It is forty years since the first New Zealand flatworms were first recorded in Ireland. The species Arthurdendyus Triangulates was first found in Northern Ireland in 1963 and came to be regarded as something of an exotic novelty in gardens around Belfast. Not much attention was paid to the flatworm until 1983, when research into the beneficial effects of native earthworms on soil fertility discovered a possible link between low earthworm populations and the presence of the New Zealand flatworm. Since then, it has been established that Arthurdendyus Triangulates is an aggressive predator of the various species of native earthworm and that it is now common throughout Northern Ireland. It has also been recorded at many locations in the Republic.

When the flatworm locates an earthworm it secretes digestive juices which dissolve the earthworm into a ‘soup’ which is then sucked up by the flatworm. Once the flatworm becomes established on a piece of ground, the native earthworm population can decline to a level at which earthworms are barely detectable. There is some evidence that the smaller species of earthworm which live deeper in the soil can survive the predatory activities of the flatworm, while those which live nearer the surface may almost completely disappear.

Soil Fertility
A number of studies have been carried out into the beneficial effects of earthworms. They are an important part of our soil ecology and are responsible for aerating and mixing the different layers of soil, assisting the breakdown of organic material and improving soil drainage. As any organic gardener knows, the earthworm is the gardener’s friend and a high population density of earthworms in the soil is a good indicator of its fertility.

The earthworm has not tended to thrive in soils which are either compacted by heavy machinery or dosed with pesticides, herbicides or large quantities of artificial fertilizers and even before the appearance of the New Zealand flatworm, there were well founded concerns that intensive methods of agriculture were having a very detrimental effect on earthworm populations.

Ongoing studies carried out over a number of years in Northern Ireland suggest that low earthworm populations lead to a build up of organic acids in the soil. In other words, the soil becomes more acidic. Land which is not turned by hand or ploughed will have a greater tendency to become waterlogged, owing to the reduced rate of earthworm related soil mixing and aeration. These processes do not occur overnight but are long-term changes which may not manifest themselves for ten years or more. One can conclude that the spread and increase in numbers of the New Zealand flatworm is not good news for the soil.

Australian Flatworm
A new species of flatworm was detected in the Scilly Isles (off the Cornish coast) in 1980 and since then has also taken up residence in Ireland, particularly in the Southwest and along the West coast. This species, Australoplana Sanguinea Alba is known as the Australian flatworm, though apparently it too comes from New Zealand. The Australian flatworm is smaller than its New Zealand cousin and appears to be less vigorous in hunting down native earthworms. As far as is known, both species feed exclusively on earthworms. While certain species of bird may eat flatworms if given the chance, the flatworms are nocturnal reclusive creatures which rarely expose themselves on the surface of the soil.

Anecdotal evidence suggests that there are a number of flatworm hotspots around Ireland and that the spread of flatworms may have been inadvertently assisted by nurseries selling potted plants in which the flatworm was already resident. While the evidence is not conclusive, it seems extremely likely that the flatworm has been dispersed countrywide as a result of human activity of one sort or another and probably primarily through the movement of potted plants. Once established at a particular location, the flatworm will migrate slowly onto adjacent land.

There is no indication that the flatworms can be eradicated once they become established. Any effective chemical measures which might work would have a very detrimental effect on other soil fauna. The only option for control seems to be physically collecting the flatworms and destroying them. While this is unlikely to result in the flatworms being eradicated from any particular location (especially if they are present on adjacent land) it will at least keep the flatworm population in check and allow the earthworm population some possibility of recovering.

Collecting and Trapping Flatworms
It is quite easy, if somewhat unpleasant, to collect flatworms. The mucus they secrete can be an irritant, so gloves may be advisable. Flatworms like undisturbed resting sites, and in particular can be found under big stones, large plant pots and under mulching materials such as newspaper or sheets of polythene. In particular they seem to favour black polythene, possibly because it is both damp and dark underneath.

These locations can be checked on a regular basis, perhaps every two or three days or once a week. In addition, proactive measures can be taken. Small pieces of black polythene can be deployed around the garden as flatworm ‘traps’. They should be weighed down and inspected on a regular basis. Once positive identification is made, any captured flatworms should be destroyed.

Identification
Both the Australian and New Zealand flatworms are easily identified. They cannot easily be mistaken for any other garden creature though occasionally people have confused them with either native earthworms or the garden leech.

The NZ flatworms are dark purplish brown with a buff or sandy coloured edge. The underside is also pale buff. At rest they are about 6cm (two and a half inches) long and 1cm (just under half an inch) wide but when fully extended they can be as long as 30cm (one foot). NZ flatworm eggs are shiny and black and resemble oval blackcurrants. They are about 1cm long. Each egg can contain up to about a dozen young flatworms.

The Australian flatworm is an orangey pink colour and about one third to one half the size of the NZ species

The eggs are much smaller; only about 2mm (one tenth of an inch) in diameter.
The Australian flatworm has not been studied to the same extent as its larger brethren.

Other Methods of Control
Heat treatment may be effective for suspect potted plants. The pots or rootballs should be immersed in hot water of about 30-34ºC (Celsius) for ten or fifteen minutes. The temperature will have to be measured accurately or there is a risk of killing the plant.
Another suggested method of control is to stand the plants in a warm room at a minimum temperature of 26.5ºC for 24 hours.
Flatworms also do not like extreme cold and are much more vulnerable to frost than earthworms. Digging over ground the day before an expected hard frost may also have a detrimental effect on flatworm populations, though it certainly won’t eradicate them.

Research and Further Information
To the best of our knowledge, there is no research into flatworms being carried out anywhere in the Republic of Ireland, either by the State or independently. This extremely poor state of affairs probably accurately reflects the general attitude of both the Department of Agriculture and the main farming organisations towards organic farming and horticulture. There is a great need for research to be carried out (in the Republic) into both the long term effects of non indigenous flatworms on native earthworm populations and also on how this impacts upon soil fertility.

In Northern Ireland research has been ongoing for over twenty years. Paul Moore, of the Applied Plant Science Division of Northern Ireland’s Dept. of Agriculture and Rural Development, provided us with lots of valuable background information for this article. His colleague, Dr Archie Murchie kindly supplied the photographs which have been used in this article.

Paul would welcome any data relating to the distribution of flatworms in the Republic. Please refer to 'Things to Do' for his contact details.

Research into flatworms is also being carried out in England and in Scotland. So far, the NZ and Australian flatworms have not become established in continental Europe. This is thought to be due to the colder continental Winters, warmer Summers and generally less humid conditions. Conditions in Ireland and Western Scotland are particularly favourable to the flatworm. Flatworms have also been recorded in the Faeroe Islands, which are situated midway between the North of Scotland and Iceland. Certain European countries have regulations which require export nurseries to be certified flatworm free.