An insect that once held promise as a natural pest controller was branded the most invasive species in Britain yesterday by researchers.

The harlequin ladybird has taken just four years to spread across England and to make inroads into Scotland and Wales, a feat that took the grey squirrel a century to achieve.

Since 2005 more than 20,000 sightings have been recorded of the ladybird, which threatens to take over from many of the 46 native British species of ladybird.

Its progress has been tracked by the Harlequin Ladybird Survey, an online survey overseen by the Centre for Ecology and Hydrology. Thousands of members of the public took part and it was funded in part by the Government.

Peter Brown, of the centre, said: “It’s the most invasive species in Britain. It is perhaps equalled by the horse chestnut leaf-miner but nothing else comes close.”

Harlequin ladybirds, Harmonia axyridis, were first identified in Britain in 2004 when one was seen in a pub garden in Sible Hedingham, Essex. Last month the species was reported in Orkney.

So serious is the problem that the ladybird is the subject of a special issue of the journal BioControl. More than 50 scientists from Europe and North America contributed to the journal to share knowledge about its impact.

The horse chestnut leaf-miner is an insect first seen in southwest London in 2002. It has spread rapidly and now infests about 20 per cent of horse chestnut trees, causing leaf loss.

Helen Roy, who works at the centre and who edited the journal, said: “Through this online survey we have been able to track its movements and are now beginning to understand more complex aspects of the ecology of the harlequin ladybird.”

Because it eats so many aphids, its staple diet, as well as other ladybirds, it has threatened the number of native ladybirds and species, such as lacewings, which also eat aphids. It also threatens aphid numbers. The two-spot and seven-spot ladybirds are particularly threatened.

The harlequin ladybird’s voracious appetite for aphids attracted interest in its use as a biological pest control but after it was released in several European countries in the 1980s and 1990s it rapidly became established and spread widely.

Mr Brown said: “Ladybirds are very popular but this one is a great concern in terms of his risk to biodiversity.”

Alien species

Grey squirrel Came to Britain from North America in 1876. Found in most of England and Wales and moving into Scotland. Larger than native reds and carries a virus which devastates the latter

American signal crayfish Native white-clawed crayfish are being eradicated by the larger, more aggressive American species, which also carries disease deadly to the native animal

American mink Brought here for their fur in 1929 but escaped into the wild where they have had a severe impact on water vole numbers – leading to a 90 per cent fall in the population

Times recommends

• Energy-saving ideas from switched-on professor
• Rooks are as clever with tools as chimpanzees
• Burping of the lambs blows roast off menu