70.127 BF1782a

Cryptic Fern Horisme radicaria

(La Harpe, 1855)

Wingspan 25-32 mm.

This is a recent introduction to the British list of Lepidoptera, although there is some evidence to suggest that it has been resident for some time but confused with Horisme tersata (Fern). Until 2020 all the specimens have been recorded in Kent and Sussex, which together with the documented expansion of its range on mainland Europe suggests it is a recent colonist that has spread northwards across the channel.

Differences between this species and H. tersata have been described by Smith and Clancy (Ent. Rec. J. Var. 131: 261-268) but in summary, H. radicaria lacks a discal spot (usually present in H. tersata), has a much more obvious apical streak and the forewing tone is brown rather than ochreous. Additionally, H. radiacaria has more contrasting forewing markings compared with the usually washed-out ill-defined features of H. tersata. Confirmation by genitalia dissection, particularly in males, is relatively straightforward.

As with other Horisme species, the main larval foodplant is Clematis vitalba. It is double brooded, flying in May/June and late July to late August/early September in mainland Europe.

Description: P.Jewess