Moth Report #21

The Delicate, The Golden-rod Pug and the Barred Rivulet

Welcome to my new subscribers (indeed, to all my subscribers!). This introductory blurb (in italics) will remain much the same from post to post; please skip it if you’ve read previous posts.

This newsletter will remain free, I’ve no intention of converting it to a paid subscription. It’s aimed at readers who have a general interest in nature and natural history, but maybe don’t know very much about moths. It’s not really aimed at people who regularly run their own moth traps, but of course they’re welcome to read it (even to criticise it if they wish!).

I run a light trap in my garden in Eastbourne (Sussex, UK) and the main objective of this newsletter is to post photos of some of the moths (and occasionally, other insects) that are attracted to the light, and to say a little about them. On average I run the trap about one night in three, and the plan is to write one post a week reporting on that week’s trapping sessions, and featuring two or three of the moths seen in the previous couple of weeks.

In the morning I photograph any catches of special interest, then all moths and other insects caught in the trap are released (if they haven’t escaped already!).

This is my ninth year of running a garden light trap, firstly three years in Eastbourne (2017-2019), then three in Hurstpierpoint (2020-2022), and then back in Eastbourne (2023 onwards). Hopefully, yet more house moves are off the cards for the foreseeable future!

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Report for w/b 18th August

This is the first issue of the report under the new schedule of weekly editions published on Tuesdays, and it covers the week from 18th to 24th August (inclusive). But the report won’t look very different from the previous ones, because in fact I’ve only run the trap once in the week covered, on the 22nd August. (The next report will probably cover at least two trapping sessions).

Anyway, the night of 22nd August was quite mild with hardly any wind or moon, and it turned out the be the best night so far for this August, with 204 moths of 54 species. Even better, two of the moth species were ones I’ve never seen before. One of these (Oncocera semirubella) I posted a brief note about on Saturday; the other forms the third entry in the moths selected for a more detailed discussion below. This brings the total species count for the year so far to 358, of which 25 are ‘new for me’ (what birders would call ‘lifers’). This total already beats my annual totals for the last two years (357 and 350 species for 2023 and 2024 respectively), so the year looks set to be a record for this garden at least (my overall record is 398 species, achieved in 2022 in my previous garden in Hurstpierpoint; some way to go still to beat that!).

One other notable feature of this trap is that there were no Jersey Tigers (first such night since 25th July). So it looks like they’re finished for the year now. The year total is 87. I wrote about this species in an earlier report, in which I described the growth of this species over the years I’ve been trapping, with a maximum count of 64 reached last year. Well, this year’s 87 beats that, so it seems the growth is still continuing. Incidentally, I made a mistake in that earlier report, when I said I had never seen the yellow form of the moth in Eastbourne. But on checking my records later I noticed that one of the 38 I recorded in 2023 was actually a yellow one! But that remains the only one I’ve seen here as all 87 this year were red.

Anyway, on with some individual species descriptions.

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The Delicate, Mythimna vitellina

Year total to date: 1 (17th August)

This moth is generally assumed to be a migrant (the majority of Sussex records are within a few miles of the coast), although it’s thought it might sometimes succeed in breeding in the UK. It’s main flight season is in September and October, although in recent years there has been a smaller wave in June and July.

From 2017 to 2022 I typically saw one or two of these each year, always in September or October, but last year the total shot up to 11 (with 7 of them on a single night in October), and for the first time I had one of the early wave in July. The fact that this year my first one appeared in the middle of August might just be a reflection of the fact that autumn seems to be arriving early for everything this year.

The genus Mythimna is a largish genus of medium-sized, usually pale-coloured moths with some 13 UK representatives, all of which have caterpillars which feed on grasses or sedges. This is the first member of the genus to have been selected for my newsletter, but I’ve seen six other members so far this year and no doubt I’ll report on some of the others in due course. The genus name is derived from Mithymna, an ancient town on the Greek island of Lesbos. Why this town was chosen as the genus name for these moths is not recorded, but the authors of such names often liked to draw on classical themes, perhaps to lend poetic or scholarly weight to the name (or maybe, just to show off their classical knowledge!). Nonetheless, if you have a poetic frame of mind the moths in this genus do seem to have an air of understated elegance about them, which fits well with the cultured lyricism of the poetry of Sappho, herself a native of Lesbos.

