Westcott, Bucks - the year 2021 in review

The cold, dry Spring probably didn't help our Lepidoptera in 2021 and that may go some way to explain why overall numbers of butterflies and moths at Westcott were well down on the previous few years.

Butterflies

2021 was a fairly mediocre year for butterflies in the garden with only 20 species putting in an appearance, the first sighting being of a Brimstone Gonepteryx rhamni on 24th February while the last was a Red Admiral Vanessa atalanta on 2nd November.  Our small resident population of White-letter Hairstreak Satyrium w-album was recorded every day between 2nd and 18th July, with a peak count of three individuals on the 5th.  Of the other regulars, Large Skipper Ochlodes Sylvanus (six sightings), Common Blue Polyommatus icarus (one sighting) and Marbled White Melanargia galathea (eight sightings) didn’t do particularly well and there were no recorded visits at all from Small Skipper Thymelicus sylvestris, Small Copper Lycaena phlaes, Brown Argus Aricia agestis or Small Heath Coenonympha pamphilus.  On 8th November an egg of Brown Hairstreak Thecla betulae was found on young blackthorn growing less than 50 metres away from the house but of the butterfly itself there was no sign during its flight period.  As usual, the “buddleia season” brought the year’s peak daily garden counts for the four common vanessid species, which occurred on 3rd September for Small Tortoiseshell Aglais urticae (12 individuals seen at one time), on 10th August for Red Admiral (14 individuals), on 9th August for Peacock Aglais io (37 individuals) and on 7th August for Comma Polygonia c-album (4 individuals).  The buddleia also brought in seven visits from Painted Lady Vanessa cardui between 2nd and 19th August as well as two from Silver-washed Fritillary Argynnis paphia on 16th July and 10th August, all as singletons. 
 
Butterflies are very occasionally found in the overnight moth traps but in 2021 no less than six individuals from five different species were recorded in this way in the garden.  They comprised Meadow Brown Maniola jurtina on 16th July, Small White Pieris rapae on 25th July, Holly Blue Celastrina argiolus on 5th August, Red Admiral on 3rd September and Small Tortoiseshell on 6th September all to the mercury vapour light and a further Meadow Brown to the actinic light on 5th August.  Holly Blue was the only one I’ve not previously had to a light trap and it becomes the 11th butterfly species to do so in the garden and the 19th species overall within Bucks.

Holly Blue & Meadow Brown from moth traps,
 Westcott 5th August 

For the fifth year in a row there were no new butterfly species added to the garden list and the count here remains at 31, all recorded post-millennium.

The Mothing Year

This was definitely not a vintage year for moth numbers.  While the garden species tally for 2021 was a record high (712, beating 2019’s total of 699), the overall count of individual moths was 29,330 which is a reduction of nearly a quarter over the figure achieved in 2020 for a broadly similar trapping effort.  That’s quite a drop.  The table below gives garden data for the past five seasons, the first three being “normal” years and the last two being COVID years when the various lockdowns meant there was theoretically the opportunity for an increase in trapping effort (although in 2019 I seemed to manage a significant increase without the virus!).  Listed for each year are the number of nights on which traps were run, the number of “trap nights” (taking account of those occasions when more than one trap was run on a given night), the numbers of micro and macro species recorded and the final overall moth count.
 

Year	Nights	Traps	Micro	Macro	Count
2017	279	311	326	336	28189
2018	286	373	346	338	42063
2019	323	443	344	355	54400
2020	320	496	345	330	36784
2021	335	455	380	332	29330

 
Several species which usually manage to achieve very large numbers, for example Dark Arches Apamea monoglypha, Heart and Dart Agrotis exclamationis and Large Yellow Underwing Noctua pronuba, performed very poorly during 2021.  In fact Dark Arches (at 173) and Heart and Dart (at 398) produced their poorest ever totals in the garden.  Heart and Dart was down significantly even on the previous low of 499 achieved in 2008 and this is a moth whose annual counts can sometimes be up in the thousands here, for example 6,447 in 2011 and 4,975 in 2019.  A few species did buck the trend, notably the lichen-feeding Footman moths, but their increase in numbers wasn’t anywhere near sufficient to offset the reductions elsewhere.
 
