Halloween bugs

As the temperature drops, the leaves are browning, the birds are leaving and.. the ladybirds are invading? Autumn is very definitely here; I've been noticing and appreciating all the usual signs, but last week I saw something I hadn't in previous years. Sat at my desk in my uni room, I was distracted from my online lecture by the many ladybirds on the walls and window. After looking into ladybird identification, I found that most were likely to be harlequin ladybirds Harmonia axyridis, an invasive species in the UK. Native to eastern Asia, this species has spread around the world, and is now also present in North and South America, Africa and Europe. Harmonia axyridis was introduced to the US in the 1980s as a method of biological control for aphids, and was recorded in the UK in 2004, after being probably accidentally introduced. In 2015, it was declared the fastest spreading invasive species in the UK after spreading from Essex as far as Cornwall and the Shetland Islands in under a decade.

One variation (form succinea) of the polymorphic species Harmonia axyridis can be seen above. 

This species is holometabolous, meaning it undergoes a complete metamorphosis during its lifetime; from egg, to larva, pupa, then adult. The average active period of a harlequin ladybird in adult form is 30-120 days, with the generation cued to go into diapause usually surviving through winter. Diapause (the reduction of metabolic activity in response to environmental conditions) is primarily triggered by a reduction in daylight hours to under 10-13 hours. In spring, when average temperatures reach 12°C, adults lay eggs near a supply of food. Females can lay 20-30 eggs each day, some unfertilized as food for the larvae; this appears to be the only parental investment shown by Harlequin ladybirds. In most areas, H. axyridis is thought to be bivoltine - having 2 generations per year - although in favourable conditions they can breed continuously. 
In adult form, Harmonia axyridis is a predatory insectivore, equipped with mandibles.

Above: Harmonia axyridis form Spectabilis on my windowsill (20/10/20). 

Harlequin ladybirds pose a threat to our native ladybirds for a variety of reasons, such as outcompeting native species for food and even eating native ladybird eggs and larvae. Studies have shown them predating Coccinella septempunctata (the native seven-spot ladybird) and in some areas of the UK where harlequin ladybirds are present, Adalia bipunctata (the native two-spot ladybird) has declined 44% since invasion by harlequins. There is also a resistance within H. axydridis to some diseases such as a microsporidian parasite which can affect native ladybird species. Transmission can occur between the invasive and native species, without affecting H. axydridis. This species is present in a wide variety of habitats, from deciduous and coniferous forest, to grassland and marshland, with a preference for sycamore and lime trees. 

Harlequin ladybird taxonomy


In some areas, the harlequin ladybird is also known as the Halloween bug, due to the way it can be seen gathering in large numbers in houses around Halloween when looking for overwintering sites, but personally I think the threat of invasive species is far scarier than ghosts!
Above: another example of H. axyridis form succinea


Links with more info: https://www.coleoptera.org.uk/sites/www.coleoptera.org.uk/files/Ladybird%20descriptions_Info%20pack_NEW_v.5.pdf
https://animaldiversity.org/accounts/Harmonia_axyridis/
https://www.coleoptera.org.uk/species/harmonia-axyridis
https://www.buglife.org.uk/bugs/bug-directory/harlequin-ladybird/
https://www.theguardian.com/environment/2015/may/20/harlequin-ladybirds-declared-uks-fastest-invading-species
https://www.cabi.org/isc/datasheet/26515#totaxonomicTree

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