A Dangerous Dance that Works for All
Before your pungent oregano reaches your spice rack, the plant may have played a role in an inter-species drama of life, death, treachery and transformation that scientists have only recently begun to unravel.
The Large Blue, an endangered butterfly once widespread in Europe, lays its eggs on the wild oregano plant. The caterpillar feeds on the plant’s dainty flowers for about two weeks. Then it drops to the ground, where it tricks a red ant known as Myrmica into taking it into the Myrmica’s underground nest. There it spends the next 10 months cannibalizing its faux siblings in the ant colony until it swells to 50 times its original weight, turns into a pupa and emerges as a Large Blue butterfly.
A New York Time story by Nicholas Wade describes this bizarre inter-species interaction. How does the caterpillar deceive the ant into thinking it’s one of the ant’s own lost grubs? Its first tactic is timing, Wade explains. It falls from the oregano plant at night, just when the Myrmica comes out of its nest to forage. It adopts the physical posture of the Myrmica grub and even exudes a scent that mimics the grub, so the deceived ant takes it home. Once inside, the imposter mimics the little sounds made by the ants’ queen, positioning itself to start feasting on ant larvae. Wade writes that ants eat their own larvae in hard times, so the adoptee’s behavior may not seem unduly odd.
Scientists have known about the association between the Large Blue and the Myrmica for a century, but researchers have only recently figured out how the Large Blue knows to lay its eggs in proximity of the underground nests of the only ant specie that will adopt its caterpillars. Dario Patricelli and Emelio Balletto at the University of Turin in Italy and Jeremy Thomas at the University of Oxford have developed evidence that the oregano plant brings the butterfly and ant together chemically. In Proceedings of the Royal Society B, the researches explain that oregano exudes insecticide fumes to keep ants and other pests away. The Myrmica evolved to detoxify the oregano’s insecticide, called carvacrol, which allows it to flourish without other competitors. As the underground ants nip at the oreganos roots, the irritated plant doubles its carvacrol output, which signals the butterfly about the presence of its flowers and the location the Myrmica ants.
Despite its deadly ramifications, the system works well for ant, plant and butterfly, the researchers say. The oregano sacrifices some of its flowers, but benefits because the caterpillar will wipe out many of the ants that disturb its roots. The Myrmica ant loses some of its colonies, but the oregano’s emissions provide it safe space free of rival ants. The Large Blue gets a safe underground nursery for its offspring.
When Large Blue and other butterflies were nearing extinction in England, Jeremy Thomas, the Oxford ecologist, discovered one reason: changing land use was destroying the Myrmica ant habitat. These particular ants are sensitive to temperature, he found, and need warm soil just below the surface for their colonies. When farmers stopped grazing their animals on hillsides with poor soil, longer grass made the soil cooler. Thomas figured short turf in the butterfly’s habitat would aid its survival, but he found human habits hard to change. “To persuade people to change the management of nature’s reserves, and then see the results, took many years,” Thomas told the Times. “By the time I had got people to turn things around, the butterfly was gone.” Undeterred, Thomas introduced a nearly identical variety of Large Blue from Sweden, and now the Large Blue flourishes in 30 sites in Britain. Read the Times story here and the Proceedings article here.