Plant pest no match for forces of weevil

Himalayan balsam and floating pennywort smother native species and clog up habitats, costing hundreds of millions of pounds to try to remove.

The alien plants, both brought over to decorate Victorian gardens, have no natural predators in the UK, and have spread out of control.

Now plant scientists believe they have discovered two natural enemies from the plants' countries of origin that could help stop them in their tracks.

But some wildlife experts have warned that the history of releasing alien species to tackle other foreign invaders is littered with disasters.

Himalayan balsam is common in Scotland, where it has clogged up banks around major rivers such as the Tay, the Tweed and the Water of Leith. It has seed pods that fire seeds into waterways, which quickly disperse them over large distances.

Floating pennywort does not currently grow in Scotland, but is rife in England and there are fears it will soon spread north of the Border, particularly as the climate heats up.

The weevil and the fungus have been identified by scientists working for CABI – a world leader in natural control methods – in northern India and Argentina. Part-funded by the Scottish Government, they discovered a rust fungus from the Puccinia family, which attacks the stems and leaves of Himalayan balsam, rendering it lifeless.

In Argentina they came across a stem-boring weevil called Listronotus elongatus, which drills into the vulnerable stalks of the floating pennywort and kills it.

Letting alien species loose in the UK is controversial, in case they attack other plants, or, like the plants they are supposed to counter, spread out of control.

If the government grants permission for the release of the two species into the UK, it will be the only time alien creatures have been used to tackle invasive plants in Europe, apart from the recently approved use of psyllid bugs to tackle Japanese knotweed.

In the past, most introductions of alien species for pest control in other parts of the world have ended disastrously.

For example, the cane toad was let loose in Australia to eat beetles on sugar cane plantations. It failed to make any impact on the beetles but seriously threatened Australia's biodiversity by decimating native reptile species which eat the toxic toad.

In the US the seed-feeding weevil was introduced to control exotic thistles. Not only did it target these, but it also attacked the native thistles that were essential to various insects, which were wiped out.

Tony King, head of policy for the Scottish Wildlife Trust, is concerned about the idea of letting alien species loose in the UK. "Although biological control might initially seem attractive from a cost point of view, the potential unforeseen consequences could be very costly and it shouldn't be forgotten that traditional physical control methods do work."

He added that the potential trouble with letting species loose in the wild was that "their effects on the wider ecosystem can never be known until after they are released, by which time the damage is done".

However, Robin Payne, an invasive species expert with Scottish Natural Heritage, said researchers had proved how thorough they were during previous work on the psyllid, a natural predator of Japanese knotweed. Tests were carried out for five years to make sure it would only attack the target plant.

"This is new-style science for invasive species and biological control," he said. "It's a million miles away from the cane toad situation all those years ago."

He added: "Because we have got such a significant problem with invasive species we need to have a range of different weapons in our armoury.

"Biological control is just one of these but it's a very good one if you have got good agents."

Rob Tanner, lead scientist at CABI on the project to find an agent to tackle Himalayan balsam, emphasised that many years of rigorous tests, in strict laboratory conditions, would be carried out before any attempts were made to get permission to release it into the wild.

He said the rust fungus had already shown promise.

Other plants in the same family as Himalayan balsam, which include the popular garden flower Busy Lizzies, were not infected even when grown just 2cm away.

Mink to toads: other ecological backfires

IN AUSTRALIA, the huge cane toad was released in 1935 to try to control beetles in sugar cane plantations. There was no consideration of what the potential food range would be, or what impact the cane toads might have on the native fauna.

The toads now exist in their millions and threaten many native species of wildlife.

The American signal crayfish was introduced to waters in England and Wales through fish farms about 20 years ago to capitalise on its growing popularity as a delicacy in restaurants. But it has been blamed for eating young fish, including native crayfish, and destroying their natural habitat.

American mink, which were brought to the UK for fur farms in the 1950s, have destroyed huge numbers of native animals, particularly water voles and seabirds.

The North American grey squirrel was introduced in the 19th century, but has contributed to the catastrophic decline of Britain's native red squirrel.