Pyrgus andromedae (Wallengren, 1853)

Host plants:
The caterpillars live exclusively on Dryas octopetala (Avens, Rosaceae). Alleged observations on other genera (Potentilla) are based on confusion.

Habitat:
Pyrgus andromedae inhabits pastures, stony and grassy slopes in the subalpine and alpine level, usually from about 1700m above sea level (rarely even around 1400m above sea level) up to about 2600m asl. In Scandinavia lower altitudes are inhabited.

The larval habitat is located on southern and also northern slopes (if still sunny!) often in the formation of the Caricetum firmae (very low-growing sedge cushions on calcareous, shallow soil) with a lot of Dryas octopetala. In wetter terrain especially Dryas pads at elevated rocks etc. are occupied.

Life cycle:
The larval development needs two years as it is the case with Pyrgus cacaliae. The first hibernation takes place as L1 (most often) or L2, very rarely also as L3 in the lowest and warmest locations, as I observed at about 1700m in the Montafon (W-Austria) in autumn 2009.
The second hibernation is passed as pupa. A third hibernation, however, seems hardly to occur (in contrast to Pyrgus cacaliae). The adults appear in places only in odd years, at other sites, they are then at least more common. Pyrgus andromedae is able to appear very early in season due to the hibernation in the pupal stage. In the hot spring of 2007 I found already 4 males and some P. malvoides in 1700 - 1800m above sea level in the Montafon on 29 April, while on the northern slope still plenty of snow could be seen. The usual flight time is between late May and early August with peak in late June and July. Butterfly observations after 10 August are rare. The caterpillars are usually mature in July or August after the first hibernation.

Endangerment factors:
In the Alps Pyrgus andromedae is still threatened only weakly and is more or less widespread, in spite of losses due to ski tourism and other human activities which impact the habitats.

Remarks:
This boreal-alpine species had been unknown in its ecology for a long time. The Swiss Association for Nature Protection still confused the preimaginal stages complete with Pyrgus cacaliae (Volume 2 of their otherwise famous serious). The caterpillar is characterized by the well sclerotized blackish anal shield that is well developed also in the final instar. Among the Central European Pyrgus species apart from P. andromedae this is only the case with Pyrgus warrenensis. In other species this sclerotized anal shield is largely reduced, at least in last instar.

Pyrgus andromedae shows a boreal-alpine distribution and is widespread mainly in the Alps, in addition also locally in the Pyrenees and some of the western Balkans mountains. Finally it can be found in Northern Scandinavia. The butterfly also occurs in the Bavarian Alps.

Hints on determination:
Determination should be no larger problem even in the field or with good photos if you consider the combination of quite large white spots on the forewing upper side, the often typical zigzag pattern on the hind wing upper side and especially the white question mark on the lower border of hindwing bottom side that is located completely on darkish background.