Credit: J. LINO-NETO

Parasites are in the business of hijacking their hosts for their own purposes. A dramatic example is described by Amir Grosman and colleagues who studied behavioural changes induced in the Brazilian geometrid moth Thyrinteina leucocerae by a braconid parasitic wasp of the genus Glyptapanteles (A. H. Grosman et al. PLoS ONE 3, e2276; 2008).

The wasp lays up to 80 eggs in a moth caterpillar, where they hatch, grow and then exit to form pupae on nearby leaves three weeks later. At this point the caterpillar ceases its previous activity, and instead stands guard over the wasp pupae (pictured). While 'guarding' the pupae, caterpillars make violent swings of their heads towards any passing insect or other potential threat.

Grosman et al. show that this behaviour is a direct result of parasite infestation and profits the wasp. Unlike the parasitized caterpillars, their unparasitized siblings failed to take up sentry duty when placed by wasp pupae, continuing to feed as normal. Also, the head butts of parasitized caterpillars proved effective protection for the pupae, repelling nearly half of the attacks by predatory stinkbugs (Supputius cincticeps) in the laboratory.

Such behaviour has no fitness advantage for the caterpillars, who die soon after the adult wasps emerge from the pupae. But the wasps profit hugely, the presence of a guarding caterpillar almost halving the mortality of pupae in field experiments.

Although the mechanism by which these parasites influence their hosts is not clear, live wasp larvae were found in the bodies of caterpillars even after their broodmates had departed. Such 'brain worms' have also been seen with trematode and liver fluke parasites that modify the behaviour of their ant hosts.