Pot-plant trade is ‘hitchhiker pathway' for invasive flatworms, say UK experts

The Guardian

8 hours ago

They have been invading the UK for years; small mucus-covered animals which hunt in gardens, allotments and greenhouses.
The number of sightings of non-native flatworms has risen sharply over the past few years, and experts have warned they can decimate earthworm populations and degrade soil quality.
Land flatworms are non-segmented worms, which feed on a range of soil organisms from woodlice to worms. In the UK the number of non-native species has risen from two in the 1950s to 14 in 2020. Only a few of these can really be categorised as 'invasive', according to the planarian specialist Hugh Jones, because of the 'measurable damage' they inflict on ecosystems.
'There are three in the UK which I'd loosely call invasive, two of them definitely: the New Zealand flatworm, the Australian, and the Obama all eat earthworms,' he said. Once established you cannot eradicate them, only mitigate the worst of the harm.
Earthworms are ecosystem engineers. They enrich soil by passing it through their digestive systems, moving organic matter into deeper layers, and their burrows help stop compaction. According to the ecology professor Rene van der Wal, from the Swedish University of Agricultural Sciences, invasive flatworms drive down the numbers of earthworms 'to extremely low levels'. The knock-on ecosystem effects include a reduction in mole populations as their earthworm diet disappears. There is no definitive research on the extent to which this is affecting agriculture.
Populations of the New Zealand flatworm are growing in Scotland and northern England, while the Australian flatworm is spreading out from its strongholds in Lancashire, south Wales and south-west England.
The sharp increase in non-native species in recent decades is attributed to global trade, particularly in potted plants and soils, David Smith, advocacy and social change manager from the charity Buglife, told the Guardian.
Over recent years, this regulatory framework has been shifting. Post-Brexit, Britain can import potted plants from Europe, but only export bare-root. That could change with the new UK-EU trade deal, which will revise 'phytosanitary' plant health regulations, including in ornamental plants. The deal has been agreed in principle, but details are still being negotiated.
The National Farmers Union has welcomed it, suggesting that with Britain poised to re-enter the European plant health area, we could soon see an end to border control checks and phytosanitary certification for most plant products traded with the EU.
Others, however, are concerned this could accelerate the spread of invasive species, including flatworms. Of particular concern is the New Guinea flatworm, the only flatworm that features in the International Union for Conservation of Nature's 100 of the World's Worst Invasive Alien Species list. It has extirpated entire snail populations on some islands, and poses a threat to snails across Europe. So far, sightings have been reported in France but not yet in Britain.
It is, says Smith, 'a ferocious predator'. 'It's been found in greenhouses in Europe but not yet in the wild. It wouldn't take much climate change for it to move out and succeed, or to be transported to a place that's more suitable to it – some micro-climates within the UK, for instance.'
Unlike flying insects, flatworms 'rely entirely on human activity for dispersal, typically arriving hidden in soil or potted plants', Smith said. 'Current biosecurity measures are insufficient to detect and intercept them, enabling their escape into gardens and the wider countryside.'
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The soil was not policed, van der Wal said. Plants are checked at borders 'but they're being checked for what's on their surface, and on the soil's surface. Sometimes they may look into the soil itself, but essentially they're looking at the health of the plant, and not at hitchhiker species.'
Instead of deregulating the pot plant trade, which risks opening the door even wider to more harm, the trade deal could go the other way, and help close major entry points by banning all imports of soil and products containing soil.
The horticulture industry opposes this. 'They say it's easier to move plants in soil, and to sustain them whilst they're being transported and in warehouses,' Smith said. But this was how non-EU imports were already managed, and to extend the practice to the EU would be straightforward, he said.
While the invasive flatworms already in Britain are here to stay, the UK-EU trade deal offered a rare opportunity to close off a 'hitchhiker pathway' for the arrival of more invasive species, Smith said. If this is not done, he warned the risk would grow of British-based invasive flatworms being unwittingly exported to other parts of Europe, and of other species moving to Britain.
Buglife encourages anyone who finds a flatworm to submit a sighting via its PotWatch survey.