Pyrgus alveus (Hübner, 1803)

Host plants:
The caterpillars feed on Helianthemum nummularium and its related taxa. So far, however, there is no observation from Helianthemum alpestre (= oelandicum) in the Alps. Observations on Potentilla are possibly at least partially based on confusion with similar species (e.g. if syntopic with Pyrgus armoricanus), but accumulate in parts of Central Europe and should be investigated further. As the taxon accretus shows larvae on Potentilla would not be so unusual. Further observations about the taxon accretus (feeding on Potentilla and Helianthemum) are desirable from SW Europe. New observations in Central Europe, e.g. parts of the central Swabian Alb, revealed that there are indeed populations that oviposit on Potentilla (P. verna and P. heptaphylla). As in the taxon accretus in these populations that oviposit on Potentilla Helianthemum and Potentilla are usually both accepted at least in rearing.
Probably also populations of sandy grasslands e.g. in Brandenburg (Germany) live on Potentilla.

Habitat:
Pyrgus alveus inhabits nutrient-poor grasslands in the broader sense. This species prefers gappy areas with lots of rock rose, but is not limited to such sparsely vegetated areas as it is the case with Pyrgus cirsii or Pyrgus serratulae. Pyrgus alveus can also reproduce in more or less closed grasslands where Helianthemum is still found in greater density. The areas are usually grazed (transhumance). If not, there must be available enough shallow places with rocks etc. or old ant hills. Nevertheless, after abandonment of grazing a long-term extinction is at least probable.

Life cycle:
The larval development is highly variable. Partially a kind of standardization of phenology is observed when the adults fly from mid-May to early or mid-July and the caterpillars overwinter in penultimate instar (mostly L4). Elsewhere, the flight time is more extended or starts later with butterflies until September or even early October. In the Alps the caterpillars overwinter as L2-4, maybe sometimes also as L1. Additionally the larvae can undergo different long dormancies and also have sometimes four and sometimes five moults (with very slow development may be even more) which has been observed in the offspring of a single female from the Northern Alps.

These findings suggest that Pyrgus alveus is still quite young and genetically highly variable, which reminds about the conditions at Zygaena filipendulae. Some variability is also typical for other Pyrgus such as Pyrgus carthami or P. armoricanus.

Endangerment: endangered

Endangerment factors:
In the Alps Pyrgus alveus can be found almost anywhere where Helianthemum nummularium agg. occurs. In contrast Pyrgus alveus is found in the low mountains (Swabian Jura etc.) increasingly rare due to habitat loss (because of human activities of all kinds and the decline of transhumance).

Remarks:
The temporary separation particularly of the taxon Pyrgus trebevicensis is no longer tenable at the present state of knowledge. Pyrgus accretus on the other hand can possibly be classified as a subspecies of P. alveus (see P. accretus).

Pyrgus alveus agg. ranges from northwest Africa across parts of Europe (in the north to the southern half of Scandinavia) to Mongolia. The exact taxonomic situation is not known in many places, especially in the east.

Hints on determination:
Pyrgus alveus can be confused in lower altitudes with Pyrgus armoricanus. Only genital section will result in reliable determination! In the higher Alps there can be confusion with the smaller Pyrgus warrenensis. In the Southwestern Alps and the Eastern Pyrenees there is another similar species (Pyrgus bellieri) which is restricted to warm slopes up to 1800m above sea level and shows as male a stouter hair tuft ad the end of the abdomen.

It is in many cases not reliable to determinate Pyrgus species by photos, at least without thorough knowledge of the given observation site.