Treble-bar Aplocera plagiata
Wingspan 37-43 mm.
In the south, there are two generations of this moth, flying in May and June, then again in August and September. In the north of England and Scotland, there is usually just one brood, flying in July and August.
On of our larger Geometrids, it has a wingspan of around 40mm. It is rather difficult to tell apart from the less common Lesser Treble-bar (A. efformata), which however has a more angled inner cross-line on the forewing.
With males, it is usually possible to separate the two species by inspecting the tip of the abdomen from below. Treble-bar has elongated, pointed claspers giving rise to a narrow abdomen tip. Lesser Treble-bar has a blunter tip to the abdomen with rather shorter, curved claspers (See photo).
It is fairly common over most of Britain, the larvae feeding on St. John's wort (Hypericum).