Week 15, Rarity: Green Hairstreak Butterfly

[If you’d like to see higher resolution images of the Green Hairstreak, they are in a set on Flickr, here]

Ok, so, the title and photographs preclude the use of intrigue and suspense! Buoyed by bunny-induced optimism, I went in search of, and found, two of the rare and elusive Green Hairstreak butterflies at Roughdown Common. The population had been discovered just before Easter by a local butterfly enthusiast (and expert, certainly when compared to me), in the course of his ongoing butterfly monitoring on the Box Moor Trust estate. He very kindly provided the following information regarding this exciting find:

The Green Hairstreak (Callophrys rubi) is designated by Butterfly Conservation as "very rare and restricted" in Hertfordshire and is only to be found in the vicinity of the Chiltern Ridge. Apart from the colony found at Roughdown, its only other established sites are at Aldbury Nowers and Hexton chalk pit. It was seen at Tring Park in 2013 after an absence of 3 or 4 years. Their rediscovery at Roughdown is very exciting news for the Boxmoor Trust who feared that they were no longer present since it is at least three years since any were last sighted. (I am in my third year of walking the Roughdown weekly transect to monitor the butterfly population for Butterfly Conservation and it was my first sighting).

About the Green Hairstreak

• It is the only butterfly found in the British Isles that has a green underside.
• Its larval food plant is primarily Common Rockrose and Common Bird's foot Trefoil.
• In the field, the behaviour of the sexes is very different. The males spend much of their time holding territory, perched waiting for passing females or other males. They actively pursue any butterfly or other flying insect that comes too close and, if another male Green Hairstreak is encountered, a lengthy and frenetic confrontation ensues. The most favoured sites are bushes such as hawthorn situated at the base of a hill, bank or slope and these are used traditionally year after year. Once a location, such as at Roughdown, has been identified, future sightings of the butterfly should be almost guaranteed.

It really was fantastic to see this species for the first time, only a few miles from home. If asked where to find a rare Hertfordshire butterfly, Hemel Hempstead would not have featured in my list of guesses! It just goes to show that dedicated monitoring of the Box Moor flora and fauna can yield surprising rewards, even if it does take years of commitment. This brings me nicely to my day’s end.

On Wednesday, as dusk fell, I returned to Roughdown Common to join Ben and Roger for the first half of the evening of their regular Box Moor moth trapping. The importance of monitoring and protecting butterflies and moths cannot be underestimated. They are a vital indicator of the health and diversity of an ecosystem but it's more than that. Butterfly Conservation sum up the reasons far more eloquently than I ever could, here. And, you can read Ben’s full report of the evening on his fascinating Moth Blog, here. A big thank you to both Ben and Roger for letting me tag along and for sharing your knowledge and infectious enthusiasm. It was a real pleasure.

Not alien landings, but two of the three moth traps on Wednesday evening. The egg cartons give the moths something to grip onto.

Lower Roughdown trap, under an oak tree

Further Roughdown trap, south-west corner