Catocala fraxini — “Blue underwing”

The Catocala, also known as underwing moths, is a very large genus of moths, counting over 300 species. Generally speaking, all of them have cryptically colored, well-camouflaged forewings that come in ‘boring’ colours; many shades grey, brown, black and white camouflage these moths quite well when they are resting against a surface such as tree bark. It is not until they lift their hindwings that their true beauty becomes visible; the hindwings of most ‘underwing’ moths are quite colourful. Red, pink, blue, orange, yellow, black and white;  the hindwings of these moths can be works of art.

Catocala fraxini, the blue underwing moth or in the United Kingdom the Clifden nonpareil, is one of those species. It is the largest Catocala species found in Europe, and the hindwings are blue. These large moths are active mid to late summer, usually in July and August. During this time, the moths hide during the day, camouflaging themselves quite well in plain sight on tree bark, or seeking shelter in cool and shaded places hotter days. In Eastern and Central Europe, this moth can be quite common. In Northern Europe however, it is a great rarity with very localised populations. This is most likely because the species is a a little bit out of it’s comfort zone in temperate north Europe, and prefer the hotter summers of central and east Europe and is often restricted to the warmest habitats. The moths are also skilled at flying, and are able to migrate greater distances, occasionally ending up in unusual places. Despite being common in some localities, the moths are rarely taken at light traps – they just seem to be less attracted to artificial lights than other moths are. There is, however, an alternative way to find them. This moth has a great appetite; yet it never visits flowers and isn’t interested in nectar. Instead, they are fructivorous – they will come to rotting and fermenting fruits that have fallen off the trees, to suck up the sugary fluids. In the wild, it is possible to attract them with a mix of something sweet (banana, peaches, syrup, sugar) and something alcoholic (beer, wine). Fermenting fruits in the wild also contain a small percentage of alcohol, so the moths are attracted to this volatile chemical. Mashed sugary fruits such as banana or peach mixed with wine or beer, can be smeared over tree bark and will attract underwing moths if the conditions are right.

Catocala fraxini adult – beautiful and azure blue, like the ocean  

In the wild, the moths can live quite long, possibly over one or two months; they are only active at night (nocturnal) and will, besides drinking the sweet fluids from wild fruits and tree sap, find a partner and pair. The eggs are then deposited on their host plants. The single and most important host plant of this species is popular tree (or cottonwood to Americans); genus Populus.  However, in a small percentage of cases, this species will lay eggs on willow (Salix) and birch trees (Betula). However, in the majority of cases, this species is associated with poplar tree (Populus). The eggs of this species overwinter, and do not hatch the same year they are laid. Instead, they spend 5 to 7 months in the egg stage, waiting for winter to arrive and pass, only to hatch in spring. In spring, the small caterpillars start feeding from the leaves of the host plants, and grow rapidly. They pupate in 1.5 to 2 months time – the warmer they are kept, the faster they grow. Pupae are spun in very thin, ‘lazy’ cocoons. Using a minimal amount of silk, caterpillars hide themselves between leaves (leaf litter on the ground or leaves in the tree) or in crevices in tree bark. Instead of spinning a full cocoons, larvae lazily stick leaves or debris together with their silk, and pupate in the cavity. The pupa is long, and covered with white waxy powder. Pupae hatch in 3 to 5 weeks time and the moths hatch mid to late summer (July, August).

Catocala fraxini caterpillar 

The caterpillars of this moth are very rarely encountered in the wild. The first reason: their host plant is extremely tall (Populus); older trees can be 15 to 50 metres long – one of the reasons they have been commonly used as wind breakers. The caterpillars that reside in them and feed on the foliage, are often far out of reach of humans. Secondly, their camouflage is brilliant, and caterpillars almost completely dissapear in their surroundings if pressed tightly against the tree branches. Thirdly, the caterpillars are mainly nocturnal. They do eat during the day, but scarcely. Only in the darkness do they become confident enough to move a lot. Caterpillars are often only spotted when they leave the trees in to pupate in leaf litter on the ground, but this is rare.

  • Difficulty rating  Moderate (Not very hard, but you need experience)
  • Rearing difficulty: 6.5/10 (From egg to pupa)
  • Pairing difficulty: 7/10 (Archieving copulations)
  • Host plants:  Most importantly poplar tree (Populus) but also Birch (Betula), Willows (Salix). Literature mentions other food plants but I am not confident about these. 
  • Natural range:  Europe  (Northern Europe, Central Europe, Eastern Europe); Asia (Russian far east, Japan, Korea, China). 
  • Polyphagous: yes  
  • Generations:  Univoltine (one generation)
  • Family:  Erebidae – Erebinae (true Erebid moths)
  • Pupation: Naked pupa attached to leaves/tree bark
  • Prefered climate:  Temperate (palearctic)  
  • Special notes:  Overwinters as egg
  • Estimated wingspan: 65mm-100mm (very large Catocala)
  • Binomial name: Catocala fraxini (Linnaeus, 1758)

As mentioned before on this page, the eggs of Catocala fraxini overwinter. Thus, fertile eggs must be kept cold for several months – otherwise they will never hatch. They are very strong and frost resistant however. They can be overwintered in plastic or glass tubes, envelopes, plastic containers and more. Be creative! It’s important to keep them cold. If not kept properly cold for a long time, they will never hatch – it is a biological requirement. They can be kept outdoors in winter, if your environment is temperate and you live in the palearctic realm. Otherwise, a fridge may be used.

