Sinister Parasitic Wasp Turns Adult Fruit Flies Into Living Incubators

Adult flies in the eastern United States have been living through a revolting body horror movie under our noses, but humans never noticed. That is, until a PhD student in Mississippi started looking at the bugs in his backyard.

Parasitic wasps lay their eggs in or on a live host, where they gestate until they hatch. Almost 200 species have been documented for over two centuries, but until now, every known species that used flies to host their eggs did so in their immature pupal or larval form.

Logan Moore, a PhD candidate at Mississippi State University, began collecting fruit flies in his backyard in March 2023. While Moore was originally monitoring the flies for nematode infections, he found a parasitoid wasp larva inside one adult male.

What made the finding even more surprising is that the wasp’s victims belong to an extremely common family of fruit fly called Drosophila. Moore pivoted his research and, along with some colleagues, began collecting 6,000 samples of flies in Mississippi, Alabama, and North Carolina. Around 1% of the male flies they caught were infected. Only one of 477 female fruit flies, however, had a baby wasp inside of her.

“All known parasitoid wasps of flies attack and develop inside immature life stages,” said Matthew Ballinger, an associate professor at Mississippi State University, who oversaw Moore’s research, in a statement. “Despite 200 years of research on parasitoid wasps of Drosophila and other flies, we have never come across a species that attacks the adult stage, until now.”

Moore and Ballinger concluded that the wasps laying the eggs belonged to a previously undiscovered species. In the resulting study published in Nature, they dubbed the wasp Syntretus perlmani, in honor of prominent University of Victoria fly researcher Steve Perlman.

The actual process by which a new wasp forms in the flies is straight up Lynchian. Seven to 18 days after the eggs are laid, the flies didn’t even need to be dissected to confirm infection. In males, the wasp larva was, by that point, large enough to “swell the abdomen and obstruct the view of the host’s brightly pigmented testes,” the scientists wrote.

Even worse news, if you’re a fly: The wasps appear to be fairly widespread, as the biologists compared their mitochondrial genome to a genetic database of flies. They got hits from four states, none of which they used to find samples: Georgia, Virginia, Pennsylvania and Illinois. There is currently no evidence that S. perlmani lives anywhere outside the U.S.

In the paper, Ballinger and his colleagues offered several theories for why no other species use adult flies as hosts. One possibility is that adults are way more mobile than the young versions. Another is the thicker exoskeleton a wasp would have to break through to lay their eggs. Adults are also more likely to be able to defend themselves.

If you find it cruel that the wasps are picking on flies, you’ll be relieved to know that different varieties inflict their Lovecraftian abominations on a wide variety of insects, including some non-fly adults. One example the researchers pointed to in the study is Dinocampus coccinellae, a type of wasp that can turn spotted lady beetles into a zombie-like guardian for larval wasps. Other species have been found infecting butterflies, and some even go after rival types of parasitic wasps.

It’s good to know that when it comes to sheer, visceral, horror, wasps are so inclusive.