If a job offer ever opens up for a Halloween mascot, the crypt-keeper wasp is a shoo-in. These iridescent harbingers of horror have developed a diabolical survival strategy that involves mind control and zombification. Hang on to your jack-o'-lanterns, here comes a very real and very macabre tale from the crypt. 

Found throughout the southeastern United States, Euderus set (a name fittingly derived from the Egyptian god of chaos and war) deposits its eggs inside oak-tree chambers created by parasitic gall wasps. These tiny chambers, known as galls, are like blisters that form in the trees induced by the presence of the gall wasps' young. These larvae hatch and gorge themselves on nutritive tree tissues. Once the gall-wasp larvae have matured and are ready to face the world, the adults chew their way out of the chamber. Unless, of course, they are being manipulated by a crypt-keeper.

While gall wasps have a knack for tricking their plant hosts into providing a nutrient-rich, protective home, crypt-keepers can do one better: they manipulate the manipulator. When crypt-keeper eggs hatch inside a gall, the larvae climb inside the gall wasp and set their evil, but oh-so-fascinating, plan into action. The gall wasp continues to develop as it normally would, right up until it's ready to exit the crypt. Through a feat of insidious mind control, the crypt-keepers force their victim to drill a hole that is too small for its escape. The gall wasp becomes wedged in the opening, its head plugging the hole like a wine cork. Now that the "zombie" host has done the hard work of gnawing through the tough stem of the plant, the crypt-keepers begin to eat their way through the gall wasp, eventually erupting out of its head.

The iridescent emerald sheen of the crypt-keeper wasp. Image © Scott P. Egan, Kelly L. Weinersmith, Sean Liu, Ryan D. Ridenbaugh, Y. Miles Zhang, Andrew A. Forbes

Until recently, it was thought that crypt-keepers victimised a single species: Bassettia pallida which carves out its knobbly home in sand live oak trees. Recent research, however, shows that the crypt-keepers target at least six other species of gall wasp. Researchers collected more than 23,000 galls containing over 100 species of oak gall wasps. After raising the wasps they discovered crypt-keepers parasitising 305 wasps from six different species.

There is still much to learn about crypt-keepers, which belong to a group of little-understood parasites called hypermanipulators. Parasitism is fairly common in the animal kingdom – ticks and fleas are prime examples. Hyperparasites – such as parasitic wasps that deposit their eggs in other parasitic wasps that lay their eggs in caterpillars – are also not unheard of. But crypt-keepers add an extra layer of macabre into the mix as they are able to manipulate the behaviour of another parasite that is able, in turn, to manipulate its own host. If your head is spinning, that's probably normal.

"It's safe to say these researchers have uncovered something special," Emily Meineke, a National Science Foundation postdoctoral fellow at Harvard University told Nat Geo back in 2017 when the crypt-keeper was first uncovered. "There are plenty of examples of hyperparasitism among parasitoid wasps, but manipulation of a complex behaviour, and being able to document that it improves fitness, is certainly rare."