This year has been a disaster for most of the Lycaenidae (Blue) butterflies here. Numbers remained low throughout the exceptionally dry spring and even with our relatively wet summer numbers have stayed low. One of the few blue butterflies which appears to have done reasonably well (although never common, even in good years) is the Large Blue Maculinea arion* (l'Azuré du serpolet in French).
The big blues Maculinea spp have a very particular lifecycle and ecology. Females lay eggs on the flower buds of the caterpillar host plant. Once hatched the caterpillar consumes the buds, hidden amongst the flowers for 3 or 4 weeks. Then, while still very small, it descends to the ground and waits to be adopted by an ant Myrmica spp. Once a suitable 'nanny' arrives the ant and caterpillar engage in a ritual which establishes their bond and the ant carries the little caterpillar off to its nest. The young caterpillar completely transforms the operation of the ant nest. Some Maculinea, the Large Blue included, devour the surrounding ant larvae, others just feed on the liquid the ant workers produce for their own larvae.
This extraordinary lifecycle explains the extreme vulnerability of Maculinea populations. Their existance depends on a single plant within an area (very rarely will there be two host species on a single site) and a single species of ant which is essential for the growth of the caterpillars. The host plant must be at the beginning of its flowering season and not grazed or mowed during the egglaying period and the density of the ant nests must be such that the ants and the caterpillars have a good chance of meeting. And these plants and ants have their own ecological requirements.
Here in the Touraine we are very lucky to have 3 species of Maculinea present - Alcon Blue M. alcon (l'Azuré des mouillères); Large Blue M. arion (l'Azuré du serpolet); Scarce Large Blue M. telejus (l'Azuré de la sanguisorbe). The Large Blue in the photographs was photographed this August in the field opposite our orchard.
*Syn. Phengaris arion, Glaucopsyche arion.