There’s no suggestion that this particular species is physically any more delicate than any of the others in the genus, indeed it can’t be so as it regularly migrates across the English Channel. It is likely its common name refers to the subdued markings on the wings, which are always present even though the background colour of the wings varies from pale yellow through to orange-red. The depth of the background colour has been shown to be a function of the temperatures the caterpillar experienced during development, with warmer temperatures leading to paler adults. And that brings us on to the species name, vitellina; it’s derived from the Latin vitellus, meaning ‘yolk’, so it’s actually describing the background colour of the wings.

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The Golden-rod Pug, Eupithecia virgaureata

Year total to date: 1 (13th August)

I think the time has come to grasp the nettle and talk about the pug moths. They are all placed in the genus Eupithecia; in the UK there are about 44 recorded species. They all sit with their wings flat and they have wingspans¹ varying between about 18 and 24 mm (but can be quite variable within a species). A few of the species are very distinctive, two or three of them (which I’ve never seen) are really very handsome, but many of them look incredibly similar to one another. People who are just beginning to get interested in moths are usually advised to leave the pug moths well alone until they’ve gained more experience. Indeed even seasoned moth enthusiasts have been known to express their exasperation when trying to identify some of the Eupithecia species. With experience, it’s usually possible to identify a fresh specimen, but once they get worn it’s frequently impossible. And also, many of the species (including the Golden-rod) have melanic forms, these all look virtually identical!

This difficulty in identifying many of the Eupithecia moths might have had something to do with the choice of the genus name; it comes from the Greek eu- (a prefix meaning ‘well’ or ‘good’ and pithekos (meaning ‘ape’ or ‘monkey’). It’s the monkey’s reputation for agility and the ability to play tricks on us that is the possible connection.

The specimen that generated this entry in the newsletter is a moth that appeared in my trap on 13th August, shown top left in the picture above (the date shown on the picture is wrong, but it took quite a lot of fiddling about to put the collage together and I didn’t fancy doing it all again just to move one date a few days!). I’d like to record it as a Golden-rod Pug, but as you can see it is very similar to at least the two other pugs shown on the right of the picture. There is also a question mark over the one shown bottom left, from last year. This one has all the text-book features of a Golden-rod, including the pale spot on the thorax, although this is not present in all examples so this-year’s moth can’t be ruled out just because it doesn’t have such a spot.

At the root of the problem is the fact that the Golden-rod Pug is mainly found in the north-west of England; it is quite rare in the south-east. The County Moth Recorder for Sussex, Colin Pratt, who is enormously experienced and highly regarded for his expertise regarding Sussex lepidoptera, wrote this about the Golden-rod Pug in a book published in 2020:

In imminent danger of extinction [in Sussex], the moth was last seen in West Sussex in 2008 and in East Sussex in 2013. Adults are scarce, although larvae can be locally common along a very few woodland rides and clearings. Distinctly irregular in appearance, and in decline.

So he’s always very cautious in accepting a photographic record as a Golden-rod if there are other possibilities which fit the photo among the more common pugs. He maintains that the one I’ve labelled as Golden-rod from last year is actually a weakly-marked Freyer’s Pug (cf the photo bottom right). I’ve not actually shown him the one from the 13th (yet); I’m pretty sure he won’t accept it as a Golden-rod!

Just to steer away from moths briefly, many readers will be familiar with the Holly Blue butterfly (actually there’s a pub near me called the ‘Holly Blue’!). This butterfly has two generations a year, and what is rather unusual about it is that each generation has a different foodplant. The adults which are on the wing in April and May produce caterpillars which feed (mainly) on the buds of holly bushes, then these caterpillars produce adults which fly in late July and August, and these adults give rise to caterpillars that feed (again mainly) on ivy buds. These caterpillars then pupate and the butterfly overwinters as a pupa.

