The interaction of parasitoid wasps with their hosts are often fascinatingly macabre. The interaction between the wasp Glyptapanteles sp.and its host, Thyrinteina leucocerae is no exception.
Adult female wasps oviposit directly into caterpillars of the moth. These parasitised caterpillars continue to develop along with up to 80 parasitoid larvae inside them. When the caterpillar reaches the 4th or 5th instar the parasitoid larvae tunnel out of the host to pupate. The larvae spin cocoons close to the caterpillar.
This is where things become interesting. The host then undergoes a series of behavioural changes. It ceases to feed and remains stationary near the cocoons. However, if the cocoons are approached by a potential disturbing agent, the caterpillar violently swings its head, apparently attempting to dislodge the disturbing agent.
Grosman et al compared the behaviour of parasitised and unparasitised caterpillars and the relative survival of wasps. They found that unparasitised individuals ignored wasp cocoons, as well as potential threats to the cocoons. They also found that removing parasitised caterpillars doubled the death rates of the wasps.
The authors dissected caterpillars from which parasitoids had egressed and found 1-2 active parasitoid larvae that had remained in the host. They hypothesise that the remaining larvae are responsible for the changes in host behaviour and that they represent the cost of host manipulation – some offspring are sacrificed for the survival of their kin.
The full paper can be found here: http://www.plosone.org/article/info:doi/10.1371/journal.pone.0002276