Snakes in potted olive trees ‘tip of the iceberg’ of ornamental plant trade hazards

Invasive pests are slipping unnoticed into northern Europe in huge shipments of cut flowers and potted plants, say researchers, with potential to damage food crops and the natural environment

This news piece is edited from a release by the University of Cambridge whose researchers led the study in collaboration with researchers from the University of Oxford, Netherlands Institute for Vectors, Invasive Plants, and Plant Health, The University of Hong Kong and the Reptile, Amphibian, and Fish Conservation Netherlands. 

Continental European snakes, geckos and Italian wall lizards are making their way to northern Europe undetected among imports of ornamental olive trees destined for gardens and green spaces.

These hitchhiking intruders can become invasive pests that cause extensive damage to the natural environment - as has happened in previously snake-free islands of the Mediterranean like Majorca. They’re also a red flag for a bigger problem: the range of potentially serious agricultural and environmental pests being unwittingly imported to Britain and mainland Europe on ornamental plants and cut flowers, simply because they are difficult to detect in high-volume, fast-moving shipments of plants.

In a study published today in the journal Bioscience, researchers says that despite regulations and border checks, imported cut flowers and pot plants present a growing risk because the sheer volume of trade makes it difficult to monitor and control. Insects, fungi, reptiles, spiders and various agricultural pests are being transported live across the world on ornamental plants destined to brighten up our homes and gardens.

The multi-billion dollar global market for ornamental plants is growing fast and geographically expanding, and improved standards are urgently needed, they say.

The changing climate means that disease-carrying insects like mosquitoes, which decades ago would have arrived in northern Europe and died from the cold, might now survive. It is also enabling some ornamental plants themselves to become invasive pests as growing conditions change.

Professor William Sutherland in the University of Cambridge’s Department of Zoology, who was involved in the study said

“Ornamental olive trees for sale in the UK can be over 100 years old, with many hiding places amongst their gnarly bark and the soil they’re transported in. This is incredibly risky in terms of importing pests,”

 “Adult snakes and lizards are just the tip of the iceberg. If they’re getting through, what’s the chance of us spotting small insects and fungi – the things that really cause the problems? It’s inconceivable that officials can thoroughly check an import of a million roses from Kenya, for example.”

“The sheer volume of cut flowers and ornamental plants being traded at speed around the world makes it extremely difficult to intercept all the pests and diseases they carry. Even with the best of intentions, unwanted hitchhikers are getting through customs import checks all the time,” said Dr Silviu Petrovan, a researcher in the University of Cambridge’s Department of Zoology and senior author of the paper.

Suppliers do not always operate within the law. Orchids and cacti are amongst the high-value plants sometimes illegally stripped from tropical habitats and included in shipments. Regulations to prevent the trade in protected wild plants are challenging to enforce on a large scale.

Dr Amy Hinsley, from the Department of Biology and Oxford Martin Programme on Wildlife Trade at the University of Oxford, who was first author on the paper says

“Even with a global trade in cultivated ornamental plants, there is still a market for rare species taken from the wild, and this can lead to rapid species declines, as well as increased risks that wild pests and plant diseases may enter the supply chain,’ 

Petrovan, a frog specialist, became interested in the topic when he was asked to identify a live frog found in roses in a florists’ shop in Sheffield. At first he thought it was a prank, because he didn’t recognise it as any European species. When he realised it was a tree-frog that must have arrived with the cut roses from Colombia via Ecuador, he was stunned.

“Finding a South American tree-frog in a Sheffield florist was extraordinary. It made me realise that if you can get this type of fragile small vertebrate arriving alive in a flower shipment without being noticed at customs, just how hard it must be to detect very small agricultural insect pests or their eggs,” said Petrovan.

With no comprehensive international database on the types and numbers of pests found on imported ornamental plants, it is difficult to fully assess the extent of the problem. To gain a snapshot, the team analysed records of pests found in ornamental plants at customs in The Netherlands over 2017-2018, and reported to DEFRA in the UK over 2021-2023. In both cases, over 80% of the pests intercepted were insects.

The study highlights many other concerning environmental and health issues connected with the global ornamental plant trade. These include:

• environment-harming microplastics and agrochemicals entering the soil from the growing process;
• health-harming pesticide residues affecting cut flower handlers;
• the huge volumes of water required to grow flowers that might otherwise be used to grow food - the floriculture industry in Kenya, for example, is responsible for up to 98% of the water drawn from major lakes like Lake Naivasha;
• the carbon footprint of chilling and transporting cut flowers between continents - estimated to be as high as 3kg of CO2 per flower;
• large quantities of plants being taken from the wild, including critically endangered species of cacti, succulents and orchids.

But an industry that employs so many people is not all bad: the ornamental plant trade is important for economies worldwide and supports many people and their families in rural areas. In 2022 the export value of cut flowers and foliage was US$10 billion, and for live plants and bulbs was $13 billion.

“We absolutely don’t want to encourage knee-jerk reactions that might be well-meaning, but actually cause more problems than they solve,” said Petrovan.

He added: “We need to push to make the industry more sustainable through things like certifications and better regulation, and to work with those involved in the trade to better understand the risks and how to mitigate them.”

The research was primarily funded by Arcadia and the David and Claudia Harding Foundation to Cambridge University. 

Read the study in BioScience here: https://doi.org/10.1093/biosci/biae124