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.
Large Blue female.The caterpillar overwinters in the ant nest, going dormant in time for the ants to resume their normal activity and ensuring the colony will survive until spring. Still underground in the ant nest the caterpillar transforms into a chrysalis in the spring. After it has hatched the adult butterfly must leave the ant nest rapidly, to fly free and ensure the reproduction of its species.
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.
Large Blue ovipositing on Wild Oregano Origanum vulgare.Following a series of local extinctions, naturalists have brought to public attention the strong regression of Maculinea in Europe over the course of the last decades. British readers will no doubt be familiar with the extinction and reintroduction of the Large Blue there. Hopefully this flagship group of butterflies has been instrumental in influencing policy so that management systems that are sympathetic to insect lifecycles and ecology are more widely practiced. If not, the continued survival of these remarkable creatures cannot be guaranteed.
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.