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Small Mottled Willow (above) has probably been Britain's most ubiquitous migrant moth this year. The arrival of 19 more in my trap over the past couple of days has prompted me to post the second part of this blog on our common migrant species.
Part 1 (link here) looked at the seven commonest immigrants - the ones that UK moth recorders are most likely to encounter. The present post considers the next group of eight species that sit in that 'grey zone' between rare and common, although what that means in practice will depend on where you are trapping. Moth recorders on Britain's south and east coasts will generally get better numbers than those inland, but even down here in Dorset I am very happy when any of these species turn up.
The eight moths covered in this post all had 3,000+ British and Irish records between 2010 and 2019, taken from the Atropos website. This is compiled by Sean Clancy, who does a great (and unenviable) job keeping on top of the data using information from county moth recorders. Obviously, these numbers will now be out of date - this year's Small Mottled Willow influx may well move that species into the 'very common' bracket - but still give useful pointers towards what's common and what's not.
Top of this group of eight is the Vestal (below), with 15,405 records from 2010 to 2019.
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| Vestal Rhodometra sacraria - Dorset 22.9.19 |
This is a lovely moth. Linnaeus named it sacraria (from 'holy' or 'sacred') either because its colours reminded him of priestly robes or its simple yet beautiful pattern suggested virtue or chastity. The early collectors who named it the Vestal were clearly thinking of the latter, which is both odd and yet typical of the whimsical nature of our common moth names. (Maintaining a chaste and virtuous existence will of course be the last thing on a moth's mind.)
Identification is straightforward. While the colours can vary (see below), the basic pattern never changes. When resting, the moth has an upright, tent-like posture that differs from many of the other members of its family (the Geometridae) that spread their wings flat or hold them together vertically.
Globally, the Vestal is a widespread species, occurring in Europe, Asia, Africa and South America. Early arrivals can breed in Britain, with 'home grown' second generation individuals adding to summer and autumn migrants. But there is little evidence of that they can successfully overwinter here.
An interesting feature of the Vestal is that its colour depends on the temperature experienced by the growing caterpillar. The 'classic' look of a strong pink stripe across a light straw-coloured background (as in the above photo) suggests that the caterpillar lived through high temperatures, so this moth was probably an arrival from southern Europe. Duller colours indicate that its origin may have been further north. The darker example pictured below, an autumn arrival, may well have bred in England.
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| Vestal Rhodometra sacraria (darker example) - Dorset 27.10.22 |
Britain's migrant moth of the moment, the Small Mottled Willow, comes second on the list with 8,248 records 2010-2019, but the annual average is now surely higher. This species is well known for its irregular habits - it's either feast or famine, with large numbers arriving in some years while other years see hardly any. I trapped only 13 last year (2025), while my 2026 total to date is 228!
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| Small Mottled Willow Spodoptera exigua - Dorset 12.6.25 |
At first sight, Small Mottled Willow is a rather non-descript member of the noctuid family. However, despite a wide variation in size (which we have certainly seen this year), they are easy to identify once you get your eye in. The pinky-orange oval and kidney mark are good clues, but it is the moth's posture that is the real give-away. When at rest, its wings are held in a similarly tent-like manner to the Vestal, and often partly rolled under the body, giving a generally narrow appearance. Pale Mottled Willow is perhaps the most likely confusion species, but that has wider wings that are held flat when at rest.
Known also as Beet Armyworm, Small Mottled Willow is found from North America to Australia, and causes significant damage to crops in tropical and semi-tropical lands. Its capacity to trouble farmers in Britain is severely limited by its inability to survive the colder winters here. For the time being.
Third on our list is Scarce Bordered Straw, with 7,071 records 2010-2019. Given that this moth is more likely to be seen here than its close namesake the Bordered Straw (see later), its name is rather unhelpful - which applies to a number of other British moths, as it happens.