Catocala fraxini eggs in a tube with cotton wool 

In spring, if warmed up, very tiny caterpillars will quickly hatch. The most difficult life stage in captivity for Catocala fraxini, is certainly the first instar of the very small caterpillars. When they are freshly hatched, the caterpillars are very vulnerable and sensitive. They can also run very fast, and are very prone to escaping. If the lid of their container is removed, the almost invisibly small caterpillars will run in all directions in a panic, and quickly escape the container if you are not careful. They are also very thin and easily crushed if mishandled. It is important to hatch the eggs at the correct time of the year; preferably not too early. The very early leaf of Populus is very sticky and less suitable as food for caterpillars. Try to be patient and wait until the young leaves expand and mature a little bit to hatch your eggs.

Catocala fraxini L1

The young caterpillars of Catocala fraxini walk the same way as Geometridae (“loopers”) are famous for. They only use their head/thorax and prolegs on their hind segments to move. This is infect, normal for many Erebinae species when it comes to the early instars (I have seen this in Ascaphala odorata, Thyas juno, Catocala hatchlings).  If not stressed too much and given fresh leaves, they will settle on their food plant and start eating. The small caterpillars can be raised in plastic boxes or sleeved on Populus.

Catocala fraxini L1 container 

Once the caterpillars grow bigger, move  them to a well ventilated environment. Do not treat them as silkmoths! These caterpillars hate being trapped in plastic containers and will often quickly die in them. It is only acceptable for the very small instars. Afterwards they need to be moved to well ventilaged cages ASAP, or to a rearing sleeve.

Catocala fraxini in a cage

The young caterpillars are difficult, but once they grow bigger, they become suprisingly strong and easy to raise. Once you have passed this  main obstacle, the breeding becomes quite easy. Caterpillars do well in room temperature conditions indoors, raised on cut branches of poplar tree, kept fresh by sticking them in a water-filled bottle. This way, the food plant only needs to be changed every 3 to 5 days. They can even be raised in high numbers in one cage, although it is never a good idea to push your luck and overcrowd them. The caterpillars mainly feed at night, so make sure to give them a dark night. Don’t leave them in a room that constantly has the lights turned on. They will also eat a little during the day, but not much. Most of the time, they are hidden during the day, pressed tightly against the tree branches. This species prefers a warm and dry environment. Don’t keep them or their food wet for a long time, this could make them sick.

Catocala fraxini L3(?)

Catocala fraxini L5(?) can be hard to see on the tree branches, even if they are fully grown and big 

Catocala fraxini (L5?)

The caterpillars will be fully grown in 1.5 to 2 months time (on room temp). The warmer you keep them, the faster they will grow. When they are fully grown, the caterpillars will descend to the floor of their cage, and spin a very thin cocoon under leaf litter. If you want to help the caterpillars, put some dried leaves on the floor of their cage, to make pupating easy for them. Be careful: in more rare cases, the caterpillars also spin well-hidden cocoons between the leaves of their food plant and not on the floor. Inspect the plant before throwing old food away.

Catocala fraxini cocoon; it was hidden under a leaf 

In a short time, the caterpillars pupae. The pupae of Catocala are thin, elongated and long. Curiously, they are covered in a white powder that seems to be some type of wax; this may act as a hydrophobic agent (water resistance) or dry lubricant that makes it easier for the moths to hatch smoothly.

Catocala fraxini pupa

In 3 to 5 weeks time, beautiful moths will hatch from their pupae. Catocala fraxini is as hyperactive as it is beautiful. In captivity, the risk of them escaping is very high, so be careful when handling or feeding them. One small touch is enough to send them into a frenzy, flapping wildly and spastically with their wings. Interestingly, these moths can also be startled by vibrations and sounds. Whereas most moths will flee if directly touched, Catocala fraxini react strongly to sight and vibrations. What is also interesting, is that over time, they will become calmer. It almost seems as if the moths become used to being in captivity, and old specimens are more easily handled and fed than fresh ones; this is perhaps more a form of sensory habituation, where the insects become more tolerant to stressful stimuli, than it is actual learning.

Catocala fraxini imagoes

The moths are nocturnal, and during the day they sit tightly pressed against a surface, camouflaging themselves with their forewings. In the wild this would occur on tree bark, but in captivity they can be seen sheltering against the cage walls. At night, the moths become very active and can fly for several hours.


Catocala fraxini imagoes

These moths need to feed. Their preferred food is something sugary with a trace of alcohol. Banana, peach, kiwi and many other fruits high in fructose are acceptable to them (but they don’t like apples or citrus fruits, or anything low in sugar or highly acidic). Add a little bit of wine or beer for a stronger effect. The moths can feed themselves and do not need help, just put them in a well ventilated room to help their sense of smell, and leave some food in the cage – they will find it if hungry enough. In the wild, they feed on rotten fermenting fruits and bleeding trees (tree sap can be very sweet).

Catocala fraxini eating wine/banana mash

Pairing Catocala fraxini is not very difficult, but some people can struggle with it. Important is not to treat them like most other species. Most moths are in a rush to breed in captivity, and will quickly pair in a few days if males and females are kept together in a cage. Catocala fraxini are not in a rush. The moths can live for several months if well-cared for. The trick is to feed them generously, and keep them in a well-ventilated space, with warm and breezy nights. One day, males will find the females and pair. Eggs are deposited on tree bark – or in captivity, the cage walls.

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Citations: Coppens, B. (2019); Written by Bart Coppens; based on a real life breeding experience [for citations in literature and publications] 

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