Large Blue - Biology

Biology

Large Blue caterpillars feed on wild thyme or marjoram flowers for the first few days of development. Once well fed, the caterpillar secretes sweet fluids to attract red ants of the species Myrmica sabuleti. The ants then take the caterpillar back to their nest and proceed to feed from these secretions. The caterpillar then hibernates inside the ants' tunnels. On emerging from hibernation, the caterpillar begins to eat the red ants' eggs and larvae, which it does for up to three weeks. It then suspends itself by its legs from the roof of the nest and pupates. The caterpillar spends a further three weeks transforming into the Large Blue butterfly adult. After its change from caterpillar to butterfly, the insect emerges from its chrysalis and leaves the ant nest to find a mate. Usually, red ants will escort the newly emerged butterfly to the surface, taking it to a low plant or shrub nearby. The red ants will encircle the butterfly and ward off any predators that attempt to attack the butterfly as its wings unfold and harden. When the butterfly is ready to fly, the ants will retreat back into their nest.

In addition to Myrmica sabuleti, there have been instances of the butterfly larvae using Myrmica scabrinodis but with much poorer survival rates.

In the UK and in cooler or more mountainous areas in Europe, wild thyme is the food plant, marjoram being used by populations in warmer areas. Fertilized Large Blue females will lay their eggs only on the buds of wild thyme or marjoram, which young caterpillars feed on exclusively until after the third moult, when they are ready to seek out the appropriate ant nest. At sites where Large Blues occur, there is usually a high density of such nests. When the caterpillar is ready, it will drop itself off the food-plant and wait to be picked up by a passing red ant, which will then carry it to its nest. The red ant strokes the caterpillar with its antenna, making the caterpillar produce a small drop of honeydew. This action is called "milking", which the ant continues to do until the caterpillar raises half of its body into the air. When this happens, the red ant picks the caterpillar up with its jaws and takes it into its nest. Once inside the red ant's nest, the ants will stroke the caterpillar with their antennae. After a while, the Large Blue caterpillar secretes more honeydew, on which the red ants and their young feed. Soon after this process is complete, the caterpillar will seek out the nesting chambers and proceed to eat red-ant eggs and larvae. When winter draws near, the Large Blue caterpillar begins to hibernate.

Often, the ant colony will bring in too many Large Blue caterpillars. When this happens, the caterpillars usually eat all the ant eggs and larvae, eliminating the colony. With no more ant eggs to feed on, the caterpillars starve to death. Another bizarre and unexplainable occurrence is that, often, when the ant colony produces more than one queen ant, the workers are sent to attack and eat the caterpillars.

To make itself less intrusive to the ants, a Large Blue caterpillar will assume the ants' scent and mimic their sounds. Once the caterpillar is inside its chrysalis, it will rub its head against the chrysalis walls to make a scraping sound similar to that made by the ants. Failure to adopt any of these measures puts the caterpillar at risk of being eaten by the ants.