Information and Advisory Note Number 7                                                 Back to menu

New Zealand and Australian flatworms in Scotland

1. Introduction

1.1 Background
Twelve species of terrestrial flatworm have been recorded in the British Isles. Only three of these are native, the rest have been introduced by man, usually with imported plant material. Some of the imported species have only been recorded from glasshouses but five of them have become established in the wider countryside.

In Scotland five species have been recorded so far (Table 1); another native species Mlcroplana humicola may also occur here but it has not yet been found (Jones, 1989). R hallezi was taken in hothouses in the Edinburgh Botanic Gardens before 1902 and has not been recorded since.

Table 1.
Terrestrial flatworms recorded from Scotland.

1.2 Reasons for concern
When the New Zealand flatworm was first found in the British Isles in the 1960's there was little immediate concern. However, this perception changed when it was found that this predator could have a large impact on earthworm populations, in some areas it can even cause the local extinction of earthworms. This has stimulated considerable interest in the implications for agriculture and for the natural heritage. A second introduced species, the Australian flatworm, has been spreading northwards in Britain. The impact of this species on earthworm populations is much less well known, but it may also be a cause for concern.

2. Identification

2.1 The New Zealand flatworm
Fully grown New Zealand flatworms can be up to 15cm (6") long and weigh about 2g. The body is flattened, glistening, unsegmented and it is pointed at both ends. The upper side is dark purple/brown with pale lateral margins; some specimens also have a darker stripe up the middle of the body. The flatworm has numerous eyes on the pale lateral margins at the head end. The underside is lighter in colour and the entire flatworm is covered in very sticky mucus.

2.2 The Australian flatworm
This is smaller than the New Zealand flatworm (3-8cm long) and paler in colour ranging from cream through peach to a uniform pale brown. There is a slight red tinge to the front end of the body. The head end is pointed but the hind end is more rounded. There are numerous eyes which form a row down the whole length of each side of the body.

2.3 Possible confusion
Several groups of other animals could be confused with the New Zealand and Australian flatworms. Here are some guidelines to avoid possible
1. Earthworms: these have cylindrical segmented bodies with colour evenly spread around the segments.
2 Leeches: these have a flattened segmented body with suckers at each end.
3. Native terrestrial flatworms: these have flat unsegmented bodies. They have two large eyes near the head end and they are generally smaller
than the introduced species. There are also differences in their coloration and body shape (Jones, 1988). However, firm identification depends on dissection of the flatworm, and they must be checked by an expert.

3. Distribution

3.1 New Zealand flatworm This flatworm originated from the South Island of New Zealand where it is found in cool damp areas such as woodlands and gardens It may have been more widespread on the Island in the past but the felling of the forests has restricted its range. The first confirmed records for this species in the British Isles were made in 1963 from gardens in Belfast In Scotland, it was first recorded in 1965 from plants in the Royal Botanic Gardens in Edinburgh. Both populations probably originated from infected soil imported with plants.

This flatworm has spread by two means: local dispersal has occurred by the movements of the flatworms themselves. However, the main means of dispersal into new areas seems to have been plant containers. This is borne out by its distribution, which centres on gardens and nurseries and by recent genetic work on the origins of populations in Northern Ireland.

The New Zealand Flatworm is now found throughout the mainland of Scotland and on several of the islands (Fig. 2). Outside Scotland the species has spread into England and Wales, the Isle of Man, the Faeroe Islands and Iceland. It has also spread widely within Northern Ireland and into the Irish Republic.

3.2 Australian Flatworm
This species has a confused taxonomy but is a native of Australia and New Zealand. It was first identified in the British Isles from the Scilly Isles. It has spread through England and into Wales. The first record for Scotland was recently (1995) made near Dumfries. It appears to be tolerant of higher temperatures than the New Zealand flatworm.

