A parasitoid has been described that turns its host into a bodyguard for its young, thus improving the chances of further transmission, released in a study on June 3, 2008 in the open source journal PLoS ONE.

Parasites are organisms that enter a symbiotic relationship in which the parasite, in the long run, gains resources, and the host is damaged by the presence of the parasite. Classical examples of parasites of vertebrates include the fleas and tapeworms. A parasitoid is a special type of parasite that will eventually kill its host organism.

Often, parasites can induce drastic changes in the behavior of their host organism. For example, flukes are thought to inhabit ants as an intermediate host, provoking them to climb to the tops of blades of grass in the night and early morning while their fellow uninfected ants return to the nest. Once firmly attached to the tops, they are eaten by grazing sheep, which become the flukes’ final hosts. In another example, hairworms infect certain land-insects, inducing them to suicidally jump into the water, where the adult worms can then reproduce.

These changes in behavior are often attributed to the parasite, perhaps increasing its transmission to the final host, but there are many explanations for this sort of change. For instance, if the host already shows altered behavior before infection, it is possible that this altered behavior somehow increased the chances of infection rather than being its direct result. Also, there is the consideration that behavioral changes will often induce increased attacks from other non-host animals, which would decrease the probability of transmission overall. Due to the complexity of the interactions in this sort of relationship, it is important to observe behavior-modifying parasites in natural conditions.

A research team from University of Amsterdam, the Netherlands, and the Federal University of Viçosa, Brazil, led by Arne Janssen, made observations in a project supported by WOTRO and carried out in Brazil. In it, they observed the relationship between the caterpillars of a specific moth, Thyrinteina leucocerae, that feeds on the leaves of the native guava and on eucalyptus in Brazil. When being attacked by an insect parasitoid wasp, Glyptapanteles, these caterpillars are injected up to 80 wasp eggs.

Once inside the host, the eggs of the parasitoid wasp hatch and the emerging larvae feed on the host’s body fluids. However, the caterpillar continues to behave like its uninfected siblings: feeding, moving, and growing like normal. Once the larvae are full grown, they come out of the host’s skin and begin to pupate nearby. While the host remains alive, it stops feeding and dedicates its life to the parasitoid pupae, protecting them against predators by swinging its head violently — a behavior that is highly unusual for this species. (Movies and pictures of this action can be found in the article hosted on PLoS ONE.)

Once the dult parasitoids emerge from their pupae, the moth caterpillars die. This clearly provides them no benefit. Meanwhile, caterpillars who are not parasitized do not show any behavioral changes, and continue to feed and develop into adults. In the field, the researchers observed that pupae who were guarded by caterpillars only suffered half the predation as the pupae without such protection. Therefore, the behavioral changes found in the host result in an increased survival rate for the parasitoids.

The mechanism by which the parasitoid changes the host’s behavior is not yet known. Notably, the research team discovered that between one and two parasitoid larvae remain in the host after the hatching. They speculate that these larvae may affect the overall behavior of the caterpillar, thus sacrificing themselves for the benefit of their brothers and sisters.

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Parasitoid Increases Survival of Its Pupae by Inducing Hosts to Fight Predators
Amir H. Grosman, Arne Janssen, Elaine F. de Brito, Eduardo G. Cordeiro, Felipe Colares, Juliana Oliveira Fonseca, Eraldo R. Lima, Angelo Pallini, Maurice W. Sabelis
PLoS ONE, June 2008, Volume 3, Issue 6, e2276
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Written by Anna Sophia McKenney