Invasive Alien Species in Northern Ireland

Artioposthia triangulata, New Zealand flatworm

Artioposthia triangulata
© Roy Anderson
Artioposthia triangulata
Click on map to enlarge
Artioposthia triangulata Dendy
The New Zealand flatworm is a large flatworm native, as the name suggests, to New Zealand. This animal is regarded as a major predator of our native earthworms. However, its impact in agricultural land is unclear as earthworm populations in open fields seem to recover quickly from flatworm predation. There is no doubt that in confined or shaded places such as gardens and small woods, its effect upon earthworm numbers is greater and more permanent. But the long-term effects upon soil quality are unknown.
The New Zealand flatworm is a large strap-like flattened worm with a dark brown or blackish upper surface, and with a pale brown, darker-spotted margin and underside. At rest the flatworm adopts a coiled position under paving slabs, large stones or black plastic lying on the soil’s surface (Figure 2). When moving it can be up to 150mm (6”) long (Figure 1) but at rest is 50-100mm (2-4”) long x 10mm wide. The flattened body comes to a point at each end. The head end is slender and more extendible than the hind end. On close examination a row of black dots is visible running the pale margin. These are light sensitive organs or ‘eyes’ which help the animal to navigate.
Country of origin
The New Zealand flatworm was described from Christchurch on New Zealandís South Island in 1895. It has never been recorded from the rather warmer North Island and its temperature requirements suggest a preference for cool, humid environments.
Current distribution
First recorded outside its country of origin in 1963, from a domestic garden in Northern Ireland. Two years later it was discovered in southern Scotland and has since spread rapidly in both areas. Only in Northern Ireland, however, is it frequently reported from agricultural land. In Britain most records are either for domestic gardens or for garden centres and the main concentration of records is in the area between Glasgow and Edinburgh and in the vicinity of Carlisle in northern England.
Location in Ireland
In the northern part of Ireland the flatworm occurs almost everywhere, including agricultural land, heather moor, woods and wetlands. South of a line from Donegal Bay to Carlingford Lough it is much rarer and more scattered.
As in Britain, sites in the southern part of Ireland tend to be disturbed — domestic gardens or garden centres — rather than the more natural habitats it occupies in northern counties. The difference between the two areas reflects its inability to survive soil temperatures above 23°C. In northern counties soils are cooler and wetter than in the south.
Life cycle
In mature flatworms a whitish bulge is sometimes visible three-quarters of the way down the body. This is a developing egg capsule which, when laid, is dull red in colour and about 1 cm long. The egg ‘shell’ is composed of chitin (insects use this as a body armour) which cures rapidly on exposure to air and turns to a shiny black.
Egg capsules are common in Northern Ireland under stones and other heavy objects in gardens, fields and woods. The capsule hatches after several weeks and one to several small flatworms emerge. These look like paler miniature versions of the adults and have similar feeding habits.
Wildlife and habitat impacts
Earthworms. This is a very efficient predator of native earthworms and has the ability to devastate populations as can be seen in the virtual absence of large earthworms in gardens across Northern Ireland. It is claimed to have a particularly detrimental effect upon the lobworm (Lumbricus terrestris), a favourite of the angler, now very difficult to obtain here.
Despite this, the evidence for serious damage on agricultural land is patchy. However, in many areas the density of earthworms appears to be significantly lower than densities recorded in Britain. However, this could be for reasons unrelated to the presence of the flatworm. Our soils are of a more acidic pH and higher organic matter content than those in Britain and are not ideal for worms. It is said that the common shrew is absent from Ireland because earthworms, on which it feeds, were remarkably scarce before the advent of agriculture and the ploughing up and fertilisation of the prevailing peaty soils.
Soil quality and drainage. Earthworms are known to incorporate surface organic matter into soil by dragging leaves and other surface detritus into their burrows. They also irrigate the soil with the burrows increasing the infiltration and escape of surface water.
