Oak Cloud Acrocercops brongniardella

Formerly given the vernacular name Brown Oak Slender.

The leaf mines of this species are commonly recorded but adults seem altogether much more elusive. No similar species are listed for this on various websites but I disagree - Mugwort Stilt Leucospilapteryx omissella has much in common with brongniardella and needs to be carefully eliminated, particularly with worn examples.

When I first encountered Leucospilapteryx omisella there were no photos of that species on some of the websites I used to turn to first and, as my specimens looked a lot like brongniardella and there weren't supposed to be any similar species, I identified them as such. Only later when I stumbled across a photo of omissella did I realise my possible mistake. My specimens had been rather worn and it was difficult making sure of which species they were in the absence of the specimen, not least because none of the references actually say what the differences are. Eventually I decided that all of my records were omissella, and I had to wait until 2022 before I caught my first two adult brongniardella.

I caught a Leucospilapteryx omisella the night after I caught this first Acrocercops brongniardella and comparing the two specimens the most obvious difference was size - the omisella being smaller. But there is some overlap in the stated size ranges for the two species, so this is not sufficient to establish identification. One character that appears to be consistent between all photos I've found of the two species is the first dorsal and costal strigulae are joined to form a single fascia on brongniardella whereas these are not joined on omisella. This may not be wholly reliable as on some omisella the two stigulae come very close to one another and on some brongniardella the fascia is very weak in the middle, but I suspect that even if some individuals do not conform, the angle in the middle will always be more acute in omisella than in brongniardella. Another difference that seems to be consistent, but may be difficult to discern on very worn specimens, is the background colour - paler orangey-brown on brongniardella, contrasting more strongly with the black edges of the strigulae, versus darker brown, lacking reddish-orange tones and hardly contrasting with the less distinct black edges to the strigulae in omisella. For some worn specimens it may be necessary to examine the genitalia to be completely sure (these are very obviously different indeed, for both males and females).

male Oak Cloud Acrocercops brongniardella showing its genitalia, North Elmham (Norfolk, UK), 11th July 2022

I gen detted the next one too, although by this time I was pretty confident I knew how to identify them on external characters. But this one was a much browner individual, less obviously differing from omisella in that respect (if perhaps only due to wear).

male Oak Cloud Acrocercops brongniardella, North Elmham (Norfolk, UK), 18th July 2022

I have previously recorded the leaf mines, but I don't find these easy. There are a number of other miners with vaguely similar blotch mines and I generally find them when they're old and damaged. Here's one I'm fairly confident about as it has the relatively narrow sinuous gallery as well as the larger blotch (though always welcome correction).

Oak Cloud Acrocercops brongniardella leaf mine, North Elmham Cathedral Meadows (Norfolk, UK), 20th September 2019