatory ground beetles, such as the dearg-a-daoil, or rove beetle, that find an appetite for sticky-toffee planarians.