White-letter Hairstreaks have been very under-recorded in the current five-year recording cycle, and as 2014 is the last year of it, we need to go looking for them this summer.
The red markers on the map above show the seven records which have been received since 2010. The yellow markers show those gathered between 1995 and 2009, during which time the White-letter Hairstreak was recorded in 47 locations. We do not think the butterfly has declined this drastically, but that there has been a lack of recording. Click here for a spreadsheet listing all the squares involved. To see much more detail, open this link to Google Earth (you will need to have the free Google Earth software on your computer for this to work) which allows you to zoom right in to understand the position of the relevant kilometre squares.
White-letter Hairsteaks breed on elm trees, and were very badly hit in the 1960s and 1970s when Dutch elm disease wiped out two-thirds of the country’s elm trees. They prefer wych elm, but may also use English elm and smooth elm.
They are not easy butterflies to spot, and it has been suggested that the problem is that recorders are searching at the wrong time. The best time to look is the last week of June and first week of July on, or very near, elm trees, when the males will be flying around in the tops of the trees. There cannot be any confusion with the Purple Hairstreak as, although that butterfly flies in a similar fashion, it is not out this early in the year. In July you may find the butterflies lower down, nectaring on flowers such as creeping thistle, bramble, ragwort, marjoram and hemp agrimony.
You might actually have more luck spotting the caterpillar. When it is at its biggest in June, it is 15-16mm long, and though it is very well camouflaged, it is sometimes posible to stand under the tree and look up when the sun is shining on the leaves and see the dark shape of the caterpillar in silhouette. You may also be able to spot where the leaves have been damaged by the caterpillars eating them, but there are also around five species of moth caterpillar which may be doing this – however, it might at least help you to narrow down where to look more carefully.
If you do find any White-letter Hairstreaks, please let us know. You can use the sightings form on this website, or tell us what you have seen via our Contact Form, and we’ll get back to you. If you can send us a photograph, that would be even better.