The Northern Brown Argus populations on Common rock-rose can be single brooded (Univoltine) (May/Jun) but may produce a partial 2nd brood (Bivoltine) in good years (Aug/Sept).
The first sighting of the Northern Brown Argus in 2020 were made on 20/05 at Lea Green near Grassington in VC64. This is the earliest record for the UK ever recorded for the emergence of Northern Brown Argus.
The last sighting in 2020 was at Ballowfields Nature Reserve on the 09/09 in VC65, this is likely to be an a slightly unusual second brood, given the location being a known Northern Brown Argus site and the very early first brood observed in 2020.
The eggs are laid singly, on common rock-rose leaves, typically on the upper surfaces. The female in looking for places to egg lay is seen flying close to the ground seeking out suitable foodplants tasting them with her feet until she finds a suitable place when she lays a single egg on the upperside of a leaf.
Eggs appear as squat, whitish spheres, found from mid-June on low-lying sheltered situations in the south of the county, and from early July further north and higher up. Microscopic examination reveals that eggs have a very variable reticulated surface structure. Larvae hatch out after about two weeks, and crawl to the underside of the leaf, where they eat small areas, leaving the upper leaf surface intact. The affected surface of the leaf tends to turn yellow after a short time. Green woodlouse-shaped larvae are well-marked with green chevrons and black heads. Northern Brown Argus larva have a dark green dorsal line (down the middle of the back) but such lines vary significantly. The sub-spiracular line, (low down on both sides) varies from white, with purplish pink above and below the white, to white with virtually no colours on either side. In autumn, larvae of the third instar hibernate at the bottom of the foodplant or under a leaf and become active again in early spring probably in late February, early March.
A couple weeks after feeding the larva moults in to a fourth instar. The feeding habitats now change with the larva breaking through the upper surface of the leaf edge. After around a further 10 days the larva moults for the final time into a fifth instar. When fully grown by late May it prepares to pupate. Pupation takes place on or near the ground and ants may attend both larvae and pupae. We know little about the part played by ants in Yorkshire. Sam Ellis (1997) has observed Formica lemani attending larvae of Northern Brown Argus on a Durham site, but inferred this was probably a fairly rare occurrence. There is scope for further study here.