The Trojan horse of the butterflies

A super specialist with a spectacular resume

Believe it or not, ants are among the Black-spotted Large Blue’s worst predators. Nevertheless, ants - or more precisely, certain types of ants, feed and protect the butterfly larvae. Mature, Black-spotted Large Blue butterflies only lay their eggs on a specific plant which also happens to be our particular ant’s food source. Once hatched, the young caterpillars eat like other common larval-stage Lepidoptera on plants. After this stage of their life cycle, the caterpillars then shed their outer “skin” and change their appearance, transforming their bodies so they no longer resemble caterpillars but rather…maggots.

And if you thought that was strange, wait for what’s next. The maggoty-looking butterfly larvae then drop to the ground and wait for the arrival of their enemy! In doing so, they run some risks: being eaten by other ants, birds and other predators or simply starving to death unnoticed. However, if the “right” ant comes along ̶ only two species fall into this category: the saber-thorned knot ant and the wet turf knot ant ̶ and finds a larva, it doesn't attack or eat the grub but gives it a "feel", and in the best case, adopts it. Why, you may ask. It’s because the larvae not only look deceptively like ant larvae ̶ they can curl and inflate their maggot-shaped bodies to resemble them ̶ they also act like them too, making the same sounds and using their scent glands to secrete smells that are very attractive to the ants. In other words, they camouflage themselves chemically.

The unsuspecting ants then carry the larva into their nests placing it with their own brood. If the butterfly larva has made it this far, it is set. The ants feed the imposters with sugar water who can also eat up to 600 ant larvae during their stay in the enemy’s brood chambers. As pupae – the stage between larva and adult̶ they are still chemically masked to the ants since they have acquired the colony specific odor. However, the whole endeavor is rife with perils. Caterpillars can perish in the ant nest, either because they are attacked by "scent-immune" workers, or because they plunder the ant nest until it is empty ̶ destroying their food supply. And, once the larva has undergone its metamorphosis and emerges from its cocoon, the deception can no longer be maintained: the newly transformed butterfly must flee the ant nest as quickly as possible. Only when it reaches the earth's surface does it stretch and dry its wings. It then flies off to mate and again the females lay their eggs on the right food plant: the fascinating life cycle begins anew.

An extremely endangered trickster

"Among blue butterflies, the Black-spotted Large Blue is one of the largest. The fact that it is such a trickster makes it the most exciting of the butterflies for me," says Elia Guariento, an entomologist at Eurac Research. "It is such a highly specialized species: needing just the right food plant - thyme or oregano - and just the right ant species - the saber-thorned knot ant or the wet turf knot ant - to complete its life cycle," Guariento adds. "Unlike, the widespread Small White or Small Tortoiseshell butterflies which fly for miles to access new habitats, the Black-spotted Large Blue spends its entire life in a limited area, where every condition has to be just right. That makes it very vulnerable as a species." Despite its rarity ̶ the Black-spotted Large Blue is on the global list of endangered species - Guariento has found it in meadows and pastures in South Tyrol. "In the past, extensive pastures and meadows were the norm and as such, the butterfly's particular way of life was not so far-fetched; on the contrary, the larva had the advantage of being fed and protected by the ants and therefore were less likely to be eaten," Guariento says. These days however, extensive meadows and pastures are increasingly rare habitats, and their preservation is particularly important, underlines Guariento: "What you do to preserve the landscape is not only good for the butterfly but for the entire ecosystem and everything related to it. It’s also ultimately the basis of our life and economy."