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A dark crimson underwing moth resting on an oak leaf with its deep red hindwings exposed

The dark crimson underwing is one of the rarest moths in the UK.

Credit: Hans Lang / Alamy Stock Photo

Quick facts

Common name: dark crimson underwing

Scientific name: Catocala sponsa

Family: Erebidae

Habitat: woodland with mature or ancient oaks

Diet: caterpillars feed on oak buds, catkins and foliage. Adults feed on tree sap

Predators: birds and bats

Origin: native

What do dark crimson underwing moths look like?

Adults: are one of our larger moth species, with a wingspan ranging between 58–74mm. They’re business in the front, party in the back – with cryptically camouflaged upper wings, but stunning crimson hindwings.

The dark crimson underwing can be hard to tell apart from the very similar light crimson underwing. They are, as the name suggests, usually darker in colour, but the key feature to look out for is a black, W-shaped central band on the hindwings. In the light crimson underwing, this band is only slightly wavy.

Caterpillars: are masters of disguise. They’re a slightly mottled, grey-brown colour that helps them blend perfectly into the oak twigs they crawl on. They even have a couple of small humps along their back that resemble the lumps and bumps found on oak twigs.

Close-up of the large eyes and face of a dark crimson underwing moth

Dark crimson underwings are partial to tree sap and artificial sugar solutions.

Credit: Malcolm Schuyl / Alamy Stock Photo

What do dark crimson underwing moths eat?

Adults: have a sweet tooth. They feed at sap runs on damaged trees and are partial to the sugar traps and wine ropes put out as bait by moth recorders. As these moths usually fly high up in the tree canopy from late afternoon through the night, these bait traps are one of the only ways to try and spot them.

Caterpillars: feed on the vegetation of large, mature oak trees (both sessile oak and English oak). They begin with the buds and catkins, before moving onto the leaves when these unfurl later in spring.

A dark crimson underwing caterpillar standing on a twig and blending in with its bark-like camouflage

Credit: Malcom Schuyl / Alamy Stock Photo

How do dark crimson underwing moths breed?

Adults are on the wing from late July to early September, looking for a mate. The female lays her eggs on oak trees in August, and they overwinter here until the following April, when the caterpillars hatch.

The caterpillars feed on the oak tree from April until June – munching away under cover of darkness and hiding in crevices in the bark during the day.

The caterpillars pupate in June and July, in a cocoon either attached to the oak foliage or in a gap in the tree bark.

Did you know?

The scientific name Catocala means 'beautiful below', and sponsa derives from the Latin for 'fiancée' or 'bride'.

Swedish zoologist Carl Linnaeus began naming moth species with brightly coloured hindwings in a matrimonial context - thought to reference the colourful undergarments worn by Scandinavian brides!

Where do dark crimson underwing moths live?

These moths are incredibly rare and are only found as a breeding species in parts of the New Forest in Hampshire. Even here, dark crimson underwings are hard to find as they only live in woodland with mature or ancient oak trees.

This species used to call Kent, Sussex and Wiltshire home too, but sadly, any new records away from the New Forest are individuals thought to be on migration, rather than permanent breeding residents.

A dark crimson underwing moth resting on a tree trunk and displaying the bark-like camouflage on its wings

Credit: Malcolm Schuyl / Alamy Stock Photo

Signs and spotting tips

Dark crimson underwing moths are nocturnal. They hide away during the day, using their bark-like camouflage to blend into trees. They take flight again from late afternoon and will spend the night feeding on tree sap.

The best way to try and see a dark crimson underwing is to use sweet, artificial bait that mimics their natural food source. They find home-made wine ropes (strips of absorbent fabric or rope soaked in a solution of red wine and dissolved sugar), and sugar solutions (usually a mixture of black treacle, brown sugar and brown ale) painted onto the trunks of mature oaks are irresistible.

Dark crimson underwing moths are also attracted to light, which means you could, in theory, find one in a moth trap run in the right habitat.

Did you know?

The dark crimson underwing isn't the only creature reliant on oak trees for survival. It's thought that oaks support around 2,300 wildlife species, and oak woods support more life forms than any other native woodland.

Threats and conservation

This moth is now listed as a Priority Species in the UK Biodiversity Action Plan (BAP). It’s very rare, thought to have declined alongside its favoured habitat of mature oak trees and ancient woodland.

Dark crimson underwings also specifically like more open areas of oak woodland, which are hard to come by when so many of these vital habitats are in poor condition for wildlife – neglected, overgrown, and overtaken by invasive plants.