Horror Story of A Manipulative Parasitoid Wasp & A Helpless Spider

While nature is full of wonderful, awe-inspiring stories to tell of the lives thriving from myriads of species, it also has its fair share of gore. A new study published in the Journal of Experimental Biology unravels the gruesome techniques of a parasitoid wasp larva destroying its spider host through cold-blooded manipulation to survive. The title of the paper itself gives a cringe-worthy glimpse into the horror tale: “Host manipulation by an ichneumonid spider ectoparasitoid that takes advantage of preprogrammed web-building behaviour for its cocoon protection”.

spider wasp
The spider host with its ‘parasite’ on its back, before all hell breaking loose for the former. Photo credits: Keizo Takasuka.

Parasitoids, as opposed to parasites, are definitely creepier than predators: they kill you slowly, and sometimes, in ways worse than death. The orb-weaver spider, studied by the researchers of the paper, can attest to this statement. It is fully exploited by the wasp Reclinervellus nielseni. The scientists analysed the interactions between the two to understand how parasites can alter the behaviour of their hosts.

The horror story begins with an adult wasp laying an egg on the spider’s body. When the wasp larva hatches, it sticks to the spider’s abdomen, from which it sucks fluids out. At this point, the spider has no idea as to what is awaiting it. Later, however, the spider’s behaviour begins to change. In no time, it becomes the puppet of the wasp, building the ideal conditions for the larva to pupate into an adult.

Instead of spinning its usual 2 webs – a “normal” one to catch prey and a “resting” one before it moults – the spider makes a “cocoon web” prior to the wasp larva transforming into an adult. This web bears close resemblance to the resting one in terms of strength and structure, including tiny fibrous threads which reflect UV light to protect from other insects. However, the cocoon web is stronger to provide a safer environment for the wasp.

The wasp larva appears to have instructed its host to set up a modified resting web to house the phases it needs to undergo to pupate. Therefore, instead of the spider preparing its home to moult, it is programmed – manipulated – to build a cosy space for its parasitoid.

The researchers thus observed the change in behaviour of the spider.
The spider that seems to have been hypnotised then positions itself in the middle of the web and waits for its execution by the larva.

Now, how does the wasp larva pull the strings? Apparently, it injects the spider with hormones that mimic those which control the latter’s moulting behaviour. It makes it do its bidding and when it no longer needs it, it murders it. This is the difference between parasites and parasitoids: the former rarely kill their hosts, while the latter almost always do.

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