Ecology Marvel of the Day: Large blue butterfly and the red ant Myrmica sabuleti

Extinct in Britain in the 1970's, this beautiful butterfly has been successfully reintroduced to several former haunts. But the real marvel lies in it's utter dependence upon a single ant species.

The large blue butterfly Maculinea arion was first recorded as a British species in 1795 and, even then, was considered a rare insect. Due to the loss of suitable habitat, the endemic subspecies of large blue became extinct in the British Isles in 1979, the last site being on Dartmoor in Devon.

This magnificent insect has since been "brought back from the dead" through the dedication of several conservation organisations and many individuals, using stock from Sweden. Today at least 10,000 adults can be found on reintroduction sites in the south-west of England, notably the Polden Hills in Somerset, Dartmoor and Gloucestershire.

The successful reintroduction of the large blue is, however, even more remarkable when one considers its elaborate life cycle. Large blue butterfly caterpillars exhibit a form of parasitism in which they take advantage of a host species, a red ant, Myrmica sabuleti, on whom its existence depends.

The caterpillars are able to chemically-trick the ants into taking them back to the ant nest. Once there, the caterpillar will either become a predator of the ant larvae, or beg for food by acting like an ant larva in a "cuckoo" strategy.

Since the parasitic-host relationship between the large blue and M. sabuleti is essential for the species' survival, female butterflies must lay eggs in areas where the larvae can be found by ant workers of the correct species. 

Females lay eggs on wild thyme Thymus polytrichus. After about three weeks, larvae hatch to feed on the seeds and flowers of the plant. The caterpillar will stay in the vicinity of its food plant until its 4th moult, at which time it will drop to the ground. From there the caterpillar secretes pheromones that are specific to their ant hosts and mimic the pheromones of ant larvae. By doing so the ants take the caterpillars back to their nest and tend them.

Female large blue

Once in the nest, the caterpillar will use acoustic mimicry in order to further confuse the ants and hide its true identity. The caterpillar continually emits a noise very similar to that of a queen ant, which results in them being fed by the worker ants and given preferential treatment over the ant larvae.

The caterpillars become such superior members of the nest that the ants will kill their own larvae to feed to the caterpillar and will rescue the caterpillar first in the face of danger. The large blue also becomes a predator once it is in the ant nest. It feeds on the ant pupae, predating the largest of the ant larvae first. 

However, the risks of such deception come at a price and mortality rates within the nest are high. If the mimicry is not perfect and the ants become suspicious, they will kill and eat their unwelcome guest. Also, ants in nests without a consistent supply of food are somehow much more likely to identify the large blue as an intruder.

While in the nest, large blue caterpillars acquire 99% of their final biomass, growing from an average of 1.3 mg to 173 mg. An estimated 230 large larvae and a minimum of 354 Myrmica workers are needed to ensure the survival of one butterfly, however such a large nest is very rarely found in the wild.

Here, the cunning imposter has another trick up it's sleeve: large blue butterflies are capable of withstanding starvation, beneficial in situations when the ants desert the colony and leave the caterpillar behind, which has been known to migrate to a new nest once the original is deserted. In this way, large blues may parasitize multiple Myrmica colonies.

The queen effect

It has been found that large blue butterflies are three times less likely to survive in nests that have queen ants present. In most Myrmica nests, the queen ant will lay two main batches of eggs, and the females that hatch from these eggs will either become workers or virgin queens, depending on the status of the queen in the nest. If the queen dies, the largest female larvae become virgin queens. If the queen is present and healthy, she will influence the nurse workers to neglect, starve and bite the female larvae which results in restricted growth and aids in their transition to workers.

Thus, large blue caterpillars must maintain a strict balance between mimicking the queen in the presence of workers and appearing to be a worker to avoid the queen.

Anyone wanting to see this fascinating species should visit Collard Hill in Somerset. A "Large Blue Hotline" is usually set up each year that provides an up-to-date status of the emergence at this site. Details are available on the Butterfly Conservation website.