Upper Thames Branch of Butterfly Conservation
Annual
Black Hairstreak Report 2007
by
Stuart Hodges
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Black Hairstreak Photo © David Redhead |
Despite concerns over our failure to locate some previously recorded colonies at the Black Hairstreak’s most peripheral sites there is some very good news. There is considerable evidence of a renaissance of the butterfly’s fortunes in the most closely monitored central part of its range. 5 new sites were discovered this past season (i.e. summer 2007) and 2 of these were in completely new tetrads.
Even here in this relatively strong ‘core’ area, there is some fear that under-worked woodland is becoming too thick and shady; and as a consequence losing Black Hairstreak, but many hedgerow colonies are thriving and new colonies in thick hedgerows are appearing (or being seen for the first time) in each recent season. The understanding of the importance of the hedgerow to this butterfly is relatively recent and follows pioneering research by ordinary members of the Upper Thames branch, led by Stuart Hodges (Black Hairstreak species champ - thank you all).
Work with various landowners in this core area has led to greater awareness and understanding of the need for active Blackthorn management and as a result we are fairly confident about the future of the species locally. Now work of a similar nature needs to be extended to the extremities of the butterfly’s distribution to conserve important Blackthorn stands there and if possible increase their size and number.
We are also investigating the younger life stages. Eggs are proving extremely hard to locate and on average a single egg might take 4 man hours to locate.
Oddly we see that eggs can be laid on late flowering varieties as well as on early flowering types of Blackthorn, which raises the question of the time at which their larvae emerge. One larva was seen to emerge as late as June and there is a possibility that they might enter a diapause before completing development.
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Early blossom in an otherwise bare canopy Photo © Nick Bowles |
Well camouflaged
Black Hairstreak egg Photo © Dave Wilton |
Larvae are even more difficult to locate than eggs and none has been seen yet, but pupae (which have strong resemblance to bird droppings – see below!) are more easily located (especially those on leaves at low levels in the undergrowth). Various UTB members are now reasonably adept at spotting them. Larvae also pupate on twigs, possibly less often, or more likely when they do they are harder to spot.
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Adult Black Hairstreak, freshly emerged from a pupa that had been spotted earlier Photo © Dave Wilton |
Pupae which are visible to us are obviously also visible to various predators and we know that their survival rate (the survival of those that we find) is approx 50%. Perhaps those with better camouflage, that we don’t find, fare better.
As so often the weather is felt to be a crucial factor. Late spring frosts seem implicated in reduced population size and there is correlation between numbers of hours of suitable flying time in the egg-laying period and numbers of adults in the next generation. Add to this an observed shift in first emergence towards mid June and it is evident that climate change could well become an important factor in the butterfly’s survival.
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