The number of nights on which the combined species total from the two garden traps exceeded 100 is another method of comparison that I use and is generally dependant on the weather during the peak period for moth diversity (mid-June to mid-August).  In 2021 the species count exceeded 100 on 13 occasions, all of them in July, which surprisingly was an increase of two over the previous year although it was still a disappointing result.  The highest species count on a single night occurred on 18th July with a total of 165 specific species plus four aggregates, a site record.  While it isn’t always the same date that produces the highest count of individual moths, in 2021 it did occur on the same night although at 828 individuals between the two traps this was somewhat lower than the typical four-digit sum usually achieved on the busiest night. 
 
Common Footman Eilema lurideola achieved the highest count of any species (2,029), followed by Setaceous Hebrew Character Xestia c-nigrum (1,624) and Common Wainscot Mythimna pallens (1,060).  Highest amongst the micro-moths were Chrysoteuchia culmella (716), Acentria ephemerella (524) and Agriphila tristella (310).  The “grass moths” actually had quite a poor year with only the early-flying Crambus lathoniellus and Crambus perlella putting in typical counts.  The combined total for the ten common “grass moth” species which visit the garden came to 1,275 as opposed to 3,324 the previous year.    

In 2020 nine moth species had exceeded 1,000 individuals during the year and a tenth was almost there (Dark Arches on 982), whereas in 2021 only three exceeded that threshold with one more almost there (Scarce Footman on 992).  A similar drop in numbers occurred with those noctuids treated as aggregates, the Oligia species (Minors), Mesapamea species (Common/Lesser Common Rustic) and Hoplodrina species (Uncertain/Rustic).  In fact the totals achieved by Uncertain/Rustic (776) and Common Rustic/Lesser Common Rustic (231) were the lowest here for a decade.

As mentioned earlier, the garden species count for 2021 came to 712, comprising 380 micros and 332 macros (the full list can be seen here on Google Sheets).  As usual they included a few which had not been recorded here previously and the site list has now moved up to 1,042 moth species.  The new arrivals comprised: 

 

12.039	Monopis crocicapitella *	18 Sep
14.011	Bucculatrix ulmifoliae *	20 Aug
15.002	Caloptilia cuculipennella *	7 Sep
15.011	Caloptilia falconipennella *	31 Mar
16.005	Yponomeuta rorrella	24 Jul
21.002	Lyonetia prunifoliella	28 Sep
35.004	Aproaerema taeniolella *	22 Jul
35.080	Oxypteryx unicolorella *	16 Jun
37.029	Coleophora orbitella *	16 Jun
37.043	Coleophora niveicostella *	16 Jun
37.095	Coleophora trochilella *	17 Jul
49.187	Endothenia oblongana *	20 Jul
49.350	Grapholita caecana *	10 Jun
62.010	Elegia similella *	9 Jul
62.030	Hypochalcia ahenella *	17 Jun
63.120	Schoenobius gigantella *	16 Jun
70.050	Balsam Carpet Xanthorhoe biriviata	28 Jul
70.083	Cypress Carpet Thera cupressata	18 Nov
72.074	Beautiful Marbled Eublemma purpurina	7 Sep
73.209	Red Sword-grass Xylena vetusta	9 Nov
73.330	Radford’s Flame Shoulder Ochropleura leucogaster	8 Oct
-	Japanese Silk Moth Antheraea yamamai	3 Jul

* confirmed via dissection, with grateful thanks to Peter Hall.

Micro-moths
 
As usual, during the year quite a lot of time was spent searching for leaf-mines on trees and shrubs in the garden but, while many were found, none of them proved to be new for the site so the newcomers on the list above were all adult moths.  However, I did subsequently find active leaf-rolls of Caloptilia falconipennella on our alder on 4th September.  Another very pleasing discovery, co-incidentally on the same date, was a batch of active mines of Chrysoesthia drurella on leaves of orache Atriplex, a weed growing in a redundant flower pot.  The rather striking orange, black and silver adult (a daytime flyer) has been recorded in the garden on a handful of occasions in past years but this is the first time I’ve ever seen evidence of it breeding here.

Caloptilia falconipennella, Westcott 31st March

Mines of Chrysoesthia drurella, Westcott 4th September

Bucculatrix ulmifoliae was recorded as new to Britain in 2006, reared through from unidentified mines found in the leaves of disease-resistant elms from Japan planted in a park in Farnham, Surrey.  In Bucks the adult moth was first found in 2013 thanks to the Rothamsted Insect Survey trap at Burnham Beeches and the two garden records of this species at Westcott during 2021 (a male in August and a female in September) represent the second and third records for the county.
 