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| Scarce Bordered Straw Helicoverpa armigera - Dorset 1.9.24 |
Scare Bordered Straw is one of the more tricky moths to identify because it lacks any one obvious distinguishing feature. The fact that it's a variable species doesn't help. At first sight, it is a classic member of the noctuid family (see my blog on moth family ID here), with a chunky, rather thickset appearance and a flat resting position. Consistent features are the oval and kidney marks which have fine dark outlines and central spots. Also look out for the wavy black cross-lines and the dark outer band (at the bottom of my photos). Here is a more red-brown coloured individual:
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| Scarce Bordered Straw Helicoverpa armigera (reddish-brown colour) - Dorset 27.10.22 |
Also called Old World Bollworm, Scarce Bordered Straw is a major agricultural pest in tropical and subtropical regions of Africa, Asia and Australia, as well as being found across much of Europe. Its caterpillars are however rarely seen in Britain.
Sticking with the noctuid family, our fourth species is the Pearly Underwing (below), with 5,663 records 2010-2019. Known also as the Variegated Cutworm, and is found in North and South America and East Asia as well as Southern Europe. As with the previous two species, it is an agricultural pest in warmer climates.
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| Pearly Underwing Peridroma saucia - Dorset 17.6.26 |
Like Scarce Bordered Straw, Pearly Underwing seems at first glance to be a rather undistinguished moth. But although its pattern and colouration are variable, there are several distinctive features. First, it is a large moth, its size range overlapping with that of the familiar Large Yellow Underwing. Second, as the name suggests, its hindwings have a pearl-like translucence, although this is set against dark veins and a dark band around the edge - and getting the moth to show you can be a bit of a challenge. Don't expect to see a shining white hindwing. But the best identification character of all requires you to look at the moth from the side, which reveals a narrow ridge of scales like a Mohican haircut - strictly, the thoracic crest (below).
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| Pearly Underwing Peridroma saucia (side view) - Dorset 17.6.26 |
While three of the commonest immigrant moths to Britain are classed as micro-moths (see my previous migrant moth blog), only one of the present group of eight falls within that category. It's one of my favourite species - the Olive-tree Pearl Palpita vitrealis (below), after which this wildlife blog was named. There were 5,065 records of this species between 2010 and 2019.
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| Olive-tree Pearl Palpita vitrealis -Dorset 10.8.20 |
Identification is straightforward, as the combination of plain, translucent white wings with a brown leading edge (costa) appears in no other British moth species. While the wings of the Box-tree Moth Cydalima perspectalis are translucent, they usually have thick black edges - and the moth is much larger.
Olive-tree Pearl is a member of the crambid family, many of which seem to have a yearning to travel long distances; these include two very common migrants (Rusty-dot Pearl Udea ferrugalis and Rush Veneer Nomophila noctuella) featured in my previous blog and a number of much rarer arrivals to our shores. Primarily an old world species, Olive-tree Pearl is found in Europe, Africa and Asia: I have recorded it in India (Goa), where it was a nice surprise to see a familiar moth in such exotic surroundings.
Despite its name, the caterpillars of Olive-tree Pearl feed on a variety of species including Ash, privets, forsythias, Strawberry Tree and Summer Jasmine as well as Olive. To date, there are no records of it having successfully bred in Britain, although egg-laying has been observed. Once again, the moth cannot successfully over-winter here.
Heading back into the world of macro-moths, our sixth species is Bordered Straw (below), with 3,789 records between 2010 and 2019.
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| Bordered Straw Heliothis peltigera - Dorset 21.9.20 |
Bordered Straw is a helpfully distinctive moth. Despite some variation in its background colour (see below), the overall pattern of markings is generally consistent: a dark kidney mark contrasting with a lighter and plain background. Compare these images with those of Scarce Bordered Straw earlier in this post: Bordered Straw looks 'cleaner', especially in its lighter forms.
Predominantly an old world species, the range of Bordered Straw is centred on Southern Europe, North Africa and West Asia. Unlike the other moths described in this blog post, it is sometimes seen flying during the day searching for flowers. I spotted a day-flier on Grange Heath, Dorset last month (below), showing the dark band on its hindwing which is usually hidden when the moth is at rest.
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| Bordered Straw Heliothis peltigera - day flying at Grange Heath, Dorset 22.5.26 |
Like the Vestal, as already described, the background colour of Bordered Straw is affected by the temperature experienced at the larval (caterpillar) stage, with lighter individuals likely to have bred in warmer climates. Early arrivals to our shores can breed here, giving rise to a second generation that can appear darker than the first arrivals: compare the September individual above with one from in June (below) which was almost certainly a migrant arrival. However, the moth is unable to over-winter here.
