Parasitism: Wasp uses ladybug as 'zombie bodyguard'

Although this strategy enables the wasps to protect their larvae from predation, it has a cost: the wasps pay for it in terms of fertility. The researchers have also demonstrated the reversible character of this manipulation: once the larvae have hatched, some ladybugs can recover normal behavior.

This work is published online in the journal Biology Letters.

Dinocampus coccinellae is a common parasitic wasp of the spotted lady beetle Coleomegilla maculata. Females deposit a single egg in the abdomen of their host, the ladybug, and during larval development (around twenty days) the parasite feeds on the host's tissues. Then, the wasp larva breaks out through the ladybug's abdomen, without killing it, and begins spinning a cocoon between the ladybug's legs. The ladybug, partially paralyzed, is forced to stand guard over the cocoon.

The novel manipulation strategy is intriguing in several ways: whereas the immense majority of parasitic wasps kill their host while they grow, the ladybug parasited by D. coccinellae remains alive. In addition, the behavioral manipulation occurs once the larva has left its host.

The mechanisms of this manipulation have been closely scrutinized by Frédéric Thomas' team at the Laboratoire Maladies Infectieuses et Vecteurs: Ecologie, Génétique, Evolution et Contrôle (CNRS/IRD/Universités Montpellier 1 et 2), in collaboration with researchers from the Université de Montréal. The researchers believe that the ladybug's atypical behavior results from a manipulation orchestrated by the wasp in order to be protected from predation up to the end of its larval development, in other words up to the emergence of the adult wasp. The scientists have shown in the laboratory that the cocoons of wasps guarded by a ladybug are much less vulnerable to predation than cocoons left on their own, or cocoons guarded by a ladybug killed for experimental purposes. Secretions left by the larva when it breaks out could force the ladybug to protect the cocoon once the larva has come out. The wasp larva grows by using the resources of its host. However, these resources must remain sufficient for the ladybug, since it cannot feed itself while guarding the cocoon.

This work also enabled the researchers to validate a theoretical model, according to which parasites cannot maximize both their effort of reproduction and manipulation.

The researchers have demonstrated a negative correlation between the length of the ladybug's guarding period and the fecundity of the wasp. Everything happens as if the wasp larva had to "choose" between using the resources of the ladybug to produce eggs (which will be available at the adult age) or making them last while sparing the ladybug and keeping it alive.

Finally, the researchers were surprised to observe that around 25% of the manipulated ladybugs recovered normal behavior after the emergence of the adult wasp, a very rare case of reversible manipulation.