Gardeners' taste for the exotic 'puts native species in jeopardy'

The country's continuing love affair with gardening has opened the door to an invasion of exotic pests that is killing off our natural habitat, a leading scientist will warn today.

"If we don't change our practices, worst-case scenarios could be further loss of tree or plant species for at least a generation, or degradation of entire natural ecosystems," Professor Clive Brasier will tell an audience of policy makers and industry figures.

British gardeners spend £5.4bn each year and the trade in imported plants has more than doubled in the past decade to £800m. Only 1,500 species out of 100,000 plants grown in the UK are native.

Householders seeking quick-fix solutions want mature plants, generating a trade for trees and tree ferns from as far afield as China, Australia and New Zealand. Yet few amateur horticulturists are aware of the dangers. Too many verdant immigrants are carrying "uninvited guests" that threaten Britain's forests, gardens and wildlife.

Professor Brasier, emeritus mycologist at Forest Research and visiting professor at Imperial College London will tell a Royal Horticultural Society Science Exchange that current international plant health protocols do not properly address many of the potential threats."We don't move large numbers of animals around the world for disease reasons and we shouldn't do it for plants either," he will say.

His views are backed by the Royal Horticultural Society, which fears another epidemic such as the 1970s spread of Dutch elm disease, which killed 30 million trees.

Addressing the Department for the Environment, Food and Rural Affairs-sponsored conference at Reading University, Professor Brasier will explain: "Many more plants are now being imported from overseas. Despite checks by the authorities, inevitably exotic pests and diseases slip through. In recent checks tree ferns from Australia and maple trees from China, which had been cleared for export, were found to be crawling with 'uninvited guests'."

Society, he believes, needs to change its whole mindset on plant biosecurity.

International plant health authorities publish global lists of pests and diseases that are known threats. But Professor Brasier estimates that about 80 per cent of potentially damaging organisms are not mentioned because they are harmless in their country of origin, where they have co-evolved with their hosts.

By contrast our native species are often defenceless. Many invasive pathogens, such as those causing the current alder mortality and "sudden oak death", were previously unknown to science and therefore not internationally listed.

In its native environment, an organism such as the "sudden oak death" pathogen may be harmless on a rhododendron or viburnum, but when imported it can kill indigenous trees.

There is also the possibility that some foreign pathogens - often microscopic and invisible - will hybridise to produce an entirely new disease.

Symptoms are sometimes artificially suppressed with fungicides making the infection impossible to detect and resulting in stock being given quarantine certificates indicating they are healthy when they are not.

Professor Brasier believes it would be safer for the industry to import plant material as seed or tissue cultures, which can be propagated to provide stock, while bringing in smaller numbers of plants licensed for specialist propagation.

Insect invaders

Citrus longhorn beetle (Anoplophora chinensis)

The larvae from these large, black and white beetles bore through the trunks and roots of apple, oak, poplar, maple, alder, pine and beech trees, making them susceptible to disease and wind damage

Rosemary beetle (Chrysolina americana)

This metallic green and purple pest from southern Europe has become established in Britain. Both the adults and the larvae feed on rosemary, lavender and thyme

Red lily beetle (Lilioceris lilii)

Less than a centimetre long, this red beetle with a black head damages lilies and fritillaries, primarily by defoliating them, but in heavy infestations the flowers, seed capsules and stems will also be eaten

Colorado beetle (Leptinotarsa decemlineata)

This has been found on a range of plants that it does not feed on such as Italian parsley and French salad. It lives on plants from the potato family such as tomato and pepper

Allium leaf miner (Phytomyza gymnostoma)

This 3mm-long fly with black and yellow undersides attacks onions, spring onions, garlic chives and ornamental Allium plants. It mines the leaves and bulbs, making the plants susceptible to infection