PC21
New Zealand Flatworm
The New Zealand flatworm (Arthurdendyus triangulatus) was first noted in Northern Ireland and the western parts of Scotland in the early 1960s. On the drier, eastern side of Scotland there have been fewer sightings. Flatworms have now been identified at points throughout the UK. Results of a recent survey compiled in September 2003 has shown sightings of this pest in central and southern England.
In its native New Zealand, the flatworm is found in shady, wooded areas and only on the cooler South Island. Open, sunny pasture land is too hot and dry; temperatures over 20°C are lethal to this creature. Flatworms were probably introduced into the UK in the soil in pot plants sent or brought from New Zealand.
Description of pest
When resting, flatworms will be coiled and covered in mucus. They are very distinctive and look nothing like earthworms. The upper surface is dark, purplish brown with a narrow, pale buff edge. The underside is also pale buff. They are pointed at both ends, and ribbon-flat. A mature flatworm at rest is about 1cm wide and 6cm long. When extended, it can be up to 30cm long, and proportionally narrower. Flatworm ‘eggs’ or capsules are shiny black, just under 1cm long, and like a mis-shapen blackcurrant. They contain as few as two, or as many as eleven young worms.
Several other races of flatworm have been found in the UK since the introduction of A. triangulata. They are the same shape as the common form, but come in different colours, orangey-red (Australoplana sanguinea var alba) which is common in the south-west and around Merseyside, or creamy white (Arthurdendys albidus) only known from Edinburgh.
Life cycle
Adult flatworms produce egg capsules in summer. A capsule can be between 4-11mm long and 3-8mm wide, and resemble a blackcurrant. Juvenile flatworms emerge after about a month and are creamy white/pink in colour.
Feeding habits: Flatworms seem to eat earthworms exclusively. There is no evidence that they will eat any other soil-dwelling creatures, even when the earthworm population has been reduced to below detectable levels. There is no apparent preference for one particular worm species. Flatworms are most active near the soil surface, so deeper burrowing earthworms are more likely to survive despite the presence of flatworms.
When an earthworm is located, the flatworm covers it with digestive juices. This dissolves the earthworm which is then sucked up by the predator. Estimates suggest that flatworms consume one or two earthworms per week. When its food supply has been virtually exhausted, a flatworm will shrink and wait until there are sufficient earthworms for feeding to restart. This waiting time can be as long as 12-24 months. It is not yet clear whether flatworms will move long distances to alternative feeding sites.
No need to panic
The presence of New Zealand flatworms in well-maintained gardens does not seem to be quite such a disaster as first thought. In Northern Ireland, one organic smallholder has co-existed with this creature for about 12 years. He has managed to trap and destroy large numbers of flatworm. This has enabled his earthworm population to re-establish itself. Regular additions of large quantities of organic material have created such a healthy soil that a temporary reduction in earthworm population has been tolerable.
The situation is less straightforward for farmers managing pasture land. Fields which are unploughed have become waterlogged without the regular burrowing activity of earthworms to aerate the soil.
Prevention and control
- Check your garden: The New Zealand flatworm prefers undisturbed resting sites. Look underneath large plant pots, garden ornaments and big stones. If you find one flatworm, there are probably more. Eggs are usually found between June and September. A trap can be set by weighing down a plastic sack in a damp area of the garden. Inspect the underside regularly for coiled flatworms. If found, flatworms should be destroyed by squashing, burning or immersing in salty water.
- Natural predators: Birds have eaten flatworms, with no ill effects, but the natural habitat of the pest is usually under cover, concealed from searching beaks. The larvae of ground and rove beetles prey on adult flat worms.
- Heat treatment: Recent research by commercial growers have found that flatworms in container plants can be controlled by exposing them to high temperatures, which will not cause any lasting damage to the plants. Immerse pots or whole rootballs in hot water at 34°C for 10 minutes; you will need an accurate thermometer for this. Or simply stand the plants in a warm room kept at a minimum of 26.5°C for 24 hrs or at 30°C for 12 hrs. No chemical controls exist which will not also harm earthworms.
- New sightings: Researchers are eager to know of new sightings of flatworms. Should you discover a possible flatworm in your garden, please take a digital photograph and send by email to mike.lole@adas.co.uk. When taking photos, place an object such as a pencil, or a key, alongside the creature. This gives an idea of scale. If you are unable to send a digital photograph then send a worm specimen in loose packed, damp moss to either Plant Clinic (Flatworms), ADAS Wolverhampton, Woodthorne, Wergs Road, Wolverhampton WV6 8TQ if found in England, or in Scotland - Dr J Pickup (Flatworms), SASA, East Craigs, Edinburgh EH12 8NJ. In Northern Ireland - Mr J P Moore (Flatworms), DANI, Newforge Lane, Belfast BT9 5PX.
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