4. Ecology of the New Zealand flatworm

4.1 Life history
Many of the details of the reproduction of terrestrial flatworms are unclear but we do know the broad outline. The flatworm starts life in an egg cocoon laid in the soil. The cocoon is about 5mm (Vs") long and it is bright red when it is laid but it rapidly darkens to black. Up to ten young flatworms hatch from each cocoon; they are about 30mm (114") long and creamy pink in colour. The flatworms are able to reproduce when they have grown to about 5cm (2") long, although they will continue to grow beyond this length.

4.2 Habitat
Terrestrial flatworms are prone to desiccation so they are restricted to damp habitats. During dry spells New Zealand flatworms migrate downwards through the soil and aestivate in mucus-lined cells until the weather turns wetter again. They are most active at night and avoid light, hiding under objects on the soil surface or in the soil itself. The New Zealand flatworm dies quite rapidly at temperatures higher than 20C and it is susceptible to freezing. However, by moving through cracks and burrows in the soil they are usually able to avoid these extremes.
Scotland and Northern Ireland are generally wetter than the South Island of New Zealand, and this may explain why this woodland species has been able to colonise open habitats such as pastures.

4.3 Prey
The New Zealand flatworm feeds by wrapping its body around its prey and secreting digestive juices onto it. The liquefied prey is then ingested through the mouth tube that extends from the middle of the underside When not in use, the tube is drawn back into the body of the flatworm. Studies on captive New Zealand flatworms have shown that they can consume several earthworms in a week. Part of the reason for the large impact of the flatworm on the earthworm population may be that the flatworms can follow their prey through their burrows in the soil. By contrast, other earthworm predators such as birds are largely limited to the soil surface, and those such as moles which attack worms under ground are restricted to large burrows.

We do not have firm evidence on the types of prey attacked by the flatworms because they eat most of their prey whilst concealed by the soil or by objects on the soil surface. Studies using radioisotopes have given results consistent with a diet made up entirely of earthworms but this is not conclusive. There may be some preference for earthworm species which make shallow burrows with larger diameter holes.

4 4 Impact on earthworm populations In wet areas such as south western Scotland, Northern Ireland, and the Faeroe Islands the flatworm has caused local extinctions of earthworms, although in some cases they have been found to coexist with deep-burrowing earthworms. In drier areas, such as eastern Scotland, some populations of the New Zealand flatworm coexist with relatively healthy, although reduced, populations of earthworms.

The flatworm can survive periods when no prey are available by using a process called 'degrowth': parts of organs are gradually lost and cells become smaller. This remarkable process, found in many species of flatworm, is reversible when food becomes available. As a result, the flatworms are able to persist for long periods without food and in areas with very low earthworm populations making any recovery by the earthworms much slower.

5. Flatworms, farming and the natural heritage

5.1 Effects on farming
In Scotland, at present, only 12 farms are known to be infected by the New Zealand flatworm but this may be rising. Arable forming is likely to be less affected than pasture by the loss of earthworms because earthworms tend to be less common in ploughed fields, and because the soil is turned over by ploughing.

In pasture, earthworms are important because they mix the soil, speed the decay of organic matter in the soil and improve the drainage properties of the soil. All of these affect the productivity of the soil and of the grass grown on it As a result the New Zealand flatworm could have a significant economic impact in some areas of the country.

5.2 Potential effects on toe Natural Heritage
The introduced flatworms have not yet been recorded from natural habitats and so we can only speculate on the effects in these habitats. Earthworm communities vary with habitat type, soil type and the wetness and organic content of the soil. The effects of the flatworm on these communities may differ from the effects in pasture. Factors such as soil type, pH and water potential might impose limits on the spread of the flatworms but there is no evidence for this yet. There may be little to stop them from colonising many natural habitats. Results from gardens and farmland suggest that because of toe rainfall, western areas may be more strongly affected than eastern parts of the country.

