A group of harlequin ladybirds

The ladybirds can form large clusters typically around window frames © Gilles San Martin/Flickr CC BY-SA 2.0

Why harlequin ladybirds are invading our homes

The invasive harlequin ladybirds (Harmonia axyridis) form large clusters at this time of year.

Why do the insects gather around windows? And should you be concerned about the sexually transmitted disease (STD) that some of them carry?

As the weather begins to cool, many species in the northern hemisphere will begin to wind down and start preparing for hibernation.

All species of ladybird in the UK hibernate - this is technically known as diapause. This is due to their life cycle, as the beetles you see now won't reproduce until next year.

The adults of some species will hibernate individually, finding cracks in bark or rocks in which to hunker down for the cold winter months. When the weather then starts to warm, the beetles emerge from their diapause to mate and lay eggs. The larvae then develop, pupate and hatch as adults.       

Max Barclay, Senior Curator of Beetles at the Museum, says, 'There are a number of species of ladybirds that hibernate in big clusters. Ancestrally harlequin ladybirds would probably have hibernated in big clusters in caves, hollow trees and other sheltered places.'

In absence of these they may well try and gain entrance to your home. The easiest way for them to do this is through the small gaps along the edges of loose-fitting windows.

A selection of different colour varieties of harlequin ladybird

Harlequin ladybirds are incredibly variable, but the easiest way to identify them is by their orange legs. © entomart/Wikimedia Commons

How to identify a harlequin ladybird

Ladybirds belong to the family of beetles known as Coccinellidae, of which there are at least 3,500 species worldwide.

In the UK there are 46 species known to be resident, around 26 of which are colloquially called conspicuous ladybirds. These tend to be brightly coloured, with many sporting the classic red-and-black polka dot. The remaining 20 species, called inconspicuous ladybirds, are often drab in comparison.

Harlequin ladybirds are not native - they arrived in the country in 2004. They originate from eastern Russia, China and Japan, but likely came to Britain as natural pest control via either Europe or the USA, where they were first introduced.

'Harlequin ladybirds are pretty distinctive, even though they have lots of colour forms. You can find different varieties, from orange ones, black one with red spots and more,' explains Max.

The harlequin ladybirds tend to be around the same size or slightly larger than the more familiar native seven-spot species, and like the seven-spot they also have two white spots on their thorax.    

'But whereas the native ladybirds are black on the underneath and have black legs, most of the harlequin ladybirds are orange on the underneath and have orange legs,' says Max. 

Ladybird larva

The larvae of the harlequin ladybird are very distinctive © Judy Gallagher/Flickr CC BY 2.0

Sexually transmitted disease

As was expected, after the harlequin ladybirds arrived in Britain their populations started to be brought under control by native predators and diseases. One of these is a sexually transmitted disease.

'It's called laboulbenia, and it is a fungus that forms little scales on the wing cases on the outside of the ladybird,' says Max. 'You can actually see it with your naked eye and so can tell whether the ladybird is infected.

'It just looks like a yellow crust on some parts of the ladybird's exoskeleton.'

But this is no cause for concern. The STD is native and usually infects our own ladybird species. The fungus has simply found itself another host in the invasive harlequin ladybird.

And no, it can't infect humans.   

'That is a beyond ridiculous,' laments Max. 'It is a fungus that grows on the exoskeleton and obviously we don't have one of these, and we don't have sex with ladybirds.'

Ladybird eating prey

The harlequin ladybirds are voracious predators, eating everything including native ladybird species © gbohne/Flickr CC BY-SA 2.0

Here to stay

Currently, harlequin ladybirds are mainly found in the south-east of the country, though it has slowly been pushing further north into the Midlands.

While it is thought to have a limited impact on some native species, the invasive ladybird is now so well-established that it is effectively here to stay. Some native predators of the insects, such as a parasitic wasp that lays eggs on the ladybird, have started preying on them. This means that ladybird numbers are expected to reach some form of equilibrium.

'These are not anything to panic about,' stresses Max. 'First of all they can't give you diseases, they can't hurt you and they're not going to completely exterminate our native fauna.

'They are going to find some kind of balance, even if that includes them in reasonably large numbers.'

So if you want to stop the insects from entering your home, just make sure your doors and windows are fully sealed and insulated. 

Read more