May 2020

Zombie caterpillars

What is this caterpillar doing? It seems to be acting unusually for a caterpillar…


A seemingly-normal White Looper Moth caterpillar. Photo: Veronica Cuskelly.

Wasp larvae emerge from the body of the hapless caterpillar. Photo: Veronica Cuskelly.

The caterpillar guards the wasp cocoons. Photo: Veronica Cuskelly.

The black spots on the caterpillars’ flank are ‘exit wounds’ left by the wasp larvae. Photo: Veronica Cuskelly.

These beautiful yet slightly disturbing photos show the caterpillar of the White Looper Moth Pingasa chlora (Family Geometridae). The caterpillar is certainly exhibiting unusual behaviour. Caterpillars are essentially eating machines, but rather than wandering about the plant devouring leaves like a typical hungry caterpillar, this one is doing something different. Rearing up over a cluster of cocoons which are clearly not its own, it appears to be standing guard to defend its brood!

The cocoons belong to parasitoid wasps (Family Braconidae, subfamily Microgastrinae). ‘Parasitoids’ are a subset of parasites that actively kill their hosts, rather than just feeding on them. The wasp larvae have been living inside the caterpillar, where they fed on body fluids and various organs. They were, however, careful not to inadvertently kill their host too soon. As well as providing food in the form of its living body, the caterpillar still has another job to perform!

When fully grown, the wasp larvae erupted from the caterpillar and spun cocoons together in a cluster. Usually the caterpillar would die, but in some cases it survives for an extended period. Research has shown that it actually guards and protects the very animals that have sealed its doom, presumably fending off predators and other parasites that would attack the developing wasp pupae.

Experiments have demonstrated that when parasitised caterpillars guarding wasp broods were confronted with stink bugs that attack wasp cocoons, they almost invariably thrashed their heads in the direction of the bugs. Un-parasitised caterpillars paid no attention. So, what causes this spooky behaviour?  It seems that some of the wasp larvae stay inside the caterpillar. They did not emerge with their siblings and might be responsible for ‘controlling’ the caterpillar’s behaviour. In doing so they may have sacrificed themselves to ensure the survival of their siblings.;

These caterpillars have sometimes been referred to as “zombie caterpillars”, and this ‘zombie effect’ associated with invasion and control by parasites can also be seen in other animals. Flatworms manipulate the behaviour of snails, causing them to be eaten by birds, while gordian worms command grasshoppers and mantises to enter water, allowing the worm to complete its life cycle. Even some fungi decide just where and when their insect hosts will die to optimise spore dispersal. It seems doubtful that science fiction authors could write a more outlandish story.

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