Where are they found (in Britain and Ireland)
The southern and eastern area of England, coastal Wales, with colonies in Devon and Cornwall.
What is their preferred habitat outside gardens?
Brown argus butterflies are seen in less than 10% of gardens as their preferred habitat is chalk or limestone downlands. However they can also be found in heathland, woodland rides, field margins and rough overgrown wasteland if their foodplants are available.
Resident or migrant?
Resident
When to see them?
There are normally two broods each year in central and southern England and adults can be seen from May until September. In north Wales and the north-east of England there may be only one brood with the adults emerging in early June. Both males and females fly close to the ground and often bask in the sun and males patrol their territories with darting movements.
What happens in winter?
They overwinter as caterpillars from the final brood of the year. Although there are five instars in total they go into hibernation on their foodplants after their first or second moult (second or third instar) when the caterpillars are still quite small.
What do the early stages look like?
Eggs are laid singly on the foodplant, rathe pill-shaped and pale. The first caterpillars are a very pale yellow and about 1mm. Subsequent stages are bigger and greener, reaching a final length of 11mm and developing a dark purple line down their back. The pupa is shiny and semi-transparent, formed on the ground at the base of the foodplant, but is often carried off by ants and taken underground.
What do the caterpillars eat?
The primary foodplant on chalk and limestone grasslands is common rock-rose
Helianthemum nummularium. In other habitats they use common stork's-bill
Erodium cicutarium, dove's-foot crane's-bill
Geranium molle and occasionally other
Geranium species. In Yorkshire – where the brown argus has spread rapidly – it has been suggested that there are two forms: a sedentary one found on chalk grasslands in separate large colonies and a migratory one with small, transient colonies using
Geranium species
2. .
Flowers they take nectar from
How are they doing?
The brown argus has increased both in numbers (25%) and distribution (4%) since 1976 according to the 2022 State of UK butterflies report
4.. This is thought due to climate change and also to changes in agriculture, which have allowed the annual crane’s-bills to thrive
2.. However, this increase may have come at a price as the butterfly’s range is now overlapping with the endangered
northern brown argus and the two species have been shown to hybridise.
Other interesting facts
In their later stages the caterpillars produce a secretion which attracts ants and this appears to give them some protection against predators and parasites.
References
4. State of UK Butterflies 2022
report
Page written by Judy Skinner, reviewed and compiled by Steve Head