New Zealand Flatworm

Fast Facts

Latin name: Arthurdendyus triangulatus

Notable feature: Invasive Non-Native Species (INNS) in the UK

Conservation Status: Not Evaluated

Where in the UK: This Invasive Non-Native Species (INNS) is found across the UK; records from Scotland, Northern Ireland and Northern England

New Zealand Flatworm (Arthurdendyus triangulatus) © M.Whittaker


The New Zealand Flatworm (Arthurdendyus triangulatus) is a non-native species, originating from, as the name suggests, New Zealand; it is considered invasive as it is spreading rapidly and its presence is impacting our native wildlife here in the UK.

The New Zealand Flatworm ordinarily has a dark brown body, with pale, spotted margins and a pale underside; this flatworm can measure up to 20cm in length and about 1cm wide.  The New Zealand Flatworm rests in a large flat coil.

The New Zealand Flatworm is listed as a species of concern.

This flatworm was first recorded in the UK in the early to mid 1960’s.

    • Size: 5-20cm in length
    • Life span: Undetermined
    • Diet: The New Zealand Flatworm feeds almost exclusively on earthworms (such as the Lob Worm aka Common Earthworm), in some cases, reducing earthworm populations by 20%.
    • Reproduction: Reproduces mostly by laying shiny black egg capsules about the size of a Blackcurrant, mostly in the spring; 5-8 juveniles hatch after 2-3months.  Unlike other flatworms, it is unlikely that they could reproduce by fission/fragmentation
    • When to see: Found year round in dark, damp situations such as in soil, leaf litter, at the bases of plants, or under logs, stones, plant pots and other objects.
    • Population Trend:  Increasing numbers of reports across the UK
    • Threats: There are no recorded natural enemies and no biological or pesticide control methods for non-native flatworms, the key control measure is to prevent their introduction to new areas.
    • Fun Fact:  It is an offence under the Wildlife and Countryside Act to knowingly distribute this species.
    • Bonus (not so) Fun Fact: Estimated costs to agricultural industry in Scotland alone is £10million per year.

As with other flatworm species the New Zealand Flatworm is likely to be distributed via the movement of plants and growing media in the horticultural trade.

How you can help: 

Buglife is working to increase awareness of Invasive Non-Native Species (INNS) and their, often detrimental, impacts on our native wildlife and habits.  Working with other organisations to highlight how biosecurity is everyone’s responsibility and that we can all play our part.

Join a recording scheme and log your flatworm finds – send any records/sightings to our flatworm recording scheme – Potwatch!

Follow our biosecurity guidance (at the bottom of the linked page) on flatworms to help prevent their spread and destroy any non-native flatworms that you find.

Do remember that we rely on donations to continue our work.  If you have searched, found and learnt about our incredible invertebrates on our website, please do consider Making a Donation, Becoming a Member or maybe even making a purchase in our shop.  For more ideas on how to support our work find out how to Get Involved.  Thank you 🕷