|Several parasites are known to cause a behavioural change in their hosts. Most of these are found in the insect world, where for instance nematodes change behaviour in ants. However, it has proved difficult to obtain solid data that proves that the change in behaviour confers an advantage to the parasite and a disadvantage to the host. Now scientists have found conclusive evidence of a parasite manipulating its host into acting as a bodyguard for its pupae.
The braconid wasp parasites caterpillars and cause a behavioural change that enlists them as bodyguards for the wasp pupae. Photo taken by Richard Bartz, courtesy of Wikimedia Commons.
The team of scientists comprising biologists from the University of Amsterdam in the Netherlands and the Federal University of Viҫosa in Brazil collected caterpillars Thyrinteina leucocerae infected with the parasite wasp Glyptapanteles sp. and used the emerging wasps to infect other caterpillars in the laboratory. The behaviour of these caterpillars and their parasites were then examined both in the laboratory and in the field.
Their experiments showed that infected caterpillars moved much less after the wasp larvae had left their bodies and pupated. Instead the caterpillar stayed near the pupae, stopped eating and actively defended the wasp pupae from introduced stink bugs with violent head swings toward the bug. This defensive behaviour was seen in almost 90% of the infected caterpillars, but was only observed in 5% of non-infected caterpillars, who largely ignored the stink bug. The infected caterpillars always died some time after the adult wasp had emerged from the pupae.
The scientists, furthermore, found that when the caterpillar bodyguards were removed, the mortality rate of the pupae almost doubled in the field. Thus their study shows that the induced behaviour results in an advantage for the parasite (increased survival of the pupae) and a disadvantage for the host (death).
Interestingly, the infected caterpillars show the change in behaviour even when the pupae are not present so the behaviour is not caused by the pupae. Instead the scientists observed that some wasp larvae remains in the caterpillar and perhaps manipulate the caterpillar directly. Thus it could mean that some wasp larvae sacrifice themselves to the benefit of their brothers and sisters. But this hypothesis requires further investigation.
Grosman AH, Janssen A, de Brito EF, Cordeiro EG, Colares F, et al. (2008) Parasitoid Increases Survival of Its Pupae by Inducing Hosts to Fight Predators. PLoS ONE 3(6): e2276. doi:10.1371/journal.pone.0002276.
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