Like Caloptilia syringella, which is very common and already well known from the garden, the much more local Caloptilia cuculipennella also feeds on ash and privet, both of which grow here, so hopefully the latter species is now in residence.  There are fewer than 20 records for Bucks and almost all of them from the south of the county.

Caloptilia cuculipennella, Westcott 7th September

Yponomeuta rorrella is likely to have been overlooked in the garden in previous years as I had previously thought it was not possible to separate it from the padella/malinellus/cagnagella group.  However, I am now assured that its markings are in fact consistent (unlike the other three) and with great care it can be identified on external looks mainly from the consistent shape of the grey shading on the forewing.  37 individuals were recorded here between 24th July and 20th August and three of them were given a particularly close check to ensure that they had the white stripe along the underside of the forewing costa which is indicative of this species.  The moth will no doubt have been breeding on the ancient crack willow in our front garden but unfortunately that tree is no longer with us, about a third of it having been blown down in a freak gale during November which rendered the rest of the tree unstable.  That’s a great shame and I’m sure its absence will show up in other ways during future moth seasons.

Yponomeuta rorrella, Westcott 24th July

Amongst the other new species were three Coleophorids, taking the total number known from the garden to 45.  This is a difficult group to identify and of the 28 species recorded here in 2021 I would have managed to confirm just four or five without the help of Peter Hall and his microscope.  Of the new ones, Coleophora orbitella and Coleophora trochilella are both Nationally Scarce while Coleophora niveicostella is a potential Red Data Book species.  The larval food-plants for this trio are Birch & Alder, a range of low-growing plants (including Mugwort, Wormwood, Yarrow, Tansy & Ox-eye Daisy) and Wild Thyme respectively.
 
Having had Aproaerema larseniella in the garden for the first time in 2020, I’d assumed it was the only likely species to be found here from that little family, so taeniolella this year was somewhat unexpected.  Endothenia oblongana was a pleasant surprise (only the third record for Bucks) and Grapholita caecana even more so (only one previous county record), although the latter in particular may be spreading.  Elegia similella is mainly a woodland species and is fairly well spread across the county so was expected eventually, unlike Hypochalcia ahenella which is a chalk grassland moth and must have wandered in from the Chilterns.  I find the large reed-feeding Crambid species quite difficult to tell apart on sight and the female Schoenobius gigantella on 16th June had initially been identified as a possible Donacaula mucronella (for one thing it didn’t seem large enough to be gigantella).  However I was put right by Peter Hall and he also dissected a male of the species from the garden on 22nd July.  There are well over 200 records of gigantella in Bucks but that figure is misleading because more than three-quarters of them come from the Linford Lakes Nature Reserve in northern Milton Keynes where it is obviously well established.  The remainder are mostly from other wetland sites around the county but the garden at Westcott does seem to do particularly well for moths that prefer damp places. 

Aproaerema taeniolella, Westcott 22nd July

Elegia similella, Westcott 9th July

Hypochalcia ahenella, Westcott 17th June

Schoenobius gigantella female, Westcott 16th June

Macro-moths
 
Six species were added to the garden list during the year.  Balsam Carpet is an adventive moth which, since its first discovery in the UK during the 1950s, has been slowly expanding its range alongside that of its food-plant, the invasive Orange Balsam Impatiens capensis.  Orange Balsam is certainly found along the River Thame which at its closest is only five kilometres away from here.  Cypress Carpet is also a fairly recent colonist which since the millennium has expanded its range northwards quite significantly from the original toe-hold along the south coast which was established in the 1980s.  The moth has been a regular sight in south Bucks for quite a while but up until now has rarely been recorded in the northern half of the county.  Beautiful Marbled, Red Sword-grass and Radford’s Flame Shoulder will have been migrant species here, with the first and last of that trio putting in appearances at numerous sites in southern England during the second half of the year.  Red Sword-grass is resident in the west of the UK but rare sightings in the east are usually attributed to migration - at the very least it will have been a long-distance wanderer.  The enormous Japanese Silk Moth came as a great surprise in the actinic trap on 3rd July and its source is certain to have been someone captive-rearing the species in the local area.  There are apparently places in southern Europe where escapees like this have formed local colonies in the wild but that hasn’t yet happened in the UK so far as I’m aware.

