Invasive Hitchhikers

Nature experts are calling for strict Government plant import restrictions and are urging gardeners to ‘buy British’ to avoid bringing damaging invasive species into the British Isles. 

Disease, competition for food and habitat and predation are some of the main ways that our iconic native wildlife is being harmed by invasive species. 

Red squirrel populations have fallen by 96% since the introduction of grey squirrels in 1876, water-voles have disappeared from 94% of their range, and earthworms have fallen by a fifth in some areas due to carnivorous foreign flatworms.

Invasive species (in addition to Ash dieback) that are already established in the UK via plant or soil imports, and are causing damage to UK wildlife and the economy, include:

  • Oak processionary moth. Damages our oak trees and poses a human health risk. It was imported to the UK in egg form in the canopy of trees for planting in the early 2000s.
  • Spanish slug. Nicknamed the super slug - it arrived here in potted plants and salad leaves around 2010 and is now present across the country. It’s a voracious eater of crops, wild and garden plants, and numbers are difficult to control as it’s too slimy for hedgehogs and birds to eat and resistant to poisons.
  • Red Lily beetle. First found established in the UK in the 1939, and probably a hitchhiker on imported lilies, this beetle is highly destructive to both lilies and fritillaries, many species of which are in decline in the UK.
  • Harlequin ladybird. Probably accidentally introduced through plant imports. It consumes our native ladybirds and causes a nuisance hibernating in people’s houses in the autumn.
  • New Zealand and Australian flatworms were likely introduced in potted plants and are now firmly established in the UK. They eat our earthworms, reducing populations by up to a fifth. This affects wildlife that rely on earthworms as a food source, and reduces soil fertility affecting farm productiveness.
  • Phytophthora austrocedri. This fungus-like pathogen is thought to have arrived in Britain through the plant trade. It is attacking and killing one of our rarest UK trees, Juniper, affecting birds like the goldcrest, and impacting on the Scottish Gin industry.

Other plant and soil hitchhiker species which are an imminent threat for the UK, include:

  • Asian hornet - a bee eating wasp already destroying bee populations in France. There have been more than 20 sightings here in the last 5 years and a 2020 study estimated it would cost the UK £7.6million a year in damage and control costs if it establishes.
  • The New Guinea flatworm is already found in France and is one of the “100 worst invasive alien species” in the world. It has destroyed populations of native snails across the Indo-Pacific and has the potential to do the same here.
  • Argentine ant which eliminates native ant species, harms ecosystems and has been known to cause damage to wiring. These ants have been spotted in London and Birmingham but are not thought to be established.
  • Emerald Ash Borer. a beautiful yet destructive wood-boring beetle native to East Asia but spreading across Europe. It has caused massive damage to billions of Ash trees in North America and could have a catastrophic impact on the UK’s diminished Ash population. Imported firewood is the biggest risk of introduction.
  • Two-leaf Water Milfoil, introduced to Europe through the aquarium and water gardening trades It has been found in the UK since the 1940s but is not thought to be established, yet. It can form dense underwater mats which suffocate native wildlife. It affects the fishing, swimming and boating and water industries.

Horticulture is a key avenue of concern when it comes to the introduction of invasive species to the UK.  While not responsible for all species arriving, preventing new soil and plant hitchhikers such as fungi, flatworms, ants and slugs would significantly help in protecting our native wildlife. The fungus causing Ash dieback, for example, is a rapid-spreading and high profile invasive species, which has caused great public concern. An estimated 80% of UK Ash trees are set to die.

This has prompted Government to take action in a pilot scheme that aims to combat the ever increasing threat from pest and diseases, including preventing similar invasive fungi attacking our trees. From June 2022, all trees funded under Government tree-planting schemes will be required to meet new biosecurity requirements. This means applicants for the England Woodland Creation Offer and the Future Farming Tree Health Pilot must source their trees from suppliers who are either accredited under the Plant Healthy Certification Scheme or who have passed a Ready to Plant assessment. However, experts are warning Government needs to go further to protect native trees and other wildlife, by:

  • Extending these biosecurity requirements to all trees and potted plants entering the country, not just those used in tree-planting schemes
  • Requiring imported timber, soils, and compost to undergo protective measures such as heat treatment to eradicate harmful hidden hitch-hiker species, and for plants to be transported soil free.
  • Supporting the expansion of UK tree and plant growing for domestic sale, to reduce reliance on horticultural imports.

Dr Richard Benwell, CEO of Wildlife and Countryside Link, said:

“Invasive species are one of the top threats to wildlife. They’re also bad for business, affecting fishing, farming and leisure industries alike. Here in the UK some of our most cherished species, from red squirrels to juniper trees, are suffering huge declines as they face disease, predation and competition from invasive species.

“Prevention is better than cure so it’s important to stop further invasive species before they gain a foothold. Improved biosecurity measures are vital to prevent harmful species from hitching a hidden lift into the country in plants, trees and soil.”

Craig Macadam, Conservation Director at Buglife, said:


“From carnivorous flatworms to tree-damaging moths, species hidden in imported soils, plants and timber can have a huge impact on UK wildlife. By buying home-grown British plants you can help to prevent invasive species reaching your garden and our countryside.

“While these species may be harmless in their native environments they can wreak havoc here, which is why we need much stricter controls on soil and plant imports and to boost home-grown plant production. Until there is a proven way to sterilise pot plants the Government must take effective action to only import bare-root plants and must ensure all soil & timber imports are sterilised.“

Dr Paul Walton, Head of Habitats and Species at RSPB Scotland, said:

“Non-native species impacts are one of the five main drivers of nature loss in the UK and round the world. Awareness of the issue is not always as high as it might be - yet there are simple things that people can do to help. Avoiding imported garden plants and compost (and using peat free, of course) is one. But we also need the Government to drive change in the horticulture industry to help stop invasive hitchhikers. We must move our reliance away from imported plants, and deliver extra checks and balances to stop more damaging species arriving on our shores.”

Ash dieback and similar fungi are receiving Government attention due to their highly visible and rapid impact. But other invasive species are causing severe, though less obvious harm, and need to be tackled decisively to prevent further wide-scale nature-loss and economic costs. Action on plant imports could stop multiple new invasive species establishing in Britain, preventing further wildlife decline and helping to stop the estimated £2.2billion annual cost to the economy of invasive species damage soaring further.

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