What flatworms?

Land flatworms (Platyhelminthes) are a group (class of animal) of free-living worms. They are small, flattened animals often with a ribbon like appearance and unsegmented body. They are covered with mucus and usually leave a trail. They are found throughout the world, in Britain and Ireland there are about four species considered native and probably 17 non-native species that have become established, originating from the southern hemisphere.

The majority of the non-native flatworm species prey on earthworms, slugs, snails and other soil organisms. Australian and New Zealand flatworms are two species that have become established and widespread in Britain and Ireland. Both species specialise on earthworms and they can severely reduce the populations of some earthworm species and consequently affect the soil ecosystem. However, severel other species had been accidentally introduced including two Kontikia species and there is evidence that non-native flatworms continue to be introduced including the Obama flatworm

The Australian flatworm (Australplana sanguinea) is salmon-pink, pointed at both ends and reaches 2-8cm in length. It was first recorded on the Isles of Scilly in 1980’s and since has become widespread in southern England and Wales.

The New Zealand flatworm (Arthurdendyus triangulates) reaches 20cm (8in) in length and is dark brown with a paler margin. It arrived in Britain probably with imported plants, during the 1960s and it has since become widely distributed. It feeds exclusively on earthworms and is capable of reducing earthworm populations. This has undesirable effects on soil structure and also denies earthworms as a food resource for those native animals that feed on them. This flatworm originates from New Zealand and is now thriving in Scotland, Northern England and Northern Ireland. 

The Obama flatworm (Obama nungara), was first reported in 2014, it is native to South America, and is spreading across Europe. Its name means ‘leaf animal’. It is up to 7cm long with a light brown to black coloured body with a pale middle line and small black marks across its body.  It preys on earthworm and snails. This species is considered extremely invasive, and it has been characterised as the most threatening flatworm species to the soil ecosystem and native soil organisms presently in Europe.

The black and brown Kontikia flatworms (Kontikia ventrolineta and Kontikia andersoni) are small (1-2.5 cm long and 1-2mm wide), native to Australia and New Zealand. Kontikia ventrolineata is black coloured with two narrow grey lines. It has been recorded Britain before and it is thought that was first introduced in 1840. It is known to feed on small snails and possibly slugs. Kontikia andersoni is pale brown with three lines of darker spots along its body.

Because of their potential impact to populations of earthworms and other soil animals, the introduction of non-native flatworms possess a great risk to agriculture and horticulture. As such all non-native flatworms are included under schedule 9 of the Wildlife and Countryside Act  1981, which states that it is an offence to introduce or release them into the wild. Additionally, the New Zealand flatworm is included in the Invasive Alien Species of European Union Concern list.

If you find New Zealand flatworms these can be reported to the OPAL New Zealand Flatworm Survey (external link), finding of other non-native flatworms species they can be reported to the Non-Native Secretariat. 

Symptoms

Flatworms usually found in shady and wet places on the soil surface e.g. under pots, containers, tarpaulins, and leaf litter. The lack of earthworms they can cause often goes unnoticed.

Control

• Control focuses on the prevention of the introduction to areas that are as free of flatworms: There are no recorded natural enemies and no biological or pesticide control methods for non-native flatworms and as such, the key control measure is to prevent their introduction to new areas. The main pathway of introduction is horticultural trade and other plant or soil movements. Flatworms or egg capsules can be introduced with plants in pots and containers, root-balls of trees, soil, compost or other growing media, rhizomes and bulbs.
• If you find flatworms in your garden avoid spreading them to other areas: Avoid the exchange of plants between gardeners; re-use of top soil or compost can be responsible for the  spread of flatworms. So be careful about giving growing plants to other gardeners
•  Due to the mucus, flatworms can also attach themselves onto gardening tools, plastic sheets and machinery, which can potentially be another entry pathway, ensure you follow the procedures and principles for reducing plant disease transmission.  
• Once in a garden there is nothing effective that can be done to reduce flatworm numbers. Destroying any found underneath pots or stones will remove a few, but this is likely to be only a small proportion of the population in a garden

Biology

Flatworms belong to the class of animals called Platyhelminthes, they are unsegmented worms (contrary to the segmented earthworms) and can be found in a variety of habitats throughout the world. Many species are aquatic although a few have evolved and can be found on land. Land (terrestrial) flatworms are generally smooth, slimy and have variable colours.
In Britain there are about four native species, these are generally smaller than the non-native species.
The biology and feeding preferences of most flatworms are not well known. It seems to vary with the species and available food sources. Many flatworms are thought to be generalist predators of soil organisms, capable of feeding on animals much larger than themselves, whilst some species are detrivores (feeding on dead material).
All flatworms are hermaphrodite; they have a complex system of both male and female genitalia and they produce hard-shelled egg cocoons (egg capsules).These are laid in the soil, are small (<5mm), shiny and black resembling blackcurrants. Some species are also capable to reproduce by fission, where the flatworm divides and the separated segment regenerates and produces a new individual. Similarly, to slugs and snails, they produce mucus, which not only help them to move and attack their prey but also protects them from dehydration.