Butterfly Conservation
saving butterflies, moths and our environment
Butterfly Conservation - saving butterflies, moths and our environment

White-letter Hairstreak

White-letter Hairstreak

by Jamie Burston

This captivating species has an arboreal lifestyle and is therefore somewhat elusive, as a result it is greatly under-recorded. As a result it's been hard to properly assess the distribution of the butterfly and the toll taken by Dutch Elm Disease.

The devastating effects of Dutch Elm Disease (DED) has caused widespread habitat loss within the UK, to control the spread of infection many Elm trees continue to be cut down, removing the White-letter Hairstreaks only foodplant from most of the landscape. Within the centre of Sussex, Brighton and Hove remains to be an exception, with fast acting management to destroy infected trees and stop the spread. Find out more about the story and effects of Dutch Elm Disease in it's own section below.

Part One: Elm (Ulmus)

How do you find White-letter Hairstreak colonies? White-letter Hairstreak is a species solely reliant on Elm (Ulmus) trees as a foodplant to complete its life cycle. A number of individuals will reside on a single tree consisting of both male and female White-letter Hairstreaks to form a colony. Locating and identify Elm will give you the opportunity to search for White-letter Hairstreaks. Elm occurs in variety of habitats from roadsides, parks, cemeteries, private gardens, allotments, countryside field margins and downland.

Below are illustrated photos showing the most widely occurring features which distinguish Elm from other tree species. I have include the full Elm life cycle, this of which is deeply intertwined with that of the White-letter Hairstreaks life cycle. Being able to identify Elm buds, flowers, seeds and leaves as they develop in that order will allow you to identify Elm trees throughout the year whatever the season. I haven't included the months of which flowers, seeds and leaves develop as different Elm varieties vary in the timing and speed of which they pass through the life cycle. Look for any of the following features to aid in identification -

Flower buds (refer to Figure.6): Predominantly more rounded, expressing a red hue and a fuller shape grow on the sides of twigs, the direction of growth is at an angle to that of the direction of the twig. Huntingdon Elm has particularly large, rounded and glossy flowers buds, one of Brighton and Hove's most abundant elm varieties. Please note that only mature elm is capable of producing flowers and seeds, those of younger trees will proceed to go straight into leaf.

Leaf buds Like flower buds they are found towards the tip of twigs, expressing a darker, typically more brown hue in colour. Leaf buds point in the same direction as the individual twigs, leaf buds have pointed tips and can vary in length down to the variety of elm you find. Up close, unopened leaf buds will show a lattice of protective scales that eventually peel apart as the leaves inside develop - (See photo ''5'' in Egg section below).

Elm flowers: Once the flowers buds burst your find the anthers breaking through, saturated in a rich and deep pink colour, supported by white, pink or green filaments which make up the stamen. As the flowers develop the anthers will open and turn dark brown (pictured) revealing the white pollen grains. As the flower matures they will gain an overall green appearance, produced when the stigmas and style develop to collect the pollen. Now the seeds will begin to develop. To understand flower anatomy, visit here: https://www.amnh.org/learn/biodiversity_counts/ident_help/Parts_Plants/parts_of_flower.htm

Fig 1 Elm flower

Elm seeds: Seed are contained within wings of tissue, a structure described as a Samara, the seed is contain within a central capsule, typically but not always adorned with a pink hue. Samaras will vary in colour, either from a green colouration to a pale yellow hue (like that of Wheatley Elm, another abundant elm within Brighton and Hove). At the peak of seed development, from a distance you can easily mistake the tree being covered in young leaves. After some time the samaras will turn brown in colouration. Elm generally finds it hard to self-propagate with success.

Fig 2 Elm seed

Elm leaves: The key feature which identifies elm is the occurrence of asymmetrical leaf lobes (Fig 4), one side larger than the other but the ratio between each side can vary between different varieties, always located at the base of the leaf where joining the leaf stem. Another feature of elm leaves is a toothed edge, whilst all elm varieties will have a tapered leaf tip (Fig 5), the length and pronunciation can vary greatly. The most likely species to get elm confused with is hazel, use the earlier indicators of flowers and seeds to separate the two species.

