Brown Hairstreak

I had, for some time, suspected that the Brown Hairstreak butterfly might be present in the extensive untrimmed blackthorn hedges surrounding the meadows and horse paddocks at the Naturetrek office in Chawton, but a brief glimpse of a late adult in September 2016 was all I had to show for it.

Blackthorn is the larval food-plant of the species and the females generally seek out young saplings on which to lay their eggs, normally in the crook between the stem and a twig.  

Whilst I was on leave in the first week of September 2019 my colleague, Tom Ambrose, photographed an adult on brambles and then on the 10th I watched a female in the same area of brambles and blackthorn. Presently, she began to examine the blackthorn saplings, probing with her abdomen for suitable egg-laying spots. On the 13th a second female was seen with different wing-damage to the first.

In the subsequent days, I examined the saplings and finally discovered a single egg at the base of a twig. With a magnifying lens the tiny white egg (less than 1mm wide) was seen to have a pitted surface and somewhat resembled a miniature sea-urchin.

The egg overwinters and then around April the tiny 1.3mm larva makes its exit by eating a neat circular hole in the top of the egg, before crawling to the nearest leaf bud and feeding on the leaves as they unfurl. I did not have great hope of seeing this event for myself, and it became even less likely with the outbreak of the Covid-19 virus and subsequent lockdown.

Fortunately, however, my colleague David Phillips had to make a visit to the office on the 20th of April and went to check on progress. To my delight, he found not only the opened egg case but the tiny caterpillar on the adjacent leaf!

The Brown Hairstreak is univoltine (one generation per year) and the larva passes through four stages (instars) as it grows, before pupating on the ground for about a month, with the adult finally emerging in August. The pupa is believed to be attended to and protected by ants, possibly by producing pheromones that mimic the queen ant or by exuding a sugary substance.

It is also thought that colonies of the butterfly need a ’master’ tree nearby, usually Ash, where the males defend their territories and rarely descend to lower elevations, feeding on honeydew. A mature Ash is present adjacent to this site in the hedgerow.