Zombie babysitters

It’s naptime of the living dead on guava trees in Brazil. Healthy juvenile wasps lie inside their cocoons while nearly dead caterpillars loom nearby — as bodyguards.

The Glyptapanteles wasps are parasitoids, injected as eggs by their mother into the caterpillars, explains Arne Janssen of the University of Amsterdam. The eggs hatch and the young larvae feed on the surrounding body fluids of the caterpillar. When it’s time to form a cocoon, the wasps break out through the caterpillar skin and settle in a cluster on a twig.

The caterpillar, the young of the moth Thyrinteinta leucocerae, stops feeding and stays on the spot, too. Healthy caterpillars don’t react much to approaching insects, but the ones the wasp larvae have been using as baby food develop a fighting streak. When a predatory insect or even an entomologist’s hand swoops near, the caterpillar “will start violently swinging its head from side to side,” Janssen says. The headbutting typically chases away the predator.

That caterpillar bodyguard on average halves the death rate for a cluster of wasps in cocoons during three-day tests outdoors, Janssen says. This test and others by the team make the wasp-caterpillar system one of the best documented cases of a parasitoid changing the behavior of its host, Janssen and his colleagues report in the June PLoS ONE.

Biologists have found behaviors in other species that look as if parasitoids re-engineer the host’s manners, but these impressions can be hard to substantiate. Critics have questioned whether parasitoids were attacking oddball individuals to begin with or whether the behavior benefited the caterpillar.

Janssen and his colleagues at the Federal University of Viçosa in Brazil used batches of healthy sibling caterpillars with no obvious violent tendencies for the tests. Also, he argues that the caterpillar doesn’t benefit from the bodyguard duty because its half-eaten hulk expires in a few days anyway.

The study has convinced David Biron of the French National Institute for Agricultural Research at Clermont-Ferrand. “The alteration of host behavior is spectacular,” he says.

Biron says he would like to know more about the molecular mechanisms that bring about the change.

Janssen speculates that the few larvae that remain inside the caterpillar may be secreting substances that change the behavior.

Whatever the answer is, the wasp-caterpillar interaction makes a good system for studying the evolution of behavior modification, Janssen says.

See the difference between parasitized and normal caterpillar behavior:

Zombie Caterpillar from Science News on Vimeo.

A parasitized caterpillar stays near the cocoons of the young wasps that
burst out of its body. When a potential predator intrudes, the doomed
caterpillar swings at it until it leaves. Video courtesy of PLoS One.

Ordinary Caterpillar from Science News on Vimeo.

A non-parasitized caterpillar hardly responds to a predator.
Researchers conclude that the wasps somehow changed the caterpillar’s
behavior. Video courtesy of PLoS One.

To see another example of a parasite apparently altering its host behavior, watch as a cricket infected with a hairworm throws itself into water, and to its death: