Parasitoid turns its host into a bodyguard

9 juni 2008

In a recent publication in the new electronic journal PLoS One, a research team from University of Amsterdam, the Netherlands, and the Federal University of Viçosa, Brazil, led by Arne Janssen, now offer evidence that behavioural changes of a host are beneficial to the parasite in the field.

The group studied caterpillars of a Brazilian moth, which attack trees such as the native guava and the exotic eucalyptus. Small caterpillars are attacked by an insect parasitoid wasp, which lays up to 80 eggs inside the caterpillar host. Inside the host, a cruel drama takes place: the eggs of the parasitoid hatch and feed on the body fluids of the host. The caterpillar continues feeding, moving and growing like its unparasitized 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 but 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 movies of predatory bug attack on the website of PLoS One). The caterpillar dies soon after the adult parasitoids emerge from their pupae, so there can be no benefit whatsoever for the caterpillars. In contrast, unparasitized caterpillars do not show any of these behavioural changes, but continue feeding and developing into adults.

The research team found that, in the field, parasitoid pupae which were guarded by caterpillars suffered half as much predation as those which had no bodyguard. In this case, the behavioural changes of the host do not result in increased transmission (the parasitoids do not need another host to complete their development), but in increased survival of the parasitoids. Hence, the parasitoids transform their hosts into bodyguards.

It is still unclear exactly how the parasitoid changes the behaviour of its host, but 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.

This research project was supported by WOTRO Global Science for Development