White -Letter

Hairstreak

Fieldcraft


White-letter Hairstreak - Andrew Ashworth

Early May is the time to find a potential colony on an excercise walk or cycle from your door.

Clearly this elusive, difficult species is not on the wing until the three week window in July, when there are normally better things to be doing on a sunny day than staring for ages through binoculars at a tree and not seeing anything. If lockdown continues, this year will be very different, and of all the rarer species this is potentially one that you have been walking past for years and which could be present near to where you live.


The key is to find Wych Elm. Other elm species are used but this is the favourite.

RIGHT NOW, this week, the tree is at its most obvious. Two weeks ago the flowers will have finished and were utterly inconspicuous even when in full bloom. In two weeks from now the fruits will have fallen off and you will be trying to pick out the right green leaves from the wrong ones- very difficult amongst the other trees.

The fruits which are at their peak here in Cottingham now, make the tree stand out like a sore thumb. Once you have your eye in you can spot them from half a mile—well maybe! I hope the photos help a little. It is the colour and texture of the tree created by the winged fruits that is key. Depending on the maturity they turn from a very pale green to a pale yellow .

The individual Wych Elm most likely to be a success is one that has a south facing aspect. I would ignore any on the north side of a wood or that has too much shade from surrounding trees.


Mark your trees on a map now, as you will forget where they are, if you are like me.

Once we hit July[usually week 24], especially if we are still in lockdown, get out the map and go and have a look.

The butterfly is only seen on a sunny day, always in full sun [they follow the sun round the tree] and choose a sheltered spot especially if there is even a hint of a breeze. They hate wind. Typically this means you see them midway up the tree often in a sunny “dip” in the canopy for maximum shelter.


Look at the tree from a reasonable distance ideally where you can get sky behind to silhouette the butterfly. With your naked eye watch for a fast vertical spiral of two males or a male skipping quickly from one part of the tree to another. You get about a second so be patient. It is easy to be looking at one part of a tree when the movement is a metre or two away. Do not take your eye off the perceived landing point and look with your binoculars. If no luck, do scan the tree before moving on and look for perched individuals which can stand out. Their profile is unmistakeable with practice. No other butterfly is closer to a perfect triangle than the White-letter Hairstreak [almost equilateral]. This can sometimes even be seen as a shadow through a leaf.

Hopefully you will see the white line on a very dark underwing.


Females do come lower down to egg lay when they creep about still above eye level. With most Wych Elm the lowest branches are quite high and they choose branches that are going to produce flowers