A comparatively small aggregation of Harlequin ladybirds, here aggregating in a straw hat.
It's because Britain is in the grip of an alien invasion: the non-native Harlequin ladybird, Harmonia axyridis. This large species (5-9mm long) is native to Asia (Japan, China, Russia & the Koreas), and was first found in Britain in 2003, flying the Channel after being introduced into continental Europe as a biocontrol agent, to eat aphids in hop gardens and roses. A very variable species, it comes in three forms in the UK: forms conspicua and spectabilis, which are black with two and four red spots respectively, and form succinea, which is orange with 19 black spots (although some or even all of them may be absent, or they can fuse together). Around 80% of the British Harlequins are form succinea, but even so, like snowflakes, no two are exactly alike.
But why are they still about in late autumn? And why do they form these big groups?
Well, the Harlequin is a species which can breed all year round, and has two or even three generations a year (whereas most of our 25 native species will only have one), so they take that extra time before entering overwintering. And as for the big groups? Well, each female Harlequin can lay up to 2,200 eggs under ideal conditions, so by the end of the summer there are huge numbers of them about, all looking for somewhere cozy to spend the winter.
In the native range, the Harlequin spends the winter in these groups - aggregations - in the mountains, forming groups in the mouths of caves and underneath rocks. In the introduced range - some 50 or more countries in Europe, north & south America, & Africa - houses, sheds, porches and the like take the place of these natural refuges. During the early autumn, the ladybirds form large swarms and begin to seek out overwintering locations. They find them by using a mixture of visual and chemical cues, often aggregating in buildings that are distinct from their surroundings, and pale - white attracts more than yellow, which in turn attracts more than darker colours.
Once they're in the vicinity, the swarms will seek out chemical cues laid down by their grandparent's generation, stable long-chain hydrocarbons which they can detect with their antennae - that's why aggregations form year after year on the same spot.
One last thing: Harlequins aren't the only species to form aggregations. Two-spot ladybirds will group together in houses, and many species will do it outside - Orange, Pine, & 7-spot ladybirds, to name just a few! The UK Ladybird Survey are always glad of any winter records - the website is http://www.ladybird-survey.org/
