In a joint publication in PLoS One, researchers from the University of Amsterdam and University of Viçosa (Brazil) show evidence of spectacular behavioural changes induced by a parasitic wasp in the caterpillar of a moth species.
After the wasp has oviposited eggs in the body of the caterpillar, these develop into larvae that live on the body fluids of the caterpillar. After the wasp larvae crawl out of the caterpillar to pupate, the caterpillar acts as a bodyguard to defend them from predator attacks.
A research team from the Institute for Biodiversity and Ecosystem Dynamics (IBED) of the University of Amsterdam worked together with the Entomology section of the Federal University of Viçosa (Brazil) to study such behavioural changes induced by parasites. In a recent publication in the new electronic journal PLoS One, the researchers offer evidence that behavioural changes of the host are beneficial to the parasite in the field. In Brazil, the team studied the caterpillars of a moth that feed on leaves of the native guava tree and an exotic eucalyptus.
These small caterpillars are attacked by insect parasitoid wasps, which then quickly insert up to 80 eggs into them. Inside the caterpillar host, a cruel drama takes place: the eggs of the parasitoid hatch and the larvae feed on the body fluids of the host. The caterpillar continues feeding, moving and growing like its unparasitised brothers and sisters. When the parasitoid larvae are full-grown, they emerge together through the host’s skin, and start pupating nearby. Unlike many other combinations of host and parasitoid, the host remains alive and displays spectacular changes in its behaviour: it stops feeding and remains close to the parasitoid pupae.
Moreover, it defends the parasitoid pupae against approaching predators with violent head-swings (see films via link below). The caterpillar dies soon after the adult parasitoids emerge from their pupae, so there can be no benefit whatsoever for it. In contrast, unparasitised caterpillars do not show any of these behavioural changes.
The research team found that parasitoid pupae that were guarded by caterpillars in the field suffered half as much predation as those without a bodyguard. Hence, the behavioural changes of the host result in increased survival of the parasitoids due to the host acting as a bodyguard of the parasitoid pupae. Whereas it is still unclear how the parasitoid changes the behaviour of its host, it is tempting to speculate. The research team found that one or two parasitoid larvae remained behind in the host. Perhaps these larvae affect the behaviour of the caterpillar, and sacrifice themselves for the good of their brothers and sisters.
Cause or effect?
There are many examples of parasites that induce spectacular changes in the behaviour of their host. Flukes, for example, are thought to induce ants, their intermediate host, to move up onto blades of grass during the night and early morning. There they firmly attach themselves to the substrate with their mandibles, and are thus consumed by grazing sheep, the fluke’s final host. In contrast, uninfected ants return to their nests during the night and the cooler parts of the day. Another example of behavioural change is that of terrestrial insects, parasitised by hairworms, which commit suicide by jumping into water allowing the adult hairworms to reproduce. Behavioural changes like these are thought to be induced by the parasite so as to increase its transmission to the final host, but there are alternative explanations. It is possible, for example, that the hosts already behaved differently before becoming infected. Hence, infection is a consequence of different behaviour, not its cause. Increased transmission can also be called into question: the behavioural changes of the host may result in increased attacks by other non-host animals, and this would seriously decrease the probability of transmission. Increased transmission should therefore always be tested under natural conditions. The research of the Dutch and Brazilian researchers is the most complete and convincing case for induction of behavioural changes, clearly showing that it is the parasite that profits from it.
This research was supported by the Tropical Research division of the Netherlands Organisation for Scientific Research (NWO-WOTRO).Publication information:
The online article can be viewed via: http://www.plosone.org/doi/pone.0002276. This website also includes accompanying images.
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