New Zealand Flatworm

Attack of the killer worms

Most people, in Britain, when they think of the devastation brought about by exotic species will think of grey squirrels or American mink. However these examples could pale into insignificance when compared to the potential damage possible by the latest threat… killer flatworms.

With the major increase in international trade of the last few decades, a number of exotic flatworms have been unintentionally brought into the country. These worms tend to be brought in the rootball of potted plants from other countries. Two of the potentially most damaging imports are Artioposthia triangulata, the New Zealand flatworm and the Australasian flatworm Geoplana sanguinea. The former, despite only being in the United Kingdom since 1963 (first observed in Belfast), has already developed a quite formidable reputation for its ability to decimate the endemic earthworm population. The New Zealand flatworm appears to originate from Dunedin and Christchurch in New Zealand. Geoplana, in Britain since 1980, also appears to be making an impact on local earthworm populations.

Loss of earthworms can have a dramatic effect on the ecology of an ecosystem. Earthworms play a major role in the structure and drainage of soil. They also ensure that the soil remains fertile and aerated. This is bad news for gardeners and farmers, particularly those who operate on an organic basis. Earthworms are extremely good at recycling nutrients; without them chemical fertilisers will become essential. But there is also a threat to wildlife. Many species of animal feed on earthworms. There is the obvious impact on birds such as thrushes, already under threat, but badgers, foxes, moles and other mammals also take earthworms as a major part of their diet.

So what are flatworms? Well to concentrate on the New Zealand flatworm; these killer flatworms are planarians, from the phylum Platyhelminthes. They are easily distinguished from earthworms in that their bodies are flatter, smoother and unsegmented. Flatworms come in a variety of sizes. The New Zealand flatworm can grow upto 8 inches in length. This species is also easily identified; it is narrow and strap-shaped, elongating considerably when moving. Its back is rounded and dark purple-brown in colour. It has yellow edges that are flecked with brown speckles. The front end of the worm is pinkish in colour. Its underside is also yellow in colour, again flecked with brown speckles. Both ends are pointed. Geoplana sanguinea, on the other hand, has one pointed end and a blunt end, is much smaller at 30-80mm and can be cream, white, peach or light red-brown in colour. One of the big problems with New Zealand flatworms is that they reproduce with ease, being asexual (not needing a partner for sex). They lay large shiny black eggs that produce eight or more young when hatched.

Flatworms are robust animals, although as the number of studies increase, their strengths and weakness are being discovered. The New Zealand flatworm, for instance, appears to be able to survive for over a year without feeding. When it does feed, its has no preferred prey but will eat any species of earthworm that it can find, at a rate of approximately one earthworm a week. It does this by attaching to the earthworm and secreting digestive juices and digesting the earthworm externally. Hunting tends to take place in the top 30 cm of soil or along burrows whose diameter can accommodate the flatworm. One of the early myths associated with the New Zealand flatworm; that of no predators in this country appears to be only a myth. Beetle larvae, especially Ground and Rove, seem to eat these flatworms. It also seems that the New Zealand flatworm, at least, appears to have a preferred habitat of moist clayey soil and keeps away from the more dry loamy soils. Predation activity increases when digging which opens up the structure disturbs the soil.

Distribution of the New Zealand flatworm is currently concentrated in domestic gardens, parks and garden centres throughout Britain, although they appear to have established a foothold in the pasturelands of Northern Ireland. To prevent an uncontrolled spread of these flatworms, people should take care when buying plants from garden centres. Although a recent conference organised by the OECD in New Zealand highlighted that the passing of plants between domestic gardens posed an even greater risk in the transmission of flatworms. Indeed, because of their effect on native earthworms, in 1992 they were considered to be an alien species under the Wildlife and Countryside Act 1981 and intentional release or even allowing accidental escapes is now a criminal offence. There is also the potential for a European wide ban in trade of agricultural and horticultural produce infested with the New Zealand flatworm. Always check the rootball of any plants that you buy and also check around; both inside and out, and under the pot itself. Regularly checking the garden is also useful. The New Zealand flatworm typically adopts a coiled position when at rest and can be found covered in mucus under rocks or other objects on the ground. The advised method for killing them is either to stamp on them or more beneficially to pour boiling water on them.

While we have only considered two species of flatworm here, it should be realised that many different species have been observed in Britain, both native and exotic. One of the biggest problems with recording the spread of flatworms is that so little is known about this group of animals. Even the full extent of native British species is not fully known one, Microplana humicola was only discovered in the 1980s. One of the recommendations of the OECD conference is to develop methods for the detecting, monitoring and management of terrestrial flatworms. Until further research discovers more about these interesting creatures the best advice to people is to be vigilant. Both when buying plants from garden centres and also when exchanging plants with friends and relatives.