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New Zealand flatworm

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New Zealand flatworm
Once established there are no known ways of eradicating the New Zealand flatworm hence it is very important to stop its spread.

The New Zealand flatworm (Arthurdendyus triangulatus) was first recorded form Scotland in 1965 and is now widely distributed throughout Scotland (Boag et al., 1994). All major islands are infested, for example, Arran, Skye. Harris and Lewis, Coll, Orkney, Shetland and even Fair Isle. It is an obligate predator of our native earthworms and has been shown to have the ability to decrease earthworm populations to below detectable levels in Northern Ireland (Blackshaw, 1990). In New Zealand it is confined to the South Island but there are probably over 100 other species of flatworms in New Zealand many of which a) have not been described, b) belong to the genus Arthurdendyus, c) are in the North Island and better equipped to withstand higher temperatures and therefore, if introduced into the British Isles, could become a greater problem than A. triangulatus.

There is evidence in Scotland the New Zealand flatworm spread form botanic gardens to nurseries and garden centres then to domestic gardens and only more recently to agricultural land.


New Zealand flatworms are usually found during the day, often curled up like a Swiss roll, under pieces of wood, stone or polythene lying on bare earth. They are relatively flat compared with earthworms, are pointed at both ends and covered with a sticky mucus. They can vary in colour but usually have a dark brown upper surface with a lighter beige speckled border which extends to cover the ventral surface. Flatworms can also vary greatly in shape from long and narrow (up to 15 cm) to short and relatively fat. They produce egg capsules which look like small, shiny blackcurrants.


The New Zealand flatworm requires cool damp conditions to survive, it cannot tolerate temperatures below zero or above c.20 C which is probably one of the reasons why they have becoming established in agricultural land in Ireland, north and western Scotland plus the Faroe Islands.

New Zealand flatworms are hermaphrodite with the reproductive opening on the ventral surface. The egg capsules, which can contain between one and fourteen young but usually six to seven, are often white or cherry red when laid. Observed births are usually through a spontaneous caesarean opening on the back of the flatworms which immediately heals over. The egg capsules are laid in summer under refugia similar to those where flatworms are found during the day. Hatching, it is estimated, takes place about one month later, the hatchlings being creamy white but soon turn to the colour of the adults. Probably one egg capsule is laid every seven to 10 days and can weigh 10-15% of the adult weight.

Flatworms generally lose weight in summer but increase in weight in the autumn when earthworms are more numerous. It is thought they feed by anaesthetising the earthworm extruding digestive enzymes and then sucking up the remaining “soup”. If starved they have the ability to remain alive for long periods by “degrowing”, that is, shrinking in size even down to 10% of their body mass until they find another earthworm.

Impact on below ground biodiversity

Research in Scotland has shown that while all native earthworms are eaten by the New Zealand flatworm the large anecic species such as Lumbricus terrestris are particularly at risk since they feed on the soil surface at night (Jones et al., 2001). In Ireland a long term experiment by Murchie and Gordon (2012) have shown L. terrestris numbers reduced by 75% and earthworm biomass down by 20% in infested land. Earthworms, particularly the anecic species have large, vertical burrows which open to the soil surface and therefore help soil drainage. Research has shown that when earthworms are removed then drainage is impeded (Haria et al., 1998).

Impact on above ground vegetation

No research has or is being specifically undertaken to quantify the deleterious impact the New Zealand flatworm might have on vegetation and crop yield but anecdotal evidence from farmers has suggested that there has been an increase in flooding, the establishment of rushes and lower crop yields in areas that are infested with the New Zealand flatworm.

Impact on above ground fauna

Many of our native mammals (badgers, shrews, hedgehogs, moles etc.) and birds rely on earthworms as their major source of food for at least some part of the year but there has been no investigation to quantify the deleterious impact the New Zealand flatworm has on their populations. However, observations in an area between Dunoon and Loch Eck (where 55 of 59 fields were found to be infested with flatworms) show that no moles can now be found even though they had been a problem there after World War II. Similarly in Glen Massan flatworms were found in seven fields none of which had moles while in another seven fields there were moles but no flatworms. Since about 70% of grass fields without flatworms in western Scotland have moles it would seem that in the area south of Loch Eck flatworms have been responsible for the disappearance of moles (Boag, 2000).


Once established there are no known ways of eradicating the New Zealand flatworm hence it is very important to stop its spread. Initially garden centres and nurseries were probably mainly responsible for the distributing flatworms but now it could be neighbours, relatives and friends exchanging potted plants. Gardeners should inspect any containerised plants and if they have the flatworm try to only give away cuttings, seed or bare rooted plants replanted in a known flatworm free medium.


The New Zealand flatworm continues to spread in Scotland but its detrimental impact on the environment and of below and above ground biodiversity is unknown. At present there is no systematic monitoring or research being undertaken to know what the situation or how it might be controlled.

Reporting New Zealand flatworm sightings

Gardeners are invited to report sightings of New Zealand flatworms (date, number of flatworms and postcode location) during the summer months. In the winter months live samples of flatworm can also be posted to the Institute in a sealed plastic container (for example an old-style photo film canister) along with the same reporting information.

Samples should be marked for the attention of Dr Brian Boag, The James Hutton Institute, Invergowrie, Dundee DD2 5DA and sightings can also be reported to the same address or by email to


  • Blackshaw R.P. 1990. Annals of Applied Biology 116, 689-694.
  • Boag B. 2000. Aspects of Applied Biology 62, 79-84.
  • Boag B. et al. 1994. Annals of Applied Biology 124,165-171.
  • Haria A. H. et al. 1998. Science of the Total Environment 215, 259-265.
  • Murchie A. K. and Gordon A. W. 2012. Biological Invasions 8, 1301-1316.
  • Jones H.D. et al., 2001. Annals of Applied Biology 139, 75-92.


Areas of Interest

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