Ladybirds are invading our houses this autumn, often in their thousands. What's going on?

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God, dare I say it, missed a trick when He unleashed plagues of locusts, lice, frogs and the rest on Egypt all those years ago. Britain's new pestilence is a cannibalistic and highly promiscuous species of the humble, sweet ladybird that is a native of Asia.

Harlequin ladybirds, as they are known because they come in a confusing variety of colours and designs, have already flourished in our parks and woods. And, thanks to an absence of their natural predators, they are now making moves on our homes. The really bad news is that they plan to settle in for the long, cold winter – in their thousands. If you've been picking them off your window sills in batches of five or 10, be warned: this might just be the beginning.

Scaremongering rhetoric you might think, but Matt Shardlow of Buglife, the trust that speaks up for all of Britain's invertebrates, has been answering the phone this week to worried callers whose homes are overrun with ladybirds. When I ask him if he might be overstating the case, just a little bit, he points out that I wouldn't think so if I had armies of insects marching on my kitchen.

"What these people are experiencing is a Hitchcockian The Birds sensation, where thousands of ladybirds are trying to get into their houses. We've probably had more concern about harlequins in the last two weeks than we had in the whole of last year," he says.

Although the first harlequins to hit these shores were discovered in Essex in September 2004, they have since spread to Durham, Cornwall and Wales, and have made a start on the process that entomologists have feared – the ousting of our native ladybirds.

Professor Michael Majerus, a Cambridge ladybird expert, predicted three years ago that their numbers would mushroom having watched their rapid development in the US. The American agricultural industry made the mistake of importing harlequins in the late 1980s as a form of pest control, to eat the aphids on crops. Once the bugs had exhausted the aphids they turned their attention to other species of ladybird, as well as and fruit and grape vines, before looking for a nice place to "over-winter". The problem is, once they discover a cosy spot to bed down in during the cold months, they let off pheromones to signal to all their friends to come and join them.

"I love ladybirds," says Majerus, who is Professor of Evolution at the Cambridge University's genetics department, "but I said back in 2004 that they were going to be a significant problem, not just to ecosystems but to people. Three years later I'm sitting here besieged by ladybirds and desperate people. The speed with which it has happened is beyond even what I expected."

Majerus asks anyone who finds a harlequin to post it to him. On just one day this week he received an entire post bag stuffed with them, as well as around 40 e-mails about the bugs.

Despite the benign reputation of ladybirds (exemplified by the Ladybird Books slogan "Everybody loves a ladybird"), once they form swarms – which were first spotted on the Isle of Wight last November – they can cause substantial damage. "They destroy wallpaper, curtains and carpets if they're not found," says Shardlow. "And they poo a sticky black substance everywhere." Majerus says their reflex blood, the gooey yellow stuff that seeps out of their joints, is the main problem. "It smells foul, tastes foul, and stains anything."

The journalist Annalisa Barbieri has first-hand experience of harlequins in her Suffolk home, and is frustrated that they appear impossible to get rid of. "We have had swarms of them," she says. "They fly all over the house and settle in huge clusters in the corners of window frames. They also fly at you, and they bite, but you know you can't squash them because they release their orange blood everywhere."

Barbieri was quite excited when she first spotted some ladybirds, as she had been trying to hatch some in her organic garden for her young daughter to watch. By the time she realised they were harlequins there were already huge numbers of them clustering around the windows on the outside of her house, and coming in through fissures in the wooden frames. "I actually started to get quite paranoid that someone had put a biblical curse on me," she admits.

"It was so bad last Sunday I was thinking that I don't want to live here anymore if it gets any worse. But I don't think it will, because winter is coming."

A random posting about harlequins on a blog site called Little Red Boat has attracted over 100 comments, revealing how many people living in flats in cities are also being invaded by the spotty black, red and orange bugs.

So what advice can the experts offer people like Barbieri? "Absolutely none," says Majerus, worryingly. "I will not advocate the use of chemical pesticides. I'm sure a lot of people are stamping on them, Hoovering them up and spraying anything on them that they can find. We would tell people that a better way to deal with the ladybirds is to collect them up and send them to us instead."

Barbieri is also reluctant to use toxic products inside her house but hit upon the idea of wiping away the pheromone trail left by the harlequins with a strong solution of white vinegar. She says it is easy to work out where it is because you can see hundreds of the bugs migrating across surfaces to one spot. Otherwise she waits until they are sleeping in their clusters, Hoovers them up and takes the bag to the tip where they are incinerated.

"People think I'm plucking a number out of the air," she says, "but easily and conservatively I'd say we have got rid of 2,000." She says they are quite different to British species of ladybird, calling them " ladybirds with attitude". "I tried to drown one this morning and they're pretty resilient."

As well as invading people's homes, harlequins are a threat to Britain's biodiversity. By disrupting the food chain with their apparently insatiable appetite for aphids, they have the potential to affect more than 1,000 types of insects and other organisms. If they run out of aphids to eat, they're quite happy to start munching on their fellow ladybirds instead.

Professor Majerus says around half of Britain's 46 ladybird species are now at risk. In an attempt to hold back the multicoloured tide he is trying to develop a genetically modified mite that he hopes will sterilise many of the female harlequins and begin to control their rocketing population growth.

For now, though, all he can advise is to sit tight and hope for the best. In America one common name for harlequins is the "Halloween ladybug", because they start appearing in people's houses at the same time as they're dusting off their vampire capes and scooping the flesh out of pumpkins to make lanterns and pies.

"They appear in people's houses around Halloween over there," says Majerus. "And they're doing the same over here, literally in their tens of thousands."

Help the harlequin cause by visiting www.harlequin-survey.org or contacting the Ladybird Research Group, University of Cambridge, 219d Huntingdon Road, Cambridge

Combat those creepy-crawlies

If you've been invaded by any sort of pest you can contact your local authority or a private pest control company to tackle the problem. But if you'd rather engage in some non-toxic bug control methods, here are a few ideas.

COCKROACHES

Cockroaches can cause considerable damage through chewing, and they also leave a nasty smell via their droppings and the pungent pheromone they release to attract their mates. They feed on decomposing food and are a disease risk. To reduce the likelihood of cockroaches traipsing across your work surfaces, keep food well sealed and your surfaces clean. Boric acid kills cockroaches and they will carry it back to their nests and unwittingly kill their friends as well but take care: it is toxic if swallowed by children, so be careful where you wipe it. Cockroaches also hate catnip, and a quick squirt with soapy water will kill them. You can buy non-toxic traps at www.stopthatpest.co.uk.

ANTS

The same traps will work for ants, as will soapy water and boric acid. Try mixing a cup of warm water with half a cup of sugar and two tablespoons of boric acid. Soak the solution up in cotton wool balls and place them near any trails you've seen the ants using in your home. Ants are also deterred by cucumber, mint and cloves, so if you leave any of these where you think the ants are entering, they should look for other more welcoming places to live. Peppermint tea bags are also apparently very effective. If you do know where the ants are getting in, try spreading baby powder, cayenne pepper, lemon juice, cinnamon or coffee around the spot to drive them away.

FLEAS

Fleas are usually brought into homes by cats, and your first step should target any infestations on your pets. The simplest non-toxic way to do this is to give them a warm bath and a scrub with some ordinary soap. Boric acid and non-toxic traps are also a good choice here, as is diatomaceous earth. This dust is the fossilised remains of oceanic algae whose shell is so hard and spiky that if the fleas feast on the fine dust, which you sprinkle around your house and on your pets, it will puncture their innards. It is very unpleasant for the flea, but it does work well. You can buy spray bottles of diatomaceous earth at www.greengardener.co.uk.

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