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The Story of the Butterfly That Got Adopted by A Red Ant

With unexpected infanticide and toxic chemicals, it’s a story loved by all

A Large Blue butterfly (Maculinea arion) (PJC&Co via Wikimedia Commons (CC BY-SA 3.0))
smithsonian.com

Once there was a caterpillar, hungry enough to chow down on the flower buds of a plant known for its toxic fumes. One day, the caterpillar left the plant and fell to the ground. There, an ant passing by picked it up and took it back to the ant nest to care for the little caterpillar. The caterpillar made itself at home and started eating all the larvae in the ant nest until one day it turned into a beautiful butterfly.

The story is true and the players — oregano, the Large Blue butterfly and the red ant called Myrmica — have a unique relationship, the details of which scientists have just unraveled, reports Nicholas Wade for The New York TimesThe toxic fumes and apparent betrayal by the caterpillar make the story a little less children’s book and a little more classic Mother Nature. 

The caterpillar fools the ant into thinking that it is a misplaced grub from the ant’s own nest. It does this by mimicking the posture of an ant larvae and chemically cloaking itself in a scent the seems like the ant. Once the caterpillar has been "returned" to the nest, it start clucking — a sound that imitates the ants' queen. This ensures that the ants will leave it alone when it starts munching the nest’s own larvae. As Wade writes, "The ants themselves use their larvae as a food source when times are tough, so for their queenly guest to behave like a cannibal may not strike them as all that abhorrent."

The last piece of the puzzle though comes from the toxic fumes that oregano emits. These fumes are an insecticide called carvacrol, which is supposed to keep munching insects away. (It also gives oregano the warm, pungent smell that humans value — the same chemical can be found in thyme, pepperwort and wild bergamot.) Red ants are also repealed by the smell, but they do build their nests close by oregano plants to avoid competition.

Sensing the ant nest nearby, oregano plants tend to double their carvacrol output. And it's that extra pungent output that attracts the adult Large Blue to lay their eggs. Researchers based at the University of Turin and the University of Oxford proposed this scent-guided mechanism in a study published in Proceedings of the Royal Society B

Despite the somewhat alarming nature of the tale, the story of the butterfly adopted by the ant (and the plant that grows nearby) is one of mutual advantages. Wade writes:

The oregano sacrifices more than a dozen of its flower buds to each Large Blue caterpillar, but benefits because the growing caterpillar can wipe out the ants that are irritating its roots. The Myrmica ants may lose a few colonies to Large Blue caterpillars, but this is a small price to pay for the oregano’s protection against their many ant rivals. The Large Blue exploits the oregano-Myrmica association to gain safe underground nurseries for its brood.

However, though it seems that the Large Blue comes out on top, the fact that the species relies so heavily on specific ants also makes it vulnerable. As grazing in England restricted Myrmica’s habitat, the Large Blue populations declined and eventually disappeared. When Jermey A. Thomas of the University of Oxford, one of the new study’s authors, realized this, he persuaded people to change their grazing habits and introduced a close relative of the extinct butterfly — a Large Blue from Sweden — back to the habitat.

The transplant is flourishing, taking advantage of the ants and listening to oregano’s chemical signals, to the happiness of all.

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