NATURE NOTES:
The Stuff of Science Fiction: Parasitoid Wasps in West Texas

By Andrew Stuart

A message to Nature Notes listeners: if the movie Alien, the 1979 sci-fi classic, gave you nightmares, this isn’t the episode for you.

“There are,” as Shakespeare reminds us, “more things in heaven and earth than are dreamt of in your philosophy,” and reality bears that out: the natural world doesn’t lack for the extreme, the bizarre.

When it comes to the gross-out department, few phenomena can compete with parasitoid wasps. Ranging from a fraction of a millimeter to 2 inches in length, these insects provide for their young in a lurid way: by laying their eggs on, or within, the bodies of other creatures. 

Dr. David Althoff, of Syracuse University, studies parasitoid wasps.

“There’s been the evolution of lots of toxins,” Althoff said, “specific to different insect groups or arthropod groups, where they can immobilize their prey, either permanently or for a little bit, so that they they can lay the egg.”

Some are ectoparasites – they feed from the outside. Others are endoparasites – which live within their host’s body. For all these wasps, the victim’s body serves as a surrogate womb – providing the nutrition the larvae need.

It may seem gruesome, but it’s not a niche phenomenon.

“If you look at the bees, wasps and ants, it’s the wasps that are the most diverse,” Althoff said, “and out of the wasps, it’s really the parasitoids. Every insect species that’s an herbivore is attacked by a parasitoid. If you broke up insect diversity, the most diverse would be plant-feeding insects, and right after that would be the parasitic wasps.”

West Texas is home to one of the largest and most famous. Two inches long, black with orange wings, the tarantula hawk wasp looks like trouble – and its sting is one of the most painful of any insect. 

Carrying hundreds of eggs, the female tarantula hawk patrols for tarantulas. When she finds a host, she stings it between the legs, drags it to a burrow – prepared in advance – and lays a single egg on its abdomen. Then she closes the burrow – and moves on in search of another victim.

The larva hatches, enters the tarantula, and feeds from within – initially avoiding vital organs. For this parasitic wasp, as for others, the host must stay alive till the end.

“We’re talking days if not weeks of being paralyzed,” Althoff said, “so somehow it gets into the nervous system and blocks the nerves from functioning. But just for the movement of muscles – the insect is still breathing and functioning. Not a good way to go.”

It ends when an adult wasp emerges from the spider’s abdomen.

The tarantula hawk is just one example, and our region abounds in parasitoid wasps. Althoff said there are likely hundreds of species here that have never been described or documented.

“That is one of the striking things about the Southwest,” he said. “I grew up in the East, and when I went out west and first looked at the desert system, I was like, ‘Wow – it’s just barren out here.’ But then you begin to realize as you spend more time there – no, there are lots of cool things going on. There’s a huge diversity of things, and parasitic wasps is one of those.”

Althoff also studies yucca moths – and there’s an evolutionary resonance. Yucca moths lay their eggs in the developing fruit of yucca plants. The wasps’ strategy is similar – with the difference that its host is another animal.

Indeed, there are tiny wasps in West Texas that parasitize yucca-moth caterpillars.

The wasps often have ovipositors, or egg-layers, finely suited to their victims. There are wasps in the Southwest that attack beetles feeding inside of trees.

“You’ll see them flying, and what you’ll see is the black body,” Althoff said, “and if you look closely they’ll be trailing an ovipositor sheath – to protect that egg-laying device – and in some cases that can be an inch or two inches longer than the body of the insect. It’s amazing – like trying to drill through wood with this really thin needle, but somehow they’re able to do it.”

Many adult parasitoid wasps serve as pollinators. And they’ve been used extensively as biological control – deployed in corn, soybean and sugarcane fields to attack agricultural pests.

Still, there’s no doubt parasitoid wasps are creepy. Yet in their tremendous diversity, and with a presence on Earth that far predates mammals’, this is as much their planet as it is ours.