12th August 2017
The weather is calm, cloudy and warm at Les Tourbières de Vendoire; a good day for looking for the scarce large blue Phengaris teleius butterfly, a species now restricted to only a handful of locations in western France. The French name l’azuré de la sanguisorbe is more fitting as the food plant of the freshly emerged caterpillar is the flower of great burnet Sanguisorba officials, a plant entirely restricted to wet meadows and streamsides on base rich soils. The flight period is short from mid July to early August; today the great burnet plants are still coming into flower and standing tall in the small patches of dry fen that are full of black-berried alder buckthorn Frangula alnus saplings. Trampled paths through the fen tell of a number of earlier, eager butterfly hunters.
The main path runs down to the river Lizonne, where it curves round before running back, crossing wide ditches on elegant wooden footbridges, along the straight edges of the shallow pools or tourbières formed from old peat cuttings. On a calm day in the silence of mid-August, the reflection of woodland on rush-lined water is reminiscent of a Japanese garden.
A willow tit wheezes from the alders and other tits and warblers work through the bushes mostly hidden from view. Dryad butterflies are common alongside the ubiquitous meadow browns and gatekeepers; all fluttering up on approach before settling again. A great banded grayling swoops beneath the oaks and alders on huge, black wings with a thick, white trailing edge. When it lands, the wings are immediately folded to present an understated, intricate camouflage.
Just one scarce large blue briefly flies weakly over the buckthorn in the sun then settles on a grass stem as a large, lumpen cloud cools the day. After a wait, another bright spell appears and the female exercises her wings back and forth and then suddenly is off, instantly egg laying on the base of the nearest great burnet buds. Then zig-zagging away through the fen and is lost. There are no others to be found in the fen; this female may well have been the last of the year.
The seasonal warden in the visitor centre is generous with information; and told us that there was a maximum of 35 butterflies this year; also that there were a few other colonies nearby along the Lizonne. One site had been been cut in the spring and so there was no food plant and no butterflies and possibly no more colony. Scarce large blue caterpillars hatch from the eggs to feed on the great burnet flowers, then travel down the long stems to the ground where they move into ant nests and feed on the broods through the winter. The peat bog species Myrmica scabrinodis is reported as a frequent host species. Such a specialist life cycle requires the habitat, naturally restricted to river floodplains, to be abundant and for seasonal grazing or scrub clearance in winter to permit the presence of both ants and food plant in the summer meadow. There is little space in the modern landscape for such a pernickety species today, so the name scarce large blue is, unfortunately, entirely apt.