It has been suggested that the absence of earthworms to perform these tasks may lead in time to the accumulation of a water-repelling thatch of organic matter on the soilís surface and to poorer infiltration of water into soils via wormholes. Soils with impeded drainage are already very common in Northern Ireland, so will the flatworm make things worse? We donít know the answer to this question yet, but it seems entirely possible.
Bird food. Earthworms also supply protein to the nestlings of a number of bird species. If they are in limited supply might this not affect the breeding success, for example, of some such as the song thrush? The answer seems to be no, but why this is so is unclear.
Song thrushes are doing quite well in Northern Ireland and it may be that their predilection for snails (species of Cepaea and Cornu), which are still plentiful, is the reason. At any rate, there is at present no indication that birds suffer from the poorer earthworm supply.
Human impacts
The flatworm may cause some economic impact via earthworm mortality and poorer land drainage as indicated above. Accurate valuation of the damage is not yet available.
Key vectors
There is no prima facie evidence for the mode of transport used by this pest but it seems likely that the plant trade are inadvertently responsible for its historical and continuing spread.
The original introduction to Ireland coincided with the development of a seasonal trade in daffodil bulbs and roses between New Zealand and Northern Ireland in the 1950s. Most authors (for example, Murchie et al. , 2003) attribute its arrival to inadvertent transport via this trade.
Egg capsules and juveniles can be carried in the root balls of live plants moved around by the plant trade. The eggs are drought resistant and could also survive between the scales of say, daffodil bulbs, during long-distance shipment.
What you can do as an individual
If you encounter the New Zealand flatworm and wish to record the find, report suspected occurrence to CEDaR, Botany Department, Sciences Division, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down BT18 0EU. Tel: 028 9039 5256, email[at], or the author of this article (roy.anderson[at]
Advice to specific groups relevant to this species
None at present.
Management measures
There is at present no effective control for this animal. Its distribution is limited by soil temperature and soil temperatures above 20-24°C are lethal. Its distribution in the British Isles reflects the distribution of cooler and wetter soils.
It is unlikely to become established in open places in southern Britain, for example, where soils are too warm and dry. It may be that it will decline even in Northern Ireland farmland as global warming begins to raise ambient soil temperatures.
Management groups
Research into the biology and potential control measures for flatworms is carried out by the Agri-food Biosciences Institute (AFBI), Newforge Lane, Belfast. Contact: Dr Archie Murchie, Applied Plant Science, AFBI.
Further information
HDRA, the organic organisation:
Scottish Executive:
Dr Archie Murchie, AFBI researcher:
EPPO (European PLant Protection Organisation) import standards for Arthurdendyus:
Biological and ecological studies by SERAD (Scottish Executive RUral Affaitrs Dept.):
Baird, J., Fairweather, I. and Murchie, A. K. (2005). Long-term effects of prey-availability, partnering and temperature on overall egg capsule output of 'New Zealand flatworms', Arthurdendyus triangulatus. Annals of Applied Biology 146 (3): 289-301.
Murchie, A. K., Moore, J. P., Walters, K. F. A., et al. (2003). Invasion of agricultural land by the earthworm predator, Arthurdendyus triangulatus (Dendy). Pedobiologia 47 (5-6): 920-923.
Johns, P. M. and Boag, B. (2003). The spread and distribution of terrestrial planarians (Turbellaria: Tricladida: Geoplanidae) within New Zealand. New Zealand Journal of Ecology 27 (2): 201-206.
Schrader, G., Unger, J.-G. (2003). Plant quarantine as a measure against invasive alien species: The framework of the International Plant Protection Convention and the plant health regulations in the European Union. Biological Invasions 5 (4): 357-364.
Boag, B., Yeates, G. W. (2001). The potential impact of the New Zealand flatworm, a predator of earthworms, in Western Europe. Ecological Applications 11 (5): 1276-1286.
Jones, H. D., Santoro, G., Boag, B., et al. (2001). The diversity of earthworms in 200 Scottish fields and the possible effect of New Zealand land flatworms (Arthurdendyus triangulatus) on earthworm populations. Annals of Applied Biology 139 (1): 75-92.
See the Invasive Species Ireland web site for further information -
Text written by:
Dr Roy Anderson