Fig 5 Toothed leaf edge 

Fig 4 Asymmetrical leaf lobes at stem

Fig 3 Huntingdon elm, leaves

Part Two: Egg /Ovum

Female White-letter Hairstreaks lay the majority of eggs during mid-July, this behaviour may occur into early August. When first laid the eggs appear green in colouration but later the shiny and somewhat transparent surface of the eggs turns a subtle blue tint of grey. The eggs have a beautiful structure, complete with added white fringing, in a certain light the surface of eggs will shimmer, showing tints of pink and blue. Without seeing an egg laying female the best time to search for eggs is in the Winter (December to March) when the leaves are out of the way.

My searches of eggs (I've now found around 20 White-letter Hairstreak eggs as of 2016 - 18 Feb 2017) has helped to highlight the portion of tree that is would appear females prefer to lay their eggs on. All the eggs I've found fall within the West - South - East portion of a compass, with my finding currently showing an emphasis on the South facing portion of Elm trees. The following link can help as a tool to determine where best to search for eggs on potential trees you find -http://googlecompass.com/

Important note: Based on my observation this year (2017) of Huntingdon Elm it would suggest that the season is a week ahead of last year with earlier flower bud development. I would therefore say that it's worth looking for inhabited eggs until the 3rd week of March 2017, after which I would imagine the majority of caterpillars will have emerged.

Based on first-hand experience of trying to find my very first White-letter Hairstreak egg I would suggest taking a few helpful tools along with you on your search, a hand-lens or a camera with a macro lens/setting to aid in identification. Depending on your eye sight at close range once you have seen a few eggs you should be able to correctly identify what you're looking at without any aid. Another key tool to help in your search is to understand the size of a White-letter Hairstreak egg, like other Hairstreak species the egg is very small in size, 1mm in diameter to be precise! Take a small ruler with you while searching, one of the biggest problems I first had was overestimating the size of the egg, always imagining it to be much larger, if this gets into your head your start to overlook small structure like the eggs on the Elm tree.

Important note of care: Please always be aware of where your placing your hand on branches and twigs when grabbing to steady your view, always look first where you place your hands on the tree as you never know where the eggs could be! Below should help but caution should always be used to avoid damage. Never attempt to search for eggs during windy periods or days, it poses the risk of harming eggs and the tree!

Fig 6 Egg laying positions on elm

The labelled numbers shown on (Figure.6) refer to the following photos below and the various position you may find White-letter Hairstreak eggs. Please note although only one example ''box'' is given for each position, eggs will be laid on the multiple available positions.






1: Eggs may be laid on the ''plateau'' formed to the side and base of both flower and leaf buds and the base of each twig.

2: Eggs may be laid on the twig itself, where there is a fork with another twig.

3: Eggs are most obvious to spot when laid on the ''scar band'' on twigs, created where old hard wood of the previous year meets new green shoots which hold the leaves during the summer of the current year. In winter the green twigs harden and change colour to a dark red/brown hue. As the best time to search for eggs is in winter when the leaves have gone these red/brown twigs will be what you see. The scar band is highlighted where these dark red/brown twigs which have this year's flowers, meets old, slightly lighter brown coloured twigs (see Figure.6)

4: Eggs may be laid at the base of unopened flower buds, a great advantage as the caterpillar to emerge will burrow straight into the flower buds to feed.

5: Eggs may be laid on or at the base of the unopened leaf buds, these leaf buds will later burst and the shoots will contain the years leaves and the following springs flowers buds.

Tip: Based on the number of positions female White-letter Hairstreaks may lay her eggs, to give yourself the best chance of find them it's advised to search all twigs at every feature (1-5) when looking.

