Thursday, 25 June 2015

On the Wing: The Large Blue Butterfly

Last Saturday I finally managed to see the Large Blue Phengaris (Maculinea) arion at Collard Hill Hill reserve in the Mendips. As they are on the wing for perhaps another week or so, there is not much of a time window to see one this year. As one of the rarest of British butterflies, and with one of the weirdest lifecycles of any butterfly, it is definitely one to look for.

Wild Thyme
The lifecycle starts with an egg laid by the female in the depths of a Wild Thyme flower, which hatches after around 5 days. If more than one female lays an egg on the same flower head, the smaller larvae are eaten by the older ones until only a single larva survives. The normal food of the larva for the first t2 or 3 weeks is the pollen and developing seeds. By then the larva is ready to proceed to the next stage, and drops to the ground to hide next to the soil.

In this location the ant is found by a foraging Myrmica ant. The larva produces a liquid from a gland on the back which attracts the ant, which milks it sometimes for several hours. In the process the ant transfers its scent to the larva, which then changes its appearance by hunching up and in the process makes it appear to be an ant grub. The ant is thus deceived to take the larva back to its nest and stored with its own larvae. At this point, the next hurdle in the survival strategy has to be passed. There are several species of Myrmica ant in the habitat, and while any make take a larva home, only a M.sabuleti has a reasonable chance of raising the larva. This is because the nurse ants tending the brood are more discriminating than foraging workers, and only if the larva produces exactly the right pheromones will they tolerate it rather than killing it before it has a chance to feed.

If they are successfully fooled by the larva this is a potentially lethal error, as the butterfly larva then turns carnivorous, feeding on the ant grubs. To develop to adulthood, a Large Blue larva must eat probably over a thousand grubs. This appetite is sufficient to wipe out a small colony, but the larva can survive even this. It can go long periods without food, and if necessary it waits in a small cell until a neighboring colony expands into the vacant nest, at which point it resumes feeding. Even when it becomes a chrysalis the manipulation continues – the chrysalis produces sounds and pheromones which induce the workers to treat it as if it is a queen ant.
Common Spotted Orchid
Large Blues were always rare and in scattered colonies, but a combination of overcollection by early entomologists and changes in grazing on the few sites resulted in its decline to extinction by 1979. Today a reintroduction scheme using stock from Sweden has been highly successful, with the original site at Green Down potentially supporting several thousand individuals in the course of a season. Collard Hill is not that large, but it has spread naturally to several other sites, with perhaps 25 colonies now know.
Pyramidal Orchid
A visit to Collard Hill is also good for several other butterflies. Marbled Whites are also now on the wind, as are Small Skipper, Meadow Brown, Common Blue and Ringlet. Parking is best in the layby opposite the youth hostel in Ivythorn Road. Orchids are also showing well, especially Common Spotted and Pyramidal orchids. For up to date information, check out the Collard Hill blog here:

Outside the UK, there are several species of Phengaris ranging from western Europe as far as Japan and parts of northern India. Although the details of the life cycle are unknown for many of them, they all seem to have a similar life cycle. In at least 2 European species, the larva is fed directly by the worker ants as well as feeding on the ant grubs.

(images are mine)

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