The Parasite That Compels Other Parasites to Shove Their Heads Into Holes

In 2015, Scott Egan was walking along a Florida beach when he noticed some oak trees with distinctive swellings on their branches. He recognized them as the work of gall wasps—parasitic insects that lay their eggs in plants. From within, the larvae manipulate trees into creating chambers full of nutritious tissues. Tucked away in these crypts, the young wasps can eat their fill in safety. Once they turn into adults, they chew their way out and fly away.

Egan, being a keen naturalist and an expert on gall wasps, snipped off some of the branches, took them back to his lab, and kept them in a container on his desk. After a couple of months, he noticed that a few orange insects had fallen to the bottom. Those were the gall wasps—an orange species called Basettia pallida, which had finally chewed their way out of their crypts. But not all of them made it. Egan noticed that some were stuck, their heads wedged in their own escape holes.

To find out why, Egan teamed up with Kelly Weinersmith, a parasitologist and a colleague at Rice University. They cut open the branches and realized that every stuck wasp had a companion inside its crypt—a second wasp, half the size of the first, and iridescent blue. And in every case, the blue wasp was eating the orange one.

The blue wasp was a completely new species, and Weinersmith and Egan named it the crypt-keeper wasp. It’s a stunning example of a hyperparasite—a parasite whose host is also a parasite. This lifestyle is surprisingly common, especially among wasps. Many species lay eggs in the bodies of other insects, only to have other wasps lay eggs in their young. And sometimes, hyperparasites can be parasitized by other hyperparasites, creating hierarchies of bodysnatching that can grow to four tiers.

Even by these standards, the crypt-keeper wasp is special. Parasites are incredibly common, but only some manipulate the behavior of their hosts. There are fungi that turn ants into zombies, hairworms that compel crickets to jump into water, and tapeworms that force shrimp to swarm in groups—all to help the parasites spread to their next hosts. The crypt-keeper wasp does this too, but as Weinersmith and Egan have shown, it’s one of the few known hypermanipulator­­s—parasites that manipulate the behavior of other manipulative parasites.

Somehow, the crypt-keeper wasp can find an oak tree that already contains the larva of a crypt gall wasp, and then lays an egg inside the crypt. Once hatched, its larva manipulates its orange crypt-mate into chewing an escape hole that’s smaller than usual. The orange victim then plugs the small hole with its own head, while the crypt-keeper larva devours it alive. Eventually, the crypt-keeper turns into an adult and chews its way to freedom, through the head of its roommate/larder/wall-plug.

Because of its behavior, Weinersmith and Egan gave the wasp the formal name of Eudurus set, after Set, the Ancient Egyptian god. “Set was the god of chaos and evil, and he was said to control other evil beings,” says Weinersmith. “He also locked his brother Osiris in a crypt for him to die. It kind of blew our minds how many cool connections we could find.”

After discovering the crypt-keeper, Egan went to the American Museum of Natural History and looked at old collections of gall wasps that had been gathered by Alfred Kinsey. (Yes, that Kinsey; before becoming synonymous with human sexuality, he was a prolific gall-wasp aficionado, who collected millions of the insects.) In some of the stored branches, Egan saw little heads plugging holes. We’ve found several such samples in museums around the country, going back 100 years.”

“This is the type of science I love; it leaves us hungrily asking more questions,” says David Hughes from Pennsylvania State University, who studies manipulative parasites. For example, “how does this wasp get its egg into its soon-to-be excavator? And how does it do that so precisely to stop the activity at a stage where the hole is large enough just for a head to block, but not for the body of the manipulatee to emerge?” It might secrete some kind of mind-addling chemical. Alternatively, it might just eat its host to the point when it still has enough energy to make an escape hole, but not enough to make a big one.

Whatever the method, it’s clear that the crypt-keeper benefits. It can’t effectively chew its way out of the crypt on its own; if Weinersmith and Egan resealed those head-plugged holes with fresh bark, the crypt-keepers were three times more likely to die trapped in their crypts. “This make a strong case for the adaptive significance of the manipulated host behavior,” says Shelley Adamo from Dalhousie University, who also studies parasites.

But why would the crypt-keeper force its host to plug the escape hole? It’s not clear, but Weinersmith and Egan found an enticing clue. In some cases, they saw that a third type of wasp—a fairy wasp—would emerge from infected branches. Fairy wasps are among the smallest animals alive, and some of them are hyperparasites of the family that includes the crypt-keeper.

So, perhaps the crypt-keeper uses its host to plug the crypt, so it can’t get parasitized itself by an intruding fairy wasp. Or maybe the fairy wasp uses the presence of a head-plugged hole to find a crypt-keeper wasp to target.  “We’re hoping to catch the fairy wasp this year, and get the whole system into the lab,” says Weinersmith. “It blows our minds that there could be yet another layer to all of this.”

The writer Jonathan Swift, of Gulliver’s Travels fame, said it best: