Brown argus (Aricia agestis): a blue without the blue

The brown argus, Aricia agestis, is a small butterfly of the family Lycaenidae that looks, at first glance, like a brown version of the blues it lives alongside. It belongs to the genus Aricia, and its caterpillars feed mainly on rock-rose. The species ranges across the Palearctic realm, from Denmark through Europe to the Mediterranean, west to coastal Wales, and east through Asia Minor and the Caucasus to Siberia and the Tian Shan.

Identification

The brown argus has a wingspan of 25 to 31 mm, noticeably smaller than most of the blues it is mistaken for. Both sexes are predominantly brown above, with a band of orange spots along the border of each wing, and no blue scales at all; seen at the right angle, the brown can show a faint blue sheen. Males carry a strongly marked, usually black, discocellular spot on the forewing. The underside is a greyish-brown ground colour with black spots ringed in white and a row of orange spots along the border, plus a distinctive black spot near the base of the forewing.

Telling the sexes apart takes a close look, since both show the same orange spotting; the clearest difference is the male’s bolder black discocellular spot. Unlike female blues, which keep some blue scaling at the wing base, brown argus females are brown all over, with no blue base to the wing.

Range and habitat

Within this wide range, the brown argus favours open, sunny ground. In Britain it is a butterfly of chalk or limestone downland, but it also turns up on heathland, in open woodland clearings and on coastal grassland, wherever its foodplants grow thickly enough. It is absent from Scotland, Ireland, the Isle of Man and central Wales, with a southern distribution within Britain. It is not a great wanderer, rarely moving more than a couple of hundred metres from where it emerged, so colonies tend to be small and compact, staying put from year to year once a site is occupied.

Life cycle and host plants

The caterpillars feed chiefly on common rock-rose, with common stork’s-bill and dove’s-foot crane’s-bill used as alternatives, the last two especially important where rock-rose is scarce. Females lay their eggs singly on the underside of a leaf, close to the midrib. The species overwinters as a caterpillar, in its second or third instar, tucked against the underside of a leaf, rather than as an egg or a chrysalis, and resumes feeding when its foodplant starts growing again in spring. In its final instar the caterpillar develops glands that produce secretions attractive to ants, which attend the mature larva and often bury its pupa in an underground cell, a partnership that gives the caterpillar some protection against parasites and predators.

Behaviour and flight period

Most colonies fly in two broods, in May–June and again in July–August, with a partial second brood appearing in June and early August further north where the season is shorter. There are normally two generations a year, occasionally with a partial third in southern areas when the weather is favourable. Adults spend most of their short lives close to the foodplant, and being a warmth-loving species, they favour sheltered spots or south-facing slopes for basking between bouts of nectaring. Males look for mates in one of two ways, either perching near the base of a slope to wait for a virgin female or patrolling in search of a receptive one; mating follows a brief, low flight. At night, adults roost communally on grass stems, sometimes alongside the common blue.

Similar species

• Northern brown argus (Aricia artaxerxes): the closest relative, replacing the brown argus on calcareous grassland further north in Britain. Its Scottish nominate subspecies carries a white spot on the forewing upperside that the brown argus never shows, but the English subspecies, salmacis, has this white spot greatly reduced or absent, so where the two overlap they are told apart mainly by range and habitat rather than by this one mark.
• Common blue (Polyommatus icarus): females of this species are primarily brown with a highly variable amount of blue, so a female showing little blue can look very like a brown argus at a glance; the brown argus, by contrast, never shows any blue scaling at all.