Cobweb beetle

The hairy larvae of the Cobweb beetle steals remains of dead bugs from spider webs – giving the species its fantastic name! Found throughout the UK, this Cobweb beetle’s habitat of ancient gnarled broadleaf trees is under threat. The Cobweb beetle (Ctesias serra) gets its name from the habits of its hairy caterpillar-like larval stage. The fantastic larvae live on ancient gnarled broadleaf trees, where they can be found all year round under loose webby bark, in the tunnels made by other insects and around the fringes of spider webs – close to their dinner!

Fast Facts

Latin name: Ctesias serra

Rarity in the UK: Rare / Common

Dangerous dinner

The larvae of the Cobweb beetle opt for an easy, but risky dinner – they steal the remains of dead bugs from sheet- and tube-web spider webs! The long bristles covering their bodies protect the larvae from attacks by the spiders they steal their meals from. When threatened, the Cobweb beetle larva erects and vibrates its long bristles – the spiders just can’t get their fangs through all those hairs!

A long ‘childhood’

The larval stage of the Cobweb beetle is known to last one year, although in Scotland the life cycle may take two years! As with many beetles, Cobweb beetles spend most of their life as larvae, with only a brief period as an adult. The larvae moult five times before pupating within their final skin. This then splits along the back, revealing the adult beetle.

Secretive adults

Adult Cobweb beetles are rarely seen, but can sometimes be found under bark with larvae, at the fermenting sap runs made by the Goat moth (Cossus cossus) and by sweeping vegetation under the ancient trees between May and August. Due to their secretive life, the adult ecology is not well understood. However, these beetles belong to the Dermestidae family, and as with most Dermestid beetles, adult Cobweb beetles are likely to be scavengers, eating the remains of other animals under the bark of trees.

Threatened homes

Cobweb beetles are found throughout the UK, but although they are widely distributed in England, they appear to be rare in Wales and are only known from two locations in Scotland. The beetle is threatened due to its rare and declining habitat, and is listed as Nationally Scarce.

This beetle lives on very old or decaying broadleaf trees – especially Oak, but also Plane, Poplar, Sycamore, Hawthorn, Beech and other species. It’s not the species of tree that’s important, but the condition. The tree must be ancient and over-mature, with loose bark, rotholes and other nooks and crannies. These trees provide a wealth of important habitats for other invertebrates as well – particularly those that rely on deadwood for stages of their life cycles, known as ‘saproxylic’ animals.

While Dutch elm disease was devastating for much wildlife, it created a temporary glut of habitat for species such as the Cobweb beetle. However, the Elm trees affected by the disease are now past the stage where they are suitable, and are not being replaced.

Furthermore, people have altered the age structure of Britain’s woodlands through deforestation and commercial woodland management. This has reduced the number of over-mature trees available not only now, but also for the future as it takes hundreds of years to replace these trees. While Cobweb beetles do live on over-mature trees found anywhere – whether in a woodland, hedgerow or parkland – only a semi-natural woodland offers self-replenishing habitat (ancient trees and deadwood) required to support sustainable populations. It is therefore important that we act now to restore our native woodlands, to safeguard the future of fantastic animals such as the Cobweb beetle.