“Cuckoo” Butterflies Get Ants to Raise their Kids

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There aren’t many things more beautiful than a butterfly—or many things creepier than parasites. It’s hard to think that you’d ever find one of the former that’s also one of the latter, but nature is full of surprises.

Meet the Alcon Blue butterfly, a beautiful liar. This species and many of its cousins in the genus Phengaris (sometimes known as Maculinea) raise their young the same way cuckoo birds do. Which is to say, they don’t raise them at all, and instead let other animals do the hard work of parenting for them. 

The butterflies’ hosts are industrious Myrmica ants, which adopt the Phengaris larvae and are fooled into providing them with shelter, protection, and food until the caterpillars are ready to pupate, turn into butterflies, and fly off from their foster home. Ant nests are an attractive target for parasites. They’re well-defended and have plenty of resources to leech off of, and there are some 10,000 species of insect that have evolved to exploit them in one way or another. The penalty for intruding in a nest, though, is usually dismemberment and death. A parasite needs a way to blend in if it’s going to make itself at home in the nest, and Phengaris caterpillars have a few tricks up their sleeves. 

The con starts after the caterpillar has gone through three molts while living and feeding on the flower where the egg it hatched from was laid. The caterpillar drops to the ground and waits for a group of Myrmica worker ants to march by. Ants communicate mainly through chemicals, and distinguish friends from foes by detecting blends of hydrocarbons on each other's bodies that can tell them if another ant is from the same species or nest, and even identify which caste it belongs to. The caterpillar fakes this chemical ID badge by secreting hydrocarbons that mimic those made by its host ant species. If the disguise is good enough, the workers will pick the caterpillar up, carry it back to the nest, and place it among their larvae as if it were one of their own. 

Once it’s comfortably tucked away in the nest, the caterpillar reaps the benefits of life as an ant. It spends anywhere from 10 to 22 months in the nest, eating and growing and putting on as much as 100 times its initial weight. At this stage, it employs one of two strategies, depending on its species. Some “predatory” caterpillars move to an outer area of the nest and live there, making occasional raids on the ant nurseries to binge on larvae and eggs. Others act more like cuckoo chicks and let their hosts care for them. They stay in the nursery and are fed foraged food and sometimes ant eggs by the nurse ants. 

These “cuckoo” caterpillars are also social climbers, and augment their mimicry with sound. By imitating the caste-specific sounds that ant queens make, the parasites get treated like royalty. The nurses feed them first, feed them more and, when food is scarce, give them preference over actual ant larvae, who go hungry. Other worker ants, meanwhile, respond to the caterpillar’s noises by taking up the “on-guard” posture they use when attending the queen. If the nest is disturbed or attacked, and the caterpillar is trapped under soil, the queen-like sounds will prompt workers will rescue the impostor first, even at the expense of the ant larvae and eggs. 

When the caterpillar is fully developed, it pupates and transforms into an adult butterfly. Now its stay in the ant nest is over and the jig is up. It can no longer produce the chemicals needed to disguise itself, so the butterfly has to flee the nest before its hosts realize there is a stranger among them. If any of the ants do recognize it as an intruder while it scurries towards the exit, the butterfly is covered in scales that protect it from bites until it can escape.

The ants aren’t total dupes in this situation. Over time, scientists have found, some nests that are regularly exploited by freeloading caterpillars will shift their chemical signatures, making it easier to see through the caterpillars’ imitations. This kicks off an evolutionary arms race, and Phengaris must either counter-adapt and match the new blend of chemical secretions, or find other nests, ant populations or species that it can more easily fool with what it’s got. 

Unfortunately for the ants, even the hydrocarbon shibboleth can’t protect them from another problem that the caterpillars sometimes bring with them—a wasp that parasitizes the parasites.

The wasp Ichneumon eumerus lays its eggs in certain Phengaris caterpillars. It sniffs out nearby ant nests belonging to species that normally host the caterpillars and flies to the nest entrance. There, the mother wasp determines if there’s a caterpillar inside (though scientists aren’t sure how the wasp is able to figure this out when the caterpillar is disguising itself as just another ant) and goes into the nest if she detects one. Once inside, she releases a chemical cocktail that repels the ants from her and drives them into a frenzy that causes them to attack and kill each other. As many as 80 percent of the ants in the nest can be immobilized or distracted as confusion and infighting spreads, allowing the wasp to head towards the nursery and inject the defenseless caterpillar with her eggs. When the caterpillar pupates, the eggs hatch and the wasp larvae consume the would-be butterfly from the inside out before escaping in their own fog of chemical confusion.