The parasites that control the behavior of their hosts to their advantage are a well-documented natural phenomenon, but the discovery of a previously unknown relationship between a parasitic wasp and a social spider is particularly upsetting.
Distracting unsuspecting host brains is something that some organisms do, usually for reproductive purposes. Famous examples include a fungus that controls the actions of carpenter ants, a unicellular parasite that makes the smell of cat urine irresistible to rodents and a worm that forces infected fish to perform a complex dance.
Another parasitic relationship, discovered by scientists at the University of British Columbia, can now be added to this list – and this is one of the most harmful we've seen, both in terms of complexity and of mistrust. In the new study, published today in Ecological Entomology, the lead author, Philippe Fernandez-Fournier, describes how Zatypota parasitic wasp species uses and abuses the social Anelosimus eximius spider, using it first as an escape car, then forcing him to build an incubation chamber. Oh, and then it's eating the spider.
Fernandez-Fournier accidentally came across a discovery in the Ecuadorian Amazon while studying various parasites living in the nests of A. eximius. These arachnids are considered social spiders because they live in large colonies, work together to capture prey, share parenting tasks, and rarely venture beyond the comfortable confines of their basket-shaped common nests.
These spiders have a rigid behavior, which is why Fernandez-Fournier noticed that some of these spiders, infected by a parasitic larva, were spreading one or two feet from the colony. This sighting was strange and rare in itself, but the University of British Columbia scientist also observed these same spiders begin to weave densely spun silk webs and bits of foliage.
"It was very strange because they do not do it normally, so I started taking notes," Fernandez-Fournier said in a statement.
Curious, he brought back the cocoon cloths to the laboratory. When he opened them, to his surprise, he saw a wasp grow inside. Realizing that he was on something, Fernandez-Fournier and his team continued their research, revealing a completely undocumented interaction between the two species.
Here's how it works: an adult woman Zatypota wasp lays an egg on the abdomen of one Anelosimus eximius Spiders e. After hatching the larva, she attaches to the spider and feeds on her blood. The larva grows progressively and begins to absorb much of the spider's body. Finally, the spider enters a state of "zombification", in which it no longer behaves as if it were normal. Under the influence of the larva, the spider abandons its colony and embarks on the construction of the cocoon veil. Once this forced building task is complete, the spiders remain motionless, allowing the larva to finish killing and consuming its host. Satisfied, the larva sneaks into the canvas cocoon, which she uses as an incubator for her next stage of germination. About nine to eleven days later, a fully mature wasp emerges from the cocoon. The cycle begins again, unfortunately for the next eight-legged victim.
Researchers say this strategy is unique in that parasitoid wasps were previously documented only to take solitary spiders.
"But this change in behavior is so difficult," said Samantha Straus, co-author of the study, in a statement. "The wasp completely diverts the spider's behavior and brain and makes it do something it would never do, like leaving its nest and turning on a completely different structure. It's very dangerous for these little spiders.
Straus added, "We think wasps are targeting these social spiders as they are a large host colony and a stable food source. We also found that the larger the spider colony, the more likely it was that these wasps would target it.
As for how the wasp's larva weaves its mesmerizing spell, the researchers assume that it injects a brain-altering hormone into the spider. This hormone causes the spider to think that it is at a different stage of life, or it acts as a signal that scares the spider from its colony. But these are just suppositions.
Fernandez-Fournier and Straus would now like to return to the Ecuadorian forest to learn more about these evil wasps and their hosts. In particular, they would like to know if wasps repeatedly target the same spider colonies and, if so, how this is a benefit.