A ladybird which has already caused havoc to native insects in America has been spotted near a pub in Essex.
Harmonia axyridis posed a "deadly threat" to butterflies, lacewings and ladybirds, Dr Michael Majerus of Cambridge University said.
The ladybird is an Asian species which was introduced into North America 25 years ago to fight aphids.
It has since spread to Europe and last month was discovered in the gardens of the White Lion pub in Sible Hedingham.
The ladybird, which is also known as harlequin or the multi-coloured ladybug, was seen at the pub on 19 September and identified by Dr Majerus at the university's genetics department.
Dr Majerus said: "This is without doubt the ladybird I have least wanted to see here.
"I knew it was on its way, but I hoped that it wouldn't be so soon. Now many of our ladybirds will be in direct competition with this aggressively invasive species, and some will simply not cope."
The adult H. axyridis is about 7mm long, slightly larger than the seven-spot ladybird native to the UK. It comes in a range of colours and patterns.
The one found in Essex was black with two very bold red spots and two smaller red spots. But the bugs can also be orange in colour and checked in pattern.
H. axyridis is still sold in North America as a pest control. "It is now the commonest ladybird in North America," Dr Majerus told BBC News.
"It is outcompeting pretty much all of the aphid-feeding native American ladybirds which are going through anything from a slight to a very, very serious decline.
"And there are lacewing, hoverfly larvae and even butterflies that are suffering because this thing is eating all the food, and it is also eating as secondary food butterfly eggs, and other ladybirds and lacewing larvae."
Time to defend
Despite this unwelcome and well-publicised take-over, the harlequin ladybirds are also still being sold in continental Europe by biocontrol companies. The bug now roams across France, Belgium and Holland, with numbers soaring annually.
Harlequins also feed on fruit juices as they fuel up for the winter and fruit-growers are finding that they blemish many soft fruits, reducing the value of the crop.
Wineries report the bugs also taint the vintage because of their acrid defensive chemicals.
Dr Majerus believes there is still time to save the UK from a full invasion. He urges anyone who finds the insect to send it to him with precise details on when and where the ladybird was found.
"It is critical to monitor this ladybird now, before it gets out of control and starts to annihilate our own British ladybirds," he said.
Cambridge's Evolutionary Genetics Group can be contacted at firstname.lastname@example.org