Gardeners across Sussex have been put on red alert over a killer beetle invading Britain.

The Asian harlequin ladybirds were imported to mainland Europe because of their voracious appetite for aphids.

But the harlequins are taking over and could devastate insect life, with repercussions farther up the food chain.

The fierce species can out-compete sedate homegrown ladybirds for food.

It is known to eat butterfly caterpillars and there have even been reports of the beetles biting people.

The harlequin has annihilated ladybird and butterfly populations in North America, where it was introduced 25 years ago.

Mike Russell, of the Sussex Wildlife Trust, said the beetle, Latin name Harmonia axyridis, could overwhelm British species if left unchecked.

He said: "I have been looking at research from Belgium, where it was introduced, and it particularly out-competes two-spot and seven-spot ladybirds, which are the most common in this country.

"The wildlife balance evolves over a long time. Once you bring something in, there is a good chance it does not have any serious predators and out-competes other species for food sources and breeding places.

"These ladybirds have no predators as such themselves - if they become established, they can really flourish."

Mr Russell said the trust wanted the harlequin ladybird banned from being sold to control garden greenfly.

In September, the species was spotted in the garden of a pub in Sible Hedingham, Essex.

Dr Alan Stewart, of the department of biology and environmental science at the University of Sussex, said: "Studies in the USA and in Asia show that it feeds faster than other ladybirds and may therefore deprive them of their food source.

"There is one report of this species eating caterpillars of the monarch butterfly - an American species which is not native to the UK, although it is very occasionally recorded here.

"This shows there is potential for it to eat caterpillars in general but it would be impossible to predict which British species it might go for.

"Not much can be done to stop them once they have become truly established.

"Until then, we need to be vigilant about imported materials like fruit and keep a close eye on the population that has been discovered in Essex."

Anyone who spots a harlequin ladybird can report it to the Sussex Wildlife Trust on 01273 492630.