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April 29, 2009

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Parasite turns host into bodyguard

June 5, 2008
Courtesy University of Amsterdam
and World Science staff

Many par­a­sites simply eat away at their hosts from the in­side. That may be bad enough. But some go fur­ther: they ma­ni­pu­late their hosts’ be­hav­ior to ben­e­fit them­selves, pro­duc­ing some of na­ture’s most cru­el and strange spec­ta­cles.

One no­ta­ble ex­am­ple is that of the hair­worm, which some­how in­duces its in­sect hosts to com­mit su­i­cide by jump­ing in­to wa­ter, where the hair­worms go to re­pro­duce.

In this ex­per­i­men­tal set­up, a cat­er­pil­lar knocks a small par­a­sit­ic bug off its twig. The bug is seen rap­id­ly drop­ping down at the low­er right of the twig; the cat­er­pil­lar still has­n't fin­ished its vi­o­lent swing. Above the cat­er­pil­lar are the light-colored wasp co­coons. (Cour­tesy A.H. Gros­man et al.)

Many parasites simply eat away at their hosts from the inside. That’s bad enough. But some go further: they manipulate their hosts’ behavior to benefit themselves, producing some of nature’s most cruel and strange spectacles. One notable example is that of the hairworm, which induces its insect hosts to commit suicide by jumping into water, where the hairworms go to reproduce. A new study describes yet another strange case of apparently parasite-induced behavioral changes: a creature that turns its host into its own, suicidally devoted bodyguard. After the parasitic wasp Glyptapanteles completes an early life stage as an uninvited guest in the body of a caterpillar, the caterpillar exhibits stunning changes, according to researchers. It stops eating and stays close by the wasps, which by then are cocoons. It wraps them in a protective web of silk and defends them against approaching predators with violent, relentless head-swings. It continues this until the wasps emerge from their cocoons, then it dies, according to the scientists, from the University of Amsterdam and University of Viçosa in Brazil. In experiments, when presented with a small predatory insect, “17 out of 19 parasitized caterpillars lashed out at the bug with repeated violent head-swings,” the investigators wrote in a paper describing their work. By contrast, “only one of 20 unparasitized caterpillars showed this behaviour,” they wrote. “The others hardly responded to the presence of the predator, even when it was walking on the host.” The paper is published June 4 in the online research journal PloS One. The wasp’s bizarre life cycle begins when an adult lays eggs inside the caterpillar. These develop into larvae that live on the animal’s body fluids. They eventually crawl out of the caterpillar, to become cocoons shortly in preparation for adulthood. This is when the caterpillar, known as Thyrinteina leucocerae, acts as a bodyguard to defend them from predator attacks, the researchers said. The cocoons and caterpillar live next to each other on a twig as the drama plays out. Strictly speaking, the wasp called a parasitoid, not a parasite, because it only spends part of its life cycle as a parasite. The researchers found that parasitoid cocoons guarded by caterpillars in the wild suffered half as much predation as those without a bodyguard. It’s unclear how the wasp changes the behaviour of its host, but interestingly, the investigators said, one or two parasitoid larvae normally remained behind in the host after the others left. These larvae may be the ones that change the caterpillar’s behaviour, thus sacrificing themselves for their brothers and sisters, the researchers speculated.