NATURE New Zealand flatworms – the earthworms’ enemy

Outdoor Living
Our earthworm’s foe

Andy Wilson

It is 40 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 further attention was paid until 1983, when research discovered a possible link between low earthworm populations and the presence of the flatworm. Since then, it has been established that Arthurdendyus Triangulates is an aggressive predator of the various species of native earthworm. It is now common throughout Northern Ireland and has also been widely recorded in the Republic of Ireland, including numerous locations in Co Mayo.
When the flatworm locates an earthworm it secretes digestive juices which dissolve the earthworm into a ‘soup’ which is then digested. Once the flatworm becomes established on a piece of ground, the native earthworm population can decimated. 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 completely disappear.

Why we need earthworms

There are strong links between earthworms and soil fertility. Worms 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, a high population of earthworms in the soil is a good indicator of fertility.
The earthworm has tended to decline in soils which are either compacted by heavy machinery or dosed regularly with pesticides, herbicides, artificial fertilisers or raw slurry. 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. Also, land with low earthworm populations is not so well aerated, leading to waterlogging. Both these problems are most acute in land which is not tilled. These processes do not occur overnight but are long-term changes. Owing to its impact on local earthworm populations, the spread of the flatworm is likely to lead to declining soil fertility.

Identifying a flatworm

The 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 50-70mm (two to two-and-a-half inches) long and 6-10mm wide, but when fully extended they can be as long as 30cm (one foot). The 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.

How flatworms arrived
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. For these reasons, it is advisable to be very cautious about accepting plants from friends or family, when the flatworm status of the soil used is unclear. Once established at a particular location, the flatworm will migrate slowly onto adjacent land.

Controlling NZ flatworms
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. Plant nurseries that take a responsible attitude to the flatworm will have clear protocols regarding the movement of soils and plants. However, there appears to be no legal obligation for a nursery to take any specific measures.
While certain species of bird may eat flatworms if given the chance, the flatworms are nocturnal, reclusive creatures that rarely expose themselves on the surface of the soil.
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. One possible proactive measure is to deploy large plant pots around the garden as flatworm ‘traps’. The soil surface under the pots should be inspected every few days. Once positive identification is made, any captured flatworms should be destroyed.
Heat treatment may be effective for suspect potted plants. The pots or rootballs should be immersed in hot water of about 30-34ºC for ten or fifteen minutes. The temperature will have to be measured accurately or there is a risk of killing the plant. Another method of control is to stand the plants in a warm room at a minimum temperature of 26.5ºC for 24 hours.