Long-tailed Blue; Annus Mirabilis Number Two
After a very average and sometimes disappointing summer for butterflies, we are on the cusp of some late season fireworks, courtesy of the Long-tailed Blue.
The stage is now set for an event which might rank alongside, or even surpass, the historical 2013
boeticus annus mirabilis. There is little doubt in my mind that had our August weather not collapsed, the influx of Long-tailed Blue into Sussex would have eclipsed the 2013 invasion.
Whether this statement can be applied to other parts of the South of England is unclear, as I don’t know how much effort has gone into searching out evidence of the early stages in neighbouring counties.
Reports of Long-tailed Blue numbers in Belgium and Brittany, together with sparse records in the UK, suggest that a significant movement into northernmost mainland Europe and the South of England started during the period 7 – 9 August. However, it appears that this was preceded by a smaller movement north, with the first British record (Devon) on 5 July and first Belgian record on 9 July.
I posted a couple of messages on the BC Sussex website, encouraging our members to look out for the Long-tailed Blue. A small group of dedicated enthusiasts took up the challenge; “find the pea, find the Pea Blue”.
Tracking down these primary immigrants (at least the adults) has proven challenging and time-consuming, but there is always the chance of being in the right place at the right time. At this stage in the migration/breeding cycle the butterflies are highly mobile, so almost impossible to ‘twitch’. They tend to lay a few eggs and then move on, unless there is a very large quantity of food-plant readily available.
When the main UK emergence occurs, which I predict is now only a week away, I expect the adults to behave very differently, and they should be much easier to catch up with. If my theory is correct (Thomas & Lewington, 2014, p.109), the females will mate very quickly and fly back south, carrying their fertilised eggs with them. The males will behave in a colonial manner and hang around, hoping to grab more virgin females. I suspect many males will die here, although based on limited data, it is possible that some might also make a late dash for home.
Of those sightings of adults reported to the BC Sussex website (Worthing private garden, 28 & 29 August; Seaford private garden, 30 August; Bognor Regis, Hotham Park and adjacent university campus, 5 & 6 September; Brighton Racecourse, 11 September), only the Bognor adult(s) was in fresh condition (the fringe hairs are lost very rapidly in this species). I believe that this was the first 2015 sighting of a British-born
boeticus, being the progeny of one the early pioneers.
A few more adults have been seen in Sussex, with the finders preferring not to advertise the locations for a variety of reasons. This species is as likely to turn up, perhaps more so, in urban areas, which is unsurprising bearing in mind the most regularly used food-plants. Locations relating to domestic addresses cannot be made public for obvious reasons.
A few days ago a friend called me to say he’d found a couple of Long-tailed Blues and while I was on my way to join him I heard that they had decided to mate! I arrived just in time to watch the last few minutes of this historic event.
I believe this is the fourth UK record of mating
boeticus, but more importantly, the first between adults within the immigrant wave. It seems likely that females mate before dispersing (Thomas & Lewington, 2010) and this suggestion fits with my own theory that the mated females in the autumn generation immediately fly south. However, both of these individuals were of considerable age and showed significant wear and tear, sustained over their long journey.
When we see butterflies in cop they invariably comprise either two freshly emerged individuals, or a pristine female partnered by a mature and worn male. However, exceptions do occur and I’ve witnessed elderly (very worn, rather than damaged) female Small Heath and Common Blue in cop. These must be second pairings of the female.
Furthermore, the worn female
boeticus we watched in cop was observed ovipositing within ten minutes! Despite much oviposturing, a single egg was eventually located. This therefore appears to have been a ‘top-up’, second pairing of the female. Bearing in mind the lifestyle of this species, perhaps this is not uncommon (and quite possibly advantageous).
If Jeremy’s theory is correct, and migrating females are already carrying fertile eggs, this might explain why they venture north alongside males. It would be useful to have males always in attendance, to help keep these highly fecund females in good reproductive shape, over very long and arduous journeys.
Perhaps the greatest reason for optimism, in believing that we are about to see a widespread emergence of British
boeticus, is the ease with which I have found their eggs. I first found hatched ova in Sussex on 21 August, and have been searching ever since. A week later, others joined the search, and a clear picture has emerged.
I have personally found Long-tailed Blue eggs in Bognor Regis, Worthing, Lancing, Beeding, Brighton (two sites), Newhaven and Seaford, with my tally fast approaching 100. These locations cover a 50 Km stretch of the Sussex coastline, with my strike rate suggesting that adults must have arrived across this entire front. My highest count was 9 ova on a single raceme of Broad-leaved Everlasting Pea!
I wonder how far west and east of my search area this migration front extends? One thing is certain; this must be a very large event.
To see these numbers of Long-tailed Blue entering the UK, so soon after the unprecedented 2013 influx, does suggest that the species is undergoing a significant change in either its geographical range or its behaviour. I suspect that its southern belt of permanent residency in Europe is shifting north in response to climate range, and that annual forays northwards are therefore reaching further into northern mainland Europe and, happily, into the South of England.
While so many of our resident butterflies are facing serious problems, some of which are likely to be exacerbated by climate change, it is good to know that species such as the Long-tailed Blue appear more likely to grace our shores in the future. Only two days ago another
gorganus Swallowtail was seen near Eastbourne.
The study of the British brood of Long-tailed Blue, including the emergence pattern of males and females, confirmation of pairings, evidence of egg-laying (or lack of it) and any suggestions of departure south, will hopeful start to clarify the autecology of this species, much of which remains speculation.
Female ovipositing - note ovum to left of frame (8.9.15)
Mating pair (8.9.15)
Male, Brighton Racecourse (11.9.15)
Male, Brighton Racecourse (11.9.15)
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