If earthworm populations are reduced in natural habitats there may be both direct and indirect effects:
1. Direct effects on other animals

Many vertebrate species feed on earthworms including amphibians, a number of mammals such as moles and shrews, and a wide range of birds including songbirds and rooks. In addition, there are many invertebrates which feed on earthworms including predatory slugs, leeches, beetles and parasitic flies. Some of these species are more dependent on earthworms than others and the impact of the flatworm will depend upon how far earthworm populations are reduced and what alternative food sources are available. If the reduction is not too severe these other earthworm predators may be able to coexist with the flatworm, albeit in reduced numbers. Native terrestrial flatworms might also be affected by competition or predation.

2. Indirect effects
Earthworms affect the soil structure, leaf-litter degradation rates and rates of nutrient cycling within ecosystems. If earthworm populations are reduced then all of these processes are likely to be affected and this might change the abundance of plants and animals in the affected habitat.

6. Legal status

As a result of the potential problems posed by the spread of the New Zealand flatworm, it was added to Schedule 9 of the Wildlife and Countryside Act in 1992. This makes it an offence to release the New Zealand flatworm into the wild. It is also an offence to allow it to escape into the wild unless all reasonable precautions have been taken to prevent escape.

7. Methods of Control

7.1 Reducing spread
The most important action at the present time is to reduce the spread of flatworms in plant containers This requires strict hygiene in nurseries and gardens, especially for soil which is known to be infected. If possible, plants should be planted with bare roots or from seed into clean soil. Some treatment is possible for infected plant pots: immersing pots in water overnight may make most of the flatworms leave the pots allowing them to be captured. However, any egg capsules will remain in the soil.

7.2 Chemical
There are no approved chemicals for the control of these species. Research is continuing on possible pesticides. However, there are two main difficulties: firstly, finding compounds that affect the flatworms but not earthworms, and secondly, finding a method of applying any chemical to the flatworms hidden in the soil or beneath objects on the soil surface.

7.3 Biological Control
Native British predators such as ground beetles and rove beetles will eat the New Zealand flatworm. However, since they are confined to the soil surface and coexist with relatively healthy populations of flatworms in central Scotland they are not likely to have a significant impact Several natural enemies are known from New Zealand. However, little is known about their biology and they are unlikely to be specific to the one species of flatworm. As a result, it could be difficult to find good biological control agents that attack only the target flatworms.

7.4 Trapping
The most widely used technique to control the flatworms is trapping under tiles, weighted black plastic sacks and similar objects. Anything with a smooth surface that creates a damp dark area on the soil surface is likely to attract the flatworms. These traps' are checked every few days and any flatworms can be removed and killed. They can be killed by squashing them or by dropping them into hot or very salty water. Any containers must be sealed as they are adept at escaping through surprisingly small gaps.
There have been no studies into the effectiveness of this method in reducing the populations of flatworms.

8. Further Reading

Blackshaw R. P. & Stewart V. I. (1992) Artioposthia triangulata {Dendy) a predatory terrestrial planarian and its potential impact on lumbricid earthworms. Agricultural Zoology Reviews 5, 201-219.

Boag B., Nielson R. and Palmer L F. (1993) The New Zealand flatworm Artioposthia triangulata a potential predator of earthworms in northern Europe. BCPC Monograph No. 54: Plant health and the European single market 397-402.

Boag B., Palmer L F. Nielson R. and Chambers S. J. (1994) Distribution and Prevalence of the predatory planarian Artioposthia triangulata (Dendy) (Tricladida: Terricola) in Scotland. Annals of Applied Biology 124,165-171.

Jones H. D. (1988) The status and distribution of British terrestrial planarians. Progress in Zoology 36,511-516.


Dr David Phillips, Invertebrate Ecologist


Dr David Phillips
Research and Advisory Services Directorate
Scottish Natural Heritage
2 Anderson Place
Edinburgh EH6 5NP

Dr Brian Boag
Scottish Crops Research Institute
Invergowrie, Dundee DD2 5DA


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