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Cryptic Fern
Horisme radicaria (La Harpe, 1855)
Geometridae: Larentiinae
1782a / 70.1271
Photo © Graham Geen,  Mundford - 19/05/2020 - Possible candidate (Unconfirmed)

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Similar Species
Horisme tersata
Forewing: 14-18mm
Flight: May - July ?
Foodplant:   in Europe Traveller's-joy
GB Status: -
Former Status: Unknown
Verification Grade:  Adult: 4
No confirmed Norfolk records.

The Fern Horisme tersata, has now (2019) been split in to two species in the UK. The new Cryptic Fern Horisme radicaria appears to have been hiding in plain sight for a number of years in Britain, with evidence from Europe suggesting that it is a recent/current colonist rather than a long-overlooked resident. Extremely hard to separate and dissection required to confirm any potential specimens.

Cryptic Fern has to date been confirmed on only a few occasions, the most recent being May 2020 in Kent.

Although dissection required, Cryptic Fern is noted in some specimens as lacking the dark discal spot of Fern and having a more pronounced clear black apical streak. [Ent. Rec. 131 (2019)] Cryptic Fern also appears to be on the wing a few weeks earlier then Fern in May. Please retain any potential specimens for determination. (Contact Jon Clifton)

A photograph, thought at the time to be of a Fern, taken at Mundford on 19 May 2020 (G. Geen) was recently submitted as a possible candidate. Although it is not possible to confirm from photo only, it does look like a possible Cryptic Fern, and it is now certainly worth checking and retaining any unusual specimens of Fern, as this species may be present in Norfolk. JW

Determination by Genitalia Examination Required

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