Zombifying parasites are fascinating to study, and while this phenomenon is well documented, relatively little is known about the mind-manipulating mechanisms employed by the invaders. We know what they do, but we often have no idea how they do it. Now, a new study has managed to unpick how a particular virus could be inducing a totally bizarre behavioral change in infected caterpillars. Scientists found that the virus taps into a light perception pathway in the host, causing it to literally go towards the light before dying at the top of a plant and turning into a pile of mush, spilling out a fresh supply of viral particles ready for round two.
There are many mind-blowing, albeit wholly disgusting and truly disturbing, examples of parasites turning their poor unsuspecting hosts into zombies that sound like they’re straight out of a horror movie.
There’s a worm that turns crickets suicidal, making them kamikaze into a watery grave. Jewel wasps inject mind-controlling venom straight into the brains of cockroaches, converting them into a slave that serves as a nursery for the wasp’s larva, which then eats its way through the poor host’s innards while it’s still alive. There’s even a wasp that turns caterpillars into a head banging bodyguard that protects the parasite from predators.
Another truly spectacular example of host zombification comes from members of a group of viruses called baculoviruses. These viruses trigger a fascinating phenomenon called “tree-top” disease, in which the virus causes an infected caterpillar to migrate to the top of the plant they live on just before they die. Usually, caterpillars avoid these areas during the day because they risk being spotted by predators. The decaying corpse then begins to literally melt into an oozing mass of gloop, dripping a fresh supply of virus onto the lower leaves, ready to be consumed by another hungry caterpillar.
A few years ago, researchers identified a gene in one type of baculovirus that seems to be causing this effect in its host, the Gypsy moth caterpillar, although how precisely it alters the host’s behavior is still a mystery. Now, researchers have looked at another baculovirus, and have managed to gain an insight into the processes leading to this fascinating host manipulation.
As described in Biology letters, researchers investigated the baculovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV), which infects caterpillars of the Small Mottled Willow Moth. They found that the virus changes the way that caterpillars respond to light, inducing a positive phototactic response (a posh way of saying it causes them to move towards a source of light).
They found that uninfected caterpillars move freely up and down the plant, regardless of whether it was light or dark. However, they never clambered to the very top of the plant, which would risk exposure, and always settled at a safe, lower spot before turning into immobile pupae.
Infected caterpillars, on the other hand, started their upward journey around 3 days post infection, climbing several centimeters in just over half a day, but only in the presence of light. If infected larvae were kept in the dark, however, they didn’t seem to climb as high and died lower down on the plant. The researchers therefore believe that the virus induces this bizarre change in behavior by capturing a host pathway that’s involved in light perception, causing them to migrate towards light sources.