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Environment

Trick or treat? Parasitic bugs opt for both

by Gina Vitale
October 27, 2019 | APPEARED IN VOLUME 97, ISSUE 42

 

Zombie spiders do their parasites’ bidding

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Credit: William Eberhard
Is this seat taken? A parasitic wasp larva rests on a spider’s abdomen before inducing the spider to build it a web.

As Halloween approaches, decorations of ghosts, vampires, and witches pattern walls and windows. But for William Eberhard, a staff scientist emeritus at the Smithsonian Tropical Research Institute and emeritus professor at the University of Costa Rica who has spent decades studying “zombie” spiders, the creepy-crawly ambience lasts all year long.

Certain species of wasp larvae like to form their cocoons on spiderwebs for extra protection. But because wasps can’t spin webs themselves, their larvae induce their spider hosts to do the job for them. First, a female wasp stings a spider, immobilizing it for 10–15 min while the wasp lays an egg on the spider’s abdomen, where the larva hatches and grows. When the larva is ready to make its final molt, it induces the spider’s concentration of ecdysone—a hormone that spiders produce when molting—to spike. The affected spider, tricked into thinking it’s about to molt, then spins an intricate web that it would usually create only for molting season.

After the web is spun, the nearly mature wasp overlord injects the spider with poison, finally killing it. But in terms of free will, Eberhard says, the spider has been dead all along.

“Once the spider has been stung by the female wasp, it’s effectively reproductively dead,” Eberhard tells Newscripts. “It’s maybe going to live for another couple of weeks, but it now has that egg on it, and later the larva, and so it’s done for.”

Unfortunately for the spider, it doesn’t end with death. After killing the spider, the newly hatched wasp regurgitates digestive fluid onto the host body and sucks out its insides for nutrients. Dracula, surely, would be proud.

 

Tales from the crypt

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Credit: Andrew Forbes
Crypt creeping: The female crypt-keeper wasp, shown above, lays its eggs in a crypt that’s already inhabited. This bodes poorly for the original resident.

As it turns out, wasps can be on the receiving end of parasitic interactions, too. Kelly Weinersmith, an adjunct professor at Rice University, studies interactions in which wasps are both the victims and the perpetrators.

It starts out with a species called the crypt gall wasp. A female crypt gall wasp will poke a tiny hole in the bark of a tree, laying an egg inside. The area where the wasp grows is called a crypt. In a normal life cycle, the full-grown adult wasp then burrows its way out of the crypt to begin its life.

Sometimes, though, a second type of wasp—called the crypt-keeper wasp—will come along and poke another hole into the crypt, inserting its own egg. Now, when the newly matured crypt gall wasp starts to burrow its way out of the crypt, the parasite wasp appears to manipulate it into digging a smaller-than-usual hole. And with just a part of its head peeking out from its crypt, the crypt gall wasp gets stuck, doomed to die in the confines of its chamber. The parasitic crypt-keeper wasp, after eating its way through its host’s body, then chews its way out of the crypt gall wasp’s head, creeping out to terrorize the outside world.

“I’m kind of claustrophobic, and so the whole being trapped inside of a crypt while your insides are being eaten out is just, like, it’s so creepy,” Weinersmith tells Newscripts.

The crypt gall wasp isn’t the only kind of wasp susceptible to the crypt keeper’s tricks. Recent research from Weinersmith’s team has revealed that six additional gall wasp species also fall victim to the parasitic tendencies of the crypt-keeper wasp, proving it’s a wasp-eat-wasp world.

Gina Vitale wrote this week’s column. Please send comments and suggestions to newscripts@acs.org.

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