Fig 7 Hatched egg, empty casing

During your searches you may find hatched eggs from the previous year, clearly so if found before January. If hatched eggs are found later it might suggest that this years caterpillar has emerged. Empty hatched eggs are capable of staying on the tree up to a year after the caterpillar has emerged.

Sadly with any species which overwinters in the egg stage there is the risk of disease. Below is a photo of a diseased egg (on the right) compared to the healthy appearance it had before (on the left). Disease likely set in due to poor weather conditions.

Fig 8 Healthy (left) and diseased (right) White-letter Hairstreak egg

Please let us know if you find any White-letter Hairstreak eggs within Sussex, we would like to hear from you!

Part Three: Caterpillar / Larva

Caterpillars will typically emerge from the egg during the last few weeks in February and throughout March. Observing the emergence of the caterpillars is somewhat difficult based on my observations (2017), the approach to emergence is given away when a small hole is made in the centre of the egg (Micropyle), through which you might be able to make out the caterpillars dark shiny head. It's hard to judge when caterpillars will emerge, although caterpillars have made exit holes in the top of eggs, they will hold on until darkness to make their escape to the nearest flower buds, which there then burrow into for protection and to feed. Although I haven't seen a caterpillar emerge from the egg they would be around 1mm in length, around the size of the egg they came from.

I haven't been able to locate caterpillars whilst they feed upon the buds and the following open flowers, not seeing them for an average of 30 days before I can spot my first caterpillar from their emergence. It isn't a good idea anyway to search the flowers as they are delicately attached to the tree and can easily be knocked off. During flower and seed development you should restrict contact to only holding/steadying the bare branches if needing to acquire a better view, this will avoid knocking the delicate buds off, doing so would cause habitat loss and could endanger the caterpillars, be careful and never handle the tree in windy conditions.

Below (Fig 1) illustrates bored holes in a undeveloped flower bud, possibly created by a White-letter Hairstreak caterpillar:

Fig 1

If you were able to located the current years eggs in the winter, there is still a chance of finding the caterpillars near to the eggs, waiting until the flowers have finished and the seeds begin to develop. Feeding damage on seed should be visible on most elm varieties during April, the best time of the month to search for feeding damage will vary from year to year depending on: the development of the seed, how plentiful they are in a particular year and when the majority of seed begins to shed from the tree.

White-letter Hairstreak caterpillars feeding damage on seed samaras, illustrated below. Note the small size of the caterpillar, 4mm in length in (Fig 2.5) with my hand, used to give a sense of scale, this was taken 5 April 2017:

Fig 4

Fig 3

Fig 2.5

Fig 2

Note that the characteristic holes in the seed are focused in the centre of the samaras, with entry holes made in the central seed compartment, the central seed compartment can either be pink (see Egg section - Fig 2) or remain the same colour as the paper like tissue surrounding it (see Fig 4, above).

At this point you may wish to use a hand lens or a camera with a macro setup to closely inspect the seeds and leaf buds to confirm the presence of telltale feeding damage marks.

In mature trees the flowers and seeds will develop before the leaf buds being to expand. Whilst in younger trees, flowers and seeds won't be produced and will soley develop leaves, going to leaf much sooner than mature trees.

The leaf buds typically begin to develop towards the end of seed development.

The same type of feeding damage seen on the seeds should also be seen on the expanding leaf buds when the bud scales peel apart revealing green/yellow and pink tissue (see Fig 5 with caterpillar below). Like the seeds the leaf bud feeding damage consists of small circular holes, browning at the edge (see Fig 5.5 below), the caterpillars burrow and anchor their head into the buds which they feed upon, which in turn creates these holes.

Fig 5

Fig 5.5

During 2016 I first found caterpillars on the very tips of expanding leaf buds (see Fig 6 below) matching their colouration to the buds they rest on, however this year (2017) seed production has been far more plentiful and longer lasting on Huntingdon elm. Because of the extra shelter and protection the seeds provide the caterpillars have rightly chosen to stay within the seed clusters for longer, even when leaves are expanding. During 2016 a caterpillar made full use of the shelter provided by a newly opening leaf bud (see Fig 7 below).

Fig 6

Fig 7

When the leaf buds fully open the young tender leaves will be on show (see Fig 7.5 below), depending on the amount of seed still around the caterpillars will typically rest within the leaf bud scales (see Fig 8 below), at this stage feeding damage on the leaves themselves might not be apparent. The maturing caterpillars continue to reply on the leaf bud scales to hide within or mimic, as a form of camouflage (see Fig 8.5 below), located where the leaves meet the stem.

Fig 7.5

Fig 8

Fig 8.5

Moving into May feeding damage on the developed leaves should become apparent and more noticeable. If you were not able to locate eggs earlier in winter, now is the best opportunity to search as their presence will be given away. Come mid-May the weight of the tree being in full leaf will bring the branches closer to you, in 2016 all of the caterpillars I had found within reaching distance were in the first 2 meter of the tree (measured from the ground). Huntingdon elm as a variety does have low hanging branches, so for other Elm varieties with higher out of reach branches, binoculars will become a useful tool.

The most distinctive and striking feeding pattern created by the White-letter Hairstreak caterpillars that I've observed on Huntingdon elm is illustrate in (Fig 9, 10 & 11 below):

Fig 9

Fig 10

Fig 11

The feeding damage leaves a characteristic diamond shape at the tip of the leaf. Once the leaves are the only thing on the tree it becomes easier to find and spot the caterpillars, by looking for fresh feeding damage you should be able to follow their movement on the tree, they must become nocturnal feeders as throughout 2016 I never saw them actively feeding during the day, rather just resting and moving from branch to branch in search of new leaves (see Fig 12 below). From my observations White-letter Hairstreak typically stay within a meter radius of their eggs to feed. When an Elm tree is in leaf, your best off checking the undersides of the leaves without touching the tree first, then if you spot a caterpillar, note it's position, typically the best way to turn over a leaf is by holding the very tip, where caterpillars rarely rest. Again, never touch the tree in windy conditions.

Fig 12

When the leaf bud scales which the caterpillars replied upon for shelter, dry and turn brown in colour (mid-May for Huntingdon elm) the caterpillars use the leaves in an increasing way, caterpillars by this point typically rest on underside of the leaves, positioning themselves around the asymmetrical lobes of the leaves (see Fig 13 below) and may rest on the stem of the leaves, camouflage at this point is their main attempt of protection (see Fig 13.5 below).

Fig 13

Fig 13.5

Toward the end of May, the caterpillars gear up for pupation changing their colour (see Fig 14 below), in sunlight they appear dark mauve, whilst in shade they look dark brown. At least with my observation on Huntingdon elm, unlike when feeding, they begin to stray further to find a pupation site, travelling along various branch systems to find a cosy recess or crack in the bark to pupate. Below (see Fig 15) the caterpillar here is trying out one of the various places it sampled as a pupation site.

In the instance of White-letter Hairstreak on Huntingdon elm, really the only way you would find the pupation site was if you were present to follow the caterpillar to it's final resting place on a branch. During 2016 it would appear that the caterpillars started to wondered off to pupate from around 20th May, at that time the average size of the caterpillars was 1.5cm in length. It has however been documented that on different varieties of Elm, White-letter Hairstreak caterpillars behave differently around the time of pupation, this is covered in the Chrysalis/Pupa section below. It's clear that the way White-letter Hairstreak caterpillars look and behave is highly variable and dependent on what is available to them at the time and the variety of Elm they use. A large factor which will affect the rate of development of both tree and caterpillar is the weather, this will ultimately govern the emergence date of the adult butterflies.

Fig 14

Fig 15

I hope to see White-letter Hairstreak caterpillar photos taken by our members in Sussex being posted onto our branch sightings page! It would be great to keep everyone who gets involved informed of how the caterpillars/season is developing with one another.

Get recording, thank you!

Part Four: Chrysalis / Pupa section coming soon.

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