The Brown Hairstreak (UK BAP Priority Species) is the largest of our five native hairstreaks. Once fairly widespread in England and Wales, it has since disappeared from many regions. Its strongholds are now confined mainly to southern England and Wales, including mid-west Ireland. It is thought that the main factor responsible for the continued decline of this species is the removal and/or annual flailing of blackthorn on which the females lay their eggs. The correct management of these hedgerows is essential in ensuring that this species continues to survive and expand its distribution.
There is one generation per year. The flight period usually runs from late July until late September or, in exceptional years, they can be on the wing into October. The adults emerge in the mornings and males generally appear a few days before the females. The Brown Hairstreak is a warmth-loving butterfly and is rarely seen on overcast or showery days. They spend most of their lives in the treetops or flying along woodland edges where Blackthorn (Prunus spinosa), the primary larval foodplant, is abundant. In flight, the adults are easily mistaken for Purple Hairstreaks, Gatekeepers and Speckled Woods which all fly at the same time.
Adults feed mostly on honeydew secreted by aphids at the tops of ash trees but they will also use Bramble (Rubus fruticosus agg.), Devil’s-bit Scabious (Succisa pratensis), Hemp Agrimony (Eupatorium cannabinum), Angelica (Umbelliferae), Ragwort (Senecio jacobaea) and Thistles (Cirsium spp. and Carduus spp.) as nectar sources.
In order to find a mate, both sexes will congregate high in ash “master trees” that are positioned around the breeding area. They feed mostly on aphid honeydew but will occasionally come down to feed on various other nectar sources when honeydew is scarce. Few master trees have ever been studied closely but it is believed that the males prefer bushy, moderate sized ash trees that offer a gradual decline of surrounding vegetation rather than tall, “lollipop” type ashes that stand proud of a hedgerow. Congregations of adults can occur on several trees and they have been known to use different trees from year to year.
The females are thought to remain at the master tree until their eggs have matured and they are ready to lay. They then disperse from the tree and alternate between basking in the warm sunshine, feeding from nectar sources and egg-laying. Egg-laying sites are typically in south-facing, sheltered locations at the edges of woodland or hedgerows where younger Blackthorn is heavily favoured. The female will crawl among the branches of the foodplant, searching for appropriate egg-laying sites.
As the adults are quite elusive, the butterfly’s presence is most easily confirmed by searching Blackthorn for eggs during the winter. The white, sea urchin-shaped eggs are laid singly (though sometimes in two’s and three’s) on the bark of the foodplant, typically in the forks of young prominent branches. In many cases, the females find suckering Blackthorn especially suitable. The eggs tend to be laid in sheltered areas that are exposed to the sun at varying heights, though most are found no higher than around 2 metres from the ground. Although the primary larval foodplant is Blackthorn, females will also lay their eggs on other members of the plum family, such as Bullace (Prunus domestica), Damson and Wild Plum. The larva partially develops within the egg before entering hibernation for the winter. Overwintering eggs are particularly vulnerable to annual hedge flailing as they are laid on the youngest, most prominent growth of the foodplant.
After overwintering as an egg, the larva emerges in late April or early May by cutting a small hole in the top of the egg and then immediately enters a developing leaf bud. After the first moult (there are 3 in total), the larva will often move around and feed during the day inbetween bouts of resting on a silk pad situated on the underside of a leaf. More often than not, the larva will return to the same silk pad though new pads are often spun every week or so. The bright green larvae are extremely well camouflaged and difficult to find. Despite this, in the early stages, a large percentage of larvae are killed by predators such as spiders and harvestmen, and later on by birds such as warblers and tits. After 40-60 days, the fully-fed larva finally leaves the foodplant prior to pupation, changing colour to a mottled purple to maintain its excellent camouflage as it rummages around in leaf litter to find a pupation site.
The pupa is either formed in a crevice in the ground, amongst leaf litter or at the base of a plant. Like most of the Lycaenidae family, the pupae are highly attractive to ants. It is possible that the ants bury the pupae in loose earth cells and regularly tend to them. Unfortunately, research suggests that the mortality rate is very high due to predation by small mammals such as mice and shrews. The pupa stage lasts around 4 weeks.
The above information is a mix of the authors own observations, the description text provided on UK Butterflies, the Butterfly Conservation leaflet publication, Hedgerows for Hairstreaks, and the Butterfly Conservation Brown Hairstreak Action Plan.