A female alcon blue butterfly lays its eggs (Image: David Nash) A worker ant carries an alcon caterpillar to its nest (Image: David Nash)

A beautiful butterfly is able to fool ants into rearing its young by masking them with the ants’ own smell, say researchers.


Caterpillars of the alcon blue butterfly have developed an outer coat that tricks ants into believing the young are its own, duping the ants into carrying the larvae back to their colonies to care for.

But what is more, the ant seems to “recognise” that it is being duped and one population appears to be engaging in an evolutionary arms race with the butterfly, says the team led by David Nash at the University of Copenhagen, Denmark.

The Maculinea alcon butterfly has a parasitic relationship with two species of Myrmica ants in Denmark. The butterfly’s caterpillars begin life feeding off a plant, then, still as caterpillars, they drop to the ground where they wait to be picked up by passing Myrmica ants, who take them back to their nests.

Chemical signature

It had been suggested that the ants mistake the caterpillars for one of their own larvae because their waxy waterproof coating carry similar chemicals. The ants “taste” the caterpillar’s coat with their antennae, recognise it, and treat it as one of their own young.

Nash’s most recent research confirms this theory. By comparing the composition of the waxy coating on the ants and the caterpillars, they found that the closer the caterpillars mimicked the ants, the faster their adoptive parents took them back to their nest.

The time it takes the ants to take them in is critical because once they leave their food plants the caterpillars do not feed until they are incorporated into the ant brood. Once in the nest, the ants feed the caterpillars more than they feed their own.

“This could be because they are larger. The ants might believe they have a really good larva of their own here and therefore should feed it extra,” says Nash.

Arms race

Nash and colleagues also found signs that the ants and butterflies are engaged in an evolutionary arms race.

Back in 2000, the researchers discovered that when they took an alcon butterfly larvae and introduced it into an ant population that was not normally parasitised, the caterpillars had a higher survival rate.

This suggested that regularly exploited ants have somehow adapted a defence against the butterflies.

“We have now sampled larvae of ants from parasitised populations and other populations where alcon butterflies have never been recorded,” Nash told New Scientist.

They found that the ants that are untroubled by the butterflies have very similar chemical signatures in their waxy outer coats. In contrast, ants from colonies that are often parasitised have evolved a much greater diversity in their chemical signatures. This suggests that the two species are constantly evolving new signatures in order to trick and evade trickery.

But, in a final twist, it appears that the butterflies may actually be killing off their hosts.

Host switch

The butterfly’s success may even have depleted one population of Myrmica ant, known as Myrmica rubra. And there are signs that, as a response, the butterflies are beginning to parasitise another species – one they did not exploit three years ago.

“When one host species becomes sufficiently rare, it pays the butterflies to switch to another and use that one until the adaptation by the first host species has been lost,” explains Nash.

It is still too early to say for certain if this is happening in Denmark, but Nash and his colleagues have started a long-term monitoring experiment to see if such a switch does happen in the next few years.

What is interesting, says Tommi Nyman, an expert in insect parasites at the University of Joensuu in Finland, is that the butterfly forces the ants to become more diverse. “But all victories are ephemeral because the butterfly larvae can (and must) then adapt to new chemical profiles.”

Journal reference: Science (vol 319, p 88)