Noctuidae : Noctuinae
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Fen Square-spot
Diarsia florida

(Schmidt, 1859) 2124 / 73.335
Photo © Andrew Rhodes,  Coniston Cold

Similar Yorkshire Species
Small Square-spot
Diarsia rubi
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Express Record Fen Square-spot

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Yorkshire Status: Scarce and very local resident.

Sutton & Beaumont, 1989: In many parts of Britain it is almost impossible to tell this species from D. rubi (Vieweg) since they may look very similar and both are univoltine in some areas and bivoltine in others. In Yorkshire, however, D. florida is generally single brooded, appearing in July whereas D. rubi is double brooded, appearing mainly in June and August. Also there tend to be visual differences in size and colour so we can be sure that this species does occur, although it may well be under-recorded due to general confusion and some overlap in flight times. The records suggest it is very local on low, damp ground, and P. Q. Winter notes that some VC61 records are from arable areas where only small patches of habitat exist in the form of ditches and roadside verges.

2012 (CHF): This species and Small Square-spot appear to be in the process of evolutionary separation and there is some confusion in identification. Fen Square-spot is thought to be univoltine across the country (not bivoltine in some areas as stated above) whereas Small Square-spot is bivoltine except for northern Scotland, so Fen Square-spot in Yorkshire flies in late June and throughout July, ie between the two broods of Small Square-spot. The two are not easy to tell apart but Fen Square-spot is larger, paler and brighter and likes fens and acid bogs. It is also reputed to fly later at night and rarely appears before midnight. Wing markings are identical. The genitalia are similar but subtle yet distinct differences have been noted. Most Yorkshire records come from the east of VC61 and 62 and there were three records in 2011 after a gap of four years. There are several old records from Askham Bog and Skipwith Common but it has not been seen at either site for many years.

Argus 69, 2013:The univoltine form of Small Square-spot, usually large and paler. The two species are probably in the process of evolutionary separation.

VC65. Foxglove Covert, 19.7.2013 (CHF, JCW, AJW). NEW VICE-COUNTY RECORD.

2020 (CHF): Do you believe in Fen Square-spot? I do. At least I think I do, as do most people in Yorkshire if you look at the Atlas. Not everyone would agree and the Atlas shows dots in very few other parts of the country but it does suggest it occurs in Wales and East Anglia.

It was only proposed as a distinct species in 1950 by Cockayne so in Porritt's time it didn't officially exist. He does however describe "two very fine and striking yellow forms regularly at Askham Bogs" found by Samuel Walker, which he called flava and ochracea. These specimens still exist and look very like Fen Square-spot. Sutton and Beaumont felt sure that it was a good species and thought that it was probably under-recorded. Several sites were listed, mostly on lower ground in the centre and east of the county. Continental authors also largely seem to be happy with it as well, and Noctuidae Europaea points out the larger wingspan (32-35mm against 30-34mm for Small Square-spot), the more reddish or pale reddish colouration, the striking yellow forms as described by Walker, and the lighter hindwing. Apparently in Sweden the two species are both univoltine and tricky to separate, but interestingly it suggests that there are subtle differences in the genitalia. British literature says the genitalia are identical.

So, Fen Square-spot is a larger, paler version of Small Square-spot which flies from late June to early August, ie between the two broods of Small Square-spot. It comes out when the first brood of Small Square-spot is dying down and looking tatty, and finishes about the time that the second brood is coming out. This second brood is smaller and darker than the first brood so there is no problem distinguishing it. Another feature is that Fen Square-spot is said to come out (quite specifically!) between 00:30 and 2:00am according to Skinner. I believe this to be true, as once when trapping at Ashberry Pastures, we recorded a lot of moths and all retired to bed soon after midnight, leaving some of the traps running overnight. We did not record Fen Square-spot. Examining the traps the following morning revealed 17 Fen Square-spots, so they all certainly arrived after midnight.

It really is quite a different-looking moth when you see it. You think "what the heck is that" and then the penny drops. It has a soft almost pinkish tone to it at times. It certainly seems to be a lot commoner in upland areas and is much less common when you get down to below 100m. It also seems to like damper places. Numbers have shot up since 2015. I doubt this is due to a real increase in numbers and is more likely to be due to people being more aware of it and more willing to record it. This year we received 14 records of 27 moths from 12 sites, all in the northern half of the county. I am surprised we have never had any records from VC63 and would expect it to occur in the north-western parts of the vice-county.

Retained Specimen / Photograph will be Required.

Recorded in 41 (21%) of 200 10k Squares.
First Recorded in 1900.
Last Recorded in 2022.
Additional Stats

< Small Square-spot  |  Red Chestnut >
Forewing: 15-17mm
Flight: One generation June-July
Foodplant:   Unknown
Red List Status: Data Deficient (DD)
GB Status: Unknown
Verification Grade:  Adult: 3
List Species Records   [Show All Latest]
Latest 5 Records
Date#VC10k Area
28/07/2022164SD88 - Hawes
22/07/2022164SE37 - Ripon
17/07/2022164SE27 - Kirkby Malzeard
17/07/2022164SE54 - York (S/W)
16/07/2022165SE37 - Ripon
  Immature   Adult
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