There has been a spate of news stories recently reporting the discovery of unexpected frogs, lizards and snakes in the UK, hiding in plants, cut flowers and soil imported from sunnier climes. While these events are relatively rare, and make eye-catching headlines, they are an indicator of more serious issues threatening horticulture.
If reptiles as large as lizards and snakes are getting past border checks, there’s no question we’re also welcoming smaller pests into the UK, from insects and fungi to bacteria, viruses and disease-carrying organisms, which can infect, kill and outcompete native species, and impact ecosystems and food production. In fact, the UN Environment Programme has listed invasive species as one of the top five causes of biodiversity loss worldwide.
One of the best-known diseases, ash dieback, is caused by the fungus Hymenoscyphus fraxineus, which is thought to have arrived in Europe in 2012 on imported saplings, and the latest Europe-wide data is suggesting average losses of 50% of ash trees over the next 30 years. These trees must be felled for public safety, costing the UK economy billions in the long run, according to the National Trust.
Another of the most well-known pests in the UK is the box tree caterpillar, or box moth, which defoliates topiary and hedges in a matter of days. It's originally from Asia, but was discovered in British gardens in 2011; starting in the south east but then spreading across the UK. While many gardeners plant box alternatives now, others manage the pests by removing them by hand, using biological sprays and setting moth traps.
Researchers believe around 10-12 new invasive non-native species (INNS) are established in the UK every year, taking the total number to more than 2,000 today, including everything from the grey squirrel to ash dieback. As a whole, this group costs the UK economy £4 billion annually, according to a study by CABI.
You might also like:
- Japanese knotweed: dos and don'ts
- The government is banning giant rhubarb: here's what you need to do
- Why do we hate ivy? The truth about whether it damages walls and why we need to change our view
Pests hitchhiking in soil
One aspect experts are particularly concerned about is imported soil and compost, which is transported with potted plants, and can easily hide invasive species. One area of interest for David Smith, advocacy and social change manager at the invertebrate conservation charity Buglife, is the impact of non-native land flatworms.
"Imported pot plants and soils may seem harmless, but they can smuggle in destructive invaders like non-native flatworms: silent predators that can decimate native earthworm populations and threaten soil health," he says. "Once they're here, there's no going back.” While the number of native flatworm species in the UK stands at just five, there are now between 14-16 non-native species here too.
While non-native flatworms harm native wildlife, other intruders like the Spanish slug have a destructive impact on garden plants and crops. Another species on experts’ radar is the red fire ant, currently spreading from South America across the US and Australia, and thought to be the fifth most costly invasive species worldwide. Once established, it eats invertebrates, small vertebrates and plants, and outcompetes native species, causing an estimated £4.8 billion of damage per year in the US alone.
Imported soil also brings plant-parasitic nematodes; microscopic creatures that feed on plant parts. It’s believed just one square metre of soil can contain up to 10 million nematodes, which can then infect plants with viruses and diseases such as pine wilt, which threatens European conifer forests.
The role of climate change
One reason for growing concern in this area is that invasive non-native species that may have struggled to survive in northern Europe previously are now able to thrive and breed due to milder weather brought on by climate change. For example, researchers believe half of Europe’s urban areas are now warm enough for the red fire ant to survive on arrival, including cities like London, Paris and Barcelona.
Unfortunately, the huge scale of plant and soil imports, and the small size of many invasive species, makes it difficult to detect pests as they move across the globe; the UK imports over £1.5 billion of live plants and planting materials every year.
“The global trade of ornamental plants is expanding in value and geographic spread, with logistical improvements meaning faster and cheaper transport," says Dr Silviu Petrovan, a senior research associate at Cambridge University who once identified a Colombian tree frog amongst a Sheffield florist’s roses. "But this creates a range of risks, heavily influenced by climate change.
“As temperature is a major factor driving insect distribution worldwide, there’s a strong risk that climate change will increasingly allow new invasive pests to establish and impact food production and the natural environment," he continues. "They’re also likely to increase in number as milder winters enable them to survive. Long, cold winter spells are becoming a rarity.”
Prevention or cure?
Once pests are established in an ecosystem, eradication can be almost impossible – and even if it’s not, “prevention in the first place is substantially more cost-effective for most invasive species,” says Silviu.
While some scientists believe international trade in potted plants and soil should be stopped until we’re able to sterilise them, Silviu suggests we need better data sharing and management strategies for reducing the risk of invasive species becoming established.
Looking at the biosecurity efforts undertaken by the Department for Environment, Food and Rural Affairs (Defra) and the Animal and Plant Health Agency (APHA), it’s clear to see just how much time and investment it takes to keep dangerous species under control. Tracy Wilson MBE, head of operational delivery for plant and bee health at APHA, says the Colorado potato beetle - often imported in leafy veg and frozen peas, grown and packaged on the Continent - makes her list of top 10 pests and diseases due to its severe trade impact. It can reduce a potato crop by 50%, so eradication is vital.
As the threat from the Colorado beetle is so great, potato growers have high vigilance and often spot it before it develops into a full beetle, notifying APHA, who arrive within hours and begin an “immediate, resource-intensive reaction,” says Tracy. Teams pull up plants, send samples to the lab for formal diagnosis and walk through rows of crops, scanning for evidence of feeding, and even sift soil to find young beetle larvae, finally removing its feeding material by killing the crop with herbicide. Then, a three-year contingency plan kicks in.
Following the most recent beetle finding in 2023, potato exports from Kent were banned and the team began intensively surveying a 10km radius zone, inspecting potato crops, visiting pack houses dealing with leafy veg and looking at other potential hosts, including hedgerows and allotments growing tomato plants. This year, the team has planted and surveilled a track crop adjacent to the initial site to monitor the ongoing impact.
Public awareness of pests like the Colorado potato beetle, which can easily fly from allotments and home veg patches to commercial plots, is crucial, says Tracy. That’s why it’s the topic of APHA’s Chelsea Flower Show exhibit this year.
For pest identification guides and to get a better understanding of UK pests, Tracy points anyone growing fruit and veg towards Defra’s Plant Health Portal. If you find something, “pop it in a jam jar, put the lid on, take a photograph and report it through the APHA, where it’ll be triaged quickly. You can help us continue to protect the biosecurity of Britain,” says Tracy.
What gardeners can do
On an individual level, when it comes to buying plants and compost for home gardens, the containment and control of pests is tricky, but Silviu says there are extra steps gardeners can take, as well as sticking to buying native plants. “Buying from reputable sources; prioritising seeds and bulbs over imported potted plants (especially large ones that need big root systems); and carefully monitoring pests, and reporting them when possible, are all important”.
David Smith from Buglife puts it best, and simply: “Buy locally grown or soil-free plants to help protect your garden”.
You might also like: