Brown Hairstreaks Egg Spotting

article written by Ellen Lee, TVERC Data Manager

This year I was particularly fortunate to see a female brown hairstreak butterfly laying eggs on the larval food plant, blackthorn. Adults are on the wing from late July to early October, but are notoriously elusive. This is partly due to their cryptic colouration. When they are sitting in a blackthorn bush with their wings closed, they look very much like the reddish/brown regrowth and unless they move are very difficult to see. It’s also partly to do with the fact that they spend much of their time flying around the top of prominent ash trees where they gather to mate and to feed on the honeydew produced by aphids. The best chance to see them close-up is when they come down to feed on brambles or to lay eggs. The female I saw was totally absorbed in finding just the right place on a blackthorn stem to lay her eggs and I was able to watch her for several minutes as she slowly spiralled her way along a stem. I saw that she had laid 2 eggs previously, tiny white pin-pricks, in the crook of thorns. Mainly females lay single eggs, but when I was carrying out an egg count at Milham Ford Nature Park in urban north-east Oxford, I came across this line of three eggs.

Despite blackthorn being pretty ubiquitous across the two counties, the distribution of brown hairstreaks is very much concentrated on Oxford and north-eastwards towards the Buckinghamshire boundary. This can be seen clearly in the map below which shows all the brown hairstreak records from 2000 onwards (all stages) held by TVERC.

I’m not sure why the distribution looks like this. However, the eggs have to overwinter on their blackthorn bush and hatch from about mid-April. The caterpillars stay on the blackthorn feeding and growing until pupating in the early to mid-summer. So they are highly dependent on blackthorn management. Hedge flailing in the autumn/winter is very bad news for the eggs that are very often laid on the outside of a south-facing bush. Another issue is that the adults stay within their colony and seldom wander more than a few hundred metres. Once a colony is lost, it’s not likely to be re-started by colonisation from outside. The only thing the brown hairstreak is fussy about when it comes to blackthorn is that eggs are laid on young bushes or new regrowth, so very old, unmanaged bushes are not much used to them (luckily black hairstreak like them!).

My row of three eggs was found in an urban nature park in Oxford. In recent years the green spaces in Oxford have become important for the painfully slow range expansion of this species. In an urban environment, hedgerows are less likely to be flailed at the wrong time of year and many of the places they are found are managed nature reserves and green spaces. Below is a larger scale map of Oxford showing all TVERC’s post 2000 records.

Please do get in touch if you spot any brown hairstreak eggs this winter when you are out walking. Once you’ve got you eye in they are not too difficult to spot. If you can include a photo that would be wonderful.