The Brown Hairstreak Lifecycle - By Vince Massimo


In 2011, during the course of surveying for Brown Hairstreak eggs for the Sussex Butterfly Atlas, I found a total of 3 eggs on severed Blackthorn stems. These were rescued and the resulting larvae were raised to adulthood. During this period of 95 days, I observed and photo-documented their development and this is the resulting report. For monitoring and record purposes, the larvae were given designations of L1, L2 and L3.

The larvae were raised under cover, sheltered from wind and rain, on potted Blackthorn plants, but otherwise the temperature, light and humidity levels were kept as natural as possible. The accepted method of dealing with rescued Brown Hairstreak eggs is to tie the severed stem to a living plant and the emerging larva will then crawl into an unfurling leaf bud. Current literature on this species suggested that eggs hatch in late April or early May, but in 2011, I saw a report on the UK Butterflies website that an egg being monitored in the wild in Sussex had hatched on 7th April. Upon checking my eggs the following morning, I found one already had a hole in it but fortunately the larva appeared to still be inside. At that stage, I had not yet tied any of the severed stems to the host plants and was nearly caught out by the early hatching date.

It can take a Brown Hairstreak larva the best part of a day to chew a hole large enough in the egg in order to emerge. A thick and tough eggshell is necessary in order to get through the winter, but these properties also appear to protect the developing larva against other external forces. Two of the eggs were badly abraded across parts of their surfaces (possibly due to hedge trimming damage), but nevertheless, all three larvae successfully hatched on 9th, 10th and 11th April 2011.

Although the larvae burrowed into a leaf bud (not necessarily the closest one to the egg), they emerged occasionally during the daytime. They first appeared on 14th April, by which time they were 2mm in length.

By day 12 of their development, all of the larvae had moulted into their second instar, taking on a different shape, appearance and colouration and increasing to 4mm in length. They were now to be found resting under their respective leaves, although they were occasionally active during the day.

After 17 days or so, they began to moult into their 3rd instar.

After a month they had grown to around 10mm in length and then one unexpectedly went missing. Although only half grown, it had left the food plant for some reason. Fortunately, it was relocated two and a half days later and continued feeding as soon as it was returned to the plant. Thereafter, all of my plants were netted.

After 35 days, the two largest larvae were between 12mm and 15mm in length and all were following the recognised routine of feeding during the night before returning to the undersides of their designated resting leaf during the day. When they changed resting leaves, they could be very difficult to relocate, despite the fact that I was attuned to their appearance and only had a very small plant to search. Although the larvae were a paler colour than the Blackthorn leaves and stood out when viewed in sunlight, this is not how they would normally appear in nature. Thus far, my photos had been taken from below the leaf, either with the larva moved into full sunlight or with the use of flash. This however is not how a predatory Blue Tit would see them, so I took a series of comparative shots in different lighting levels.

Whilst these go some way towards demonstrating the point, they still do not fully capture the effectiveness of their camouflage.

After 45 days, the larvae were beginning to get larger than the remaining leaves on my young plants.

On day 48, one larva went missing, but this time I knew it could not have got out of the netting, so it must have gone down into the leaf litter I had placed at the base of the plant. On day 49, I found it nestled under a dry leaf, but it had completely changed colour to a mottled purple. I had expected to see some subtle colour changes indicating that it was about to leave the plant, but this was not the case. One moment it was green and hanging under a leaf and the next it was gone. The other two larvae stopped feeding and descended into the leaf litter on their 54th and 58th days respectively.

After fully monitoring all three larvae at this stage of their development, I was able to establish the timing and sequence of events that lead to pupation.

To begin with, apart from the size and age of the individual larva, there is very little initial indication that it is going to leave the food plant and look for a pupation site. My three larvae pupated 54 to 65 days after hatching from the egg. A fully grown larva is typically 20mm in length and lime green in colour. It is still green when it descends to the ground, showing just a few dark flecks under the skin. Around 6 hours later, it has achieved a transitional colour, halfway between green and purple. Once at this point the mottled purple colouration fully asserts itself within a period of just a few hours. Thereafter, it rests under a dry leaf (or other sheltered pupation site) and does not feed again.

My understanding was that once larvae left the plant, they pupated in dry leaves, clumps of vegetation or crevices in the ground, so mine were given a choice of pupation sites and materials. These largely comprised of dry leaves over layers of dry and moist compost. Two larvae chose the dry leaves and the other preferred a cosy corner of the pupation pot.

Typically, a larva will spin a flimsy pad of silk to which it attaches its hindquarters. It then either positions itself head-down against a vertical surface or upside down flat against a horizontal surface. Once in this position, its appearance changes to a more rounded shape as it contracts. It stays like this for up to 7 days and the skin becomes increasingly transparent as it approaches pupation. Unlike the larvae of many other butterfly species which enter this stage, the shape and colour of the pupa begins to become visible through the larval skin. At the point of pupation, the larval skin becomes completely transparent and is slowly sloughed off, ending up as a compact mass attached to the rear of the pupa. The fresh pupa is pink but soon darkens to its final walnut-brown colouration.

The first hatching occurred on the morning of 5th July 2011. This was L2 which spent 32 days as a pupa and emerged as a male. At about 4 days prior to hatching, the pupa started to darken in the area around the wings and this gradually spread to the whole pupal case, which became completely black.

The adult was released back to its site of origin within a few hours. It stayed around for over an hour, occasionally taking sips of the sugar solution I had sprayed onto the leaves of the plant.

I had speculated that, based upon the relative broadness of the abdominal areas of the two remaining pupae, L1 and L3 would both be female. L3 was always a very dark pupa and gave few clues as it neared hatching. However, as L1 started to darken, the wing case area began to show signs of dark orange colouration. Both hatched as females within a few minutes of each other on the morning of 13th July 2011.

Their pupal stages lasted 30 and 32 days respectively and followed on from the 32 day development of the male (L2). As these were all reared under cover and protected from the worst of the weather, it is not clear how these conditions affected the speed of development or behaviour of the larvae. Nevertheless, these early emergences were anticipated because the eggs hatched 2-3 weeks earlier than expected. Eggs being monitored in the wild in Sussex also hatched early in 2011 and the first sighting of an adult in the wild in that year was a male in Bernwood Forest, Buckinghamshire on 11th July. This compares with national first-sighting dates of 24th July in 2010, 20th July in 2009 and 13th July in 2008.

In Summary
L1 (Female)
Hatched from egg 9th April 2011
Descended and stopped feeding 6th June 2011 (day 58)
Pupated 13th June 2011 (day 65) after 7-day interval
Emerged as adult 13th July 2011 (day 95) after 30-day interval

L2 (Male)
Hatched from egg 10th April 2011
Descended and stopped feeding 28th May 2011 (day 48)
Pupated 3rd June 2011 (day 54) after 6-day interval
Emerged as adult 5th July 2011 (day 86) after 32-day interval

L3 (Female)
Hatched from egg 11th April 2011
Appeared as 2nd instar 23rd April 2011 (day 12)
Moulted into 3rd instar 29th April 2011 (day 18)
Descended and stopped feeding 4th June 2011 (day 54)
Pupated 11th June 2011 (day 61) after 7-day interval
Emerged as adult 13th July 2011 (day 93) after 32-day interval

All three adults were returned to their original sites for release.

I would like to acknowledge the advice and encouragement received from Susie Milbank during the rearing process.

Vince Massimo
(Final Revision 9th October 2012)