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| Bordered Straw Heliothis peltigera - Dorset 25.6.25 |
Coming in at number seven, the Convolvulus Hawk-moth (below) is one of the largest moths found in Britain. It is of a similar size to the resident Privet Hawk-moth and only beaten by the massive Death's-head Hawk-moth, an extremely rare migrant species that has yet to find my garden (but I live in hope). There were 3,475 records of Convolvulus Hawk-moth between 2010 and 2019.
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| Convolvulus Hawk-moth Agrius convolvuli - Dorset 6.9.16 |
While the plain and streaky wings of Convolvulus Hawk-moth are similar to those of Pine Hawk-moth, the size difference easily separates the two species - the wing length of Pine Hawk-moth rarely exceeds 40 mm while Convolvulus can reach 55 mm. That is in the size range of Privet Hawk-moth, which also has pink stripes on its abdomen. However, Privet Hawk-moth has a distinctly black thorax (not grey) as well as dark-brown forewings and (if visible) two black bands on the hindwings, all features that are missing from Convolvulus Hawk-moth.
In Britain, Convolvulus Hawk-moths tend to arrive late in the season with numbers peaking in September. Adults are particularly attracted by the nectar of tobacco plants (Nicotiana) and can be observed feeding on flowers at dusk. Some moth enthusiasts have this plant in their gardens for that purpose; I'm afraid that I don't, but I still manage to see a few Convolvulus Hawk-moths here in Dorset each year.
Like the previous two species, Convolvulus Hawk-moth is restricted to the old world, and is distributed widely across Africa, Asia, Australia and Southern Europe - although most European records appear to relate to individuals that have travelled from Africa. Those arriving in Britain may well mainly be the offspring of these first generation migrants, hence the late arrival times. While the moth's larval foodplants (bindweeds, as the name would suggest) are frequent in Britain (especially in my garden), the climate appears too severe to allow it to over-winter here.
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| Convolvulus Hawk-moth Agrius convolvuli giving a me sharp stare - Dorset 26.9.24 |
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| Gem Nycterosea obstipata (male) - Dorset 2.5.25 |
The Gem is not a very conspicuous moth, being slightly smaller than Garden Carpet. Unusually for a moth, the male and females have different wing markings (sexual dimorphism, in the jargon). The females have dark wings, with a distinct black dot edged with white - presumably the 'gem' of the common name. (All of my Gem captures have been males, so I don't have a picture of the female. However, there's a good image on the UK Moths website here.) Males have a lighter brown or orange-brown wings, with many cross-lines. While there is also a black dot, this usually lacks the white ring, although a lighter circle is visible in my image above.
Primarily a European species, the Gem is another migrant that can breed, but not overwinter, in Britain. In Dorset, it can occur at any time between April and December; my three records were in April, August and November. While most moth migrants to our shores are more common in the south and east, the Gem seem to particularly favour these areas and has tended not to venture north of Yorkshire and Furness - or over into Ireland for that matter. Having said that, its British distribution increased significantly between 1990 and 2016 (Randle et al, 2019), so this migrant may become a more familiar sight in northern latitudes. And, I hope, in my Dorset moth trap.
Right - this 'gang of eight' concludes my brief survey of Britain's most common migrant moths. As will be seen from the Atropos website mentioned above - or indeed any good field guide - very many more moth species reach our shores, but most arrive in much smaller numbers (sometimes only single figures). While I have been lucky to see a number of real rarities (perhaps the subject of a future blog), many more migrants have eluded my trap, which is both a matter of frustration (especially when other Dorset moth recorders have been more successful - you know who you are!) but also a useful incentive to go on searching. But be warned - hunting for migrant moths can become a bit of an obsession ...
References
Emmet, A. Maitland (1991) The Scientific Names of the British Lepidoptera: their history and meaning. Harley Publications.
Parsons, M. and Clancy, S. (2023) A Guide to the Pyralid and Crambid Moths of Britain and Ireland. Atropos Publishing.
Randle, Z. et al. (2019) Atlas of Britain and Ireland's Larger Moths. Pisces Publications.
See also the field guides and websites listed in a separate blog post (notably Waring & Townsend and the Dorset Moths website).
Atropos migrant moth totals are here.
Global range data is from GBIF.
Text and images (c) Mike Hetherington