Balsam Carpet, Westcott 28th July

Red Sword-grass, Westcott 9th November

Antheraea yamamai, Westcott 3rd July

Surprising no-shows during the year included Frosted Green Polyploca ridens, Spruce Carpet Thera britannica, Engrailed Ectropis crepuscularia, Dusky Brocade Apamea remissa, Nutmeg Anarta trifolii, Dot Moth Melanchra persicariae and Double Dart Graphiphora augur.  Most of those species only visit in small numbers anyway but the first ever garden absence of Engrailed is a bit strange.  I was also just a little surprised not to get Oak Processionary Thaumetopoea processionea again after several males appeared here over the previous three seasons.  On the other hand it was good to see the return of Dotted Fan-foot Macrochilo cribrumalis, Pinion-streaked Snout Schrankia costaestrigalis and Tawny Pinion Lithophane semibrunnea, all three of them after an absence of a few years.  

Despite the overall reduction in moth numbers mentioned earlier, a few common macro-moth species managed to achieve their highest ever annual counts here during 2021 and, apart from the Footman species already mentioned, they included Poplar Hawk-moth Laothoe populi (83), Blood-vein Timandra comae (147), Barred Straw Gandaritis pyraliata (122), Common Marbled Carpet Dysstroma truncata (263), Green Carpet Colostygia pectinataria (220), Willow Beauty Peribatodes rhomboidaria (255), Light Emerald Campaea margaritaria (274), Common Emerald Hemithea aestivaria (72), Pale Prominent Pterostoma palpina (97), Snout Hypena proboscidalis (196), Yellow-tail Euproctis similis (228), Poplar Grey Subacronicta megacephala (101) and Short-cloaked Moth Nola cucullatella (104).  It wasn’t all bad news.
 
On the night of 13th August there was a very unexpected visitor to the actinic trap which had presumably flown in while chasing down what it hoped would be its dinner.  I’ve mentioned before that we have a significant population of Brown Long-eared Bats Plecotus auritus in the local area and they are quite naturally attracted to the insect activity around the garden lights, sometimes two or three at a time, but this is the first occasion that I’ve found one actually inside a trap.

Brown Long-eared Bat in moth trap, Westcott 13th August

 
Migrant moths
 
Fairly poor showings here by the regular migrant moths were balanced by the appearance of three completely new species for the garden, so it ended up being quite a good migrant year for a site so far away from the coast.  The table below gives totals seen and first/last noted dates:

 

Plutella xylostella	18	4 Jun	12 Nov
Udea ferrugalis	12	4 Oct	15 Dec
Nomophila noctuella	15	9 May	8 Oct
Hummingbird Hawk-moth	2	3 Aug	-
Gem	1	14 Aug	-
Four-spotted Footman	1	4 Aug	-
Beautiful Marbled	1	7 Sep	-
Silver Y	75	24 Feb	31 Dec
Red Sword-grass	1	9 Nov	-
Dark Sword-grass	9	29 Jul	3 Nov
Radford’s Flame Shoulder	1	8 Oct	-

Four-spotted Footman, Westcott 4th August

Beautiful Marbled, Westcott 7th September

Radford's Flame Shoulder, Westcott 8th October

Hummingbird Hawk-moth Macroglossum stellatarum had a very disappointing season with just a single moth seen making a daytime visit to our buddleia on 3rd August and then another, most likely the same individual, entering the actinic moth-trap that very night.  Gem Nycterosea obstipata and Four-spotted Footman Lithosia quadra have put in previous appearances in the garden but they are still amongst the rarer migrant visitors here.  Vestal  Rhodometra sacraria failed to appear for the second year running and I had no luck getting Palpita vitrealis or Delicate Mythimna vittelina which both seemed to be around in good numbers elsewhere in the south-east of the UK during the year.

Dark Spectacle Abrostola triplasia (one first-brood individual on 9th June and four second-brood appearances between 3rd and 17th September) has not been included in the above list because it seems likely to be resident locally now.  This has already happened to White-point Mythimna albipuncta, a former migrant which has become established in Bucks over the past decade.  2021 saw 25 first brood individuals of White-point visit the garden traps between 28th May and 7th July and then 222 second brood between 11th August and 10th October.  Of course, migration by a species doesn’t suddenly stop once it becomes established but I’m sure the majority of garden records here will now be locally bred moths.