The Large Blue butterfly, Phengaris arion has a fascinating and complex life cycle as a social parasite: eggs are laid on wild thyme flowers and, on hatching, the larvae (caterpillars) spend the first few weeks feeding on the flower head tissues with developing seeds (if two eggs hatch on the same flower head, then one larva is said to eat the other!). After the fourth growth moult, the surviving larva falls from the flower onto the ground and produces a chemical which attracts red ants Myrmica sabuleti by mimicking the smell of young ants. Considered to be an ‘escapee’ from their nest, the ants move the butterfly larva underground where it spends the next eleven months killing and feeding on their young in the brooding chamber while still being cared for by the worker ants.
Research has indicated that the butterfly larva further integrates into the ant colony by producing sounds similar to that made by the Myrmica queen – so in the event of nest disturbance, the raised status of the larva ensures it is cared for before that of the ants’ own brood! The larva starts feeding on the biggest and leaves the smallest of the ant brood to grow, thereby ensuring it has a constant supply of food until it is ready to pupate. After spending most of its life underground, the adult butterfly finally emerges from the ant nest and climbs the nearest vegetation where its wings are inflated. Mating occurs and the cycle starts again.
The Large Blue became extinct in the UK in 1979; however it has since been successfully reintroduced as part of a major conservation project. There are five large, core populations which have begun to naturally colonise key areas, but they may still be susceptible to slight changes in their habitat whether from change in management or climate. If thyme plants do not flower when the females are ready to lay their eggs, if the eggs are not laid near a red ant nest or if that nest is not large enough to sustain the butterfly larva, then the Large Blue is unlikely to survive in that location.