What, you might wonder, is the relevance of this information about the Holly Blue butterfly to the Golden-rod Pug? Well, this pug also has two generations a year, and like the Holly Blue the foodplant is different for the two generations. The second generation flies in late July and August, and the caterpillars resulting from that generation feed on the flowers of Golden-rod (and also those of Ragwort). These caterpillars then pupate, and overwinter as a pupa, giving rise to the next year’s first generation of adults in May, but there’s a degree of confusion about what foodplants the resulting caterpillars use. They’ve been found on the leaves of various deciduous trees, but never in sufficient numbers to confirm a preference for particular species. Some 50 years ago or so the first generation of adults was typically the larger, with only a small second generation, but today the pattern seems to have reversed and the July/August generation is larger than the May generation.

And finally, the species name, virgaureata. One of the foodplants, the European Goldenrod, has the scientific name Solidago virgaurea, so the species name for the moth is linking it to the foodplant. The species name for the plant, virgaurea, is Latin for ‘goldenrod’: virga; rod or wand (compare with ‘virgule’, the name for the forward slash punctuation mark “/”), and aurea; golden. The final -ata of the species name implies some form of possession. So the moth’s species name says something like ‘belonging to the goldenrod plant’.

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The Barred Rivulet, Perizoma bifasciata

Year total to date: 1 (22nd August)

This moth belongs to a small group of six moths with ‘rivulet’ in their (English) common name, of which this is the third I’ve seen. These species belong to the larger group described as ‘carpet’ moths, which I wrote about in a previous edition. The word rivulet refers to the wavy whitish line which crosses the wings, with a darker line in the centre, which is present to a greater or lesser extent in all six species. This particular species actually has two such lines, hence its species name bifasciata, meaning ‘double-banded’ (it could alternatively mean ‘two-faced’, but I don’t think that would apply in this case!). The genus name Perizoma also refers to the banding, since Peri- means ‘about’ and -zoma means ‘girdle’ or ‘belt’. While there don’t appear to be many words in European languages derived directly from the Greek zoma, its Latin form (zona) has given rise to ‘zone’ and its many derivatives.

The caterpillars of the Barred Rivulet feed on the seeds of a plant called Red Bartsia. Although not a rare plant, in Sussex it is mainly a downland plant, and I became aware of it only a couple of years ago when I found some growing on a small area of chalk downland near Eastbourne.

Red Bartsia

Then this year, there’s suddenly masses of it growing on an adjacent chalk meadow; the plant experts I know say they’ve not seen it growing in such profusion before. This leads me to wonder whether there’s any link between the sudden explosion in the growth of the foodplant, and the fact that I’ve seen my first ever Barred Rivulet in the same year. Because although the moth has a fairly standard single generation each year, overwintering as a pupa with adults appearing in July and August, it has an unusual feature in that the pupae have been observed to wait for more than one winter before emerging. So it’s perhaps just possible that the pupae will wait until the weather conditions are optimal for the Red Bartsia to thrive, before deciding whether to emerge or whether to wait another winter before doing so!

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Well, that’s it for this edition. The next one is scheduled for Tuesday, 2nd September.

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1. In the days before digital photography, when pictures in books always showed the moths in their ‘set’ position, i.e. with the wings held out horizontally, the size of a moth was invariably expressed in terms of the wingspan. But these days most moth photos show the moth in its natural resting position (like in the photo of the Delicate at the start of this edition), where the concept of a wingspan is less useful. So most books these days indicate the size of the moth using a measurement of the forewing length, measured along the costa (the leading edge) of one of the forewings. But for pugs, which always sit in more or less the classical ‘set’ position, the wingspan is still sometimes used; it’s easier to measure than the forewing length. (But neither method works very well when it comes to the females of those species where the females are wingless (like the Vapourer)!)