Saturday, 23 July 2011

Parasitic wasps turn ladybirds into their bodyguards

ResearchBlogging.orgI have covered the ladybird parasitoid Dinocampus coccinellae before. Some recent research, however, has uncovered some fascinating aspects of this little wasp's manipulation of its host behaviour. The parasitoid wasp, below, injects a single egg on a ladybird using its ovipositor (visible in the top photo of a just emerged D. coccinellae).
After hatching, the larva feeds on its host internal organs, and after about 20 days, she emerges from the ventral plates of the ladybird to pupate. She spins a cocoon that tethers the ladybird to the substrate (top photo) and pupates inside. Unlike many parasitoids, Dinocampus does not kill its host. But the ladybird cannot escape, not only she is tethered, in addition, before emerging from the ladybird's body, the larva is thought to produced some chemicals that affect the ladybird's brain and compels it to sit still, and to twitch when disturbed. The parasitized ladybird colours and twitching were through to afford the parasitoid some protection from predators while in the cocoon. Fanny Maure and her collaborators provided the much needed evidence for this hypothesis in some laboratory experiments. They infected ladybirds - Coleomegilla maculata, a Canadian species - with Dinocampus coccinellae, and once the cocoons were spun under the ladybirds, they split the coccoons into three groups. In the first group of cocoons, they removed the ladybird, in the second, they killed the ladybird and in the third they left the ladybird untouched in the usual position on top of the cocoon. Then they exposed their cocoons to a predator, third instar green lacewing larvae Chrysoperla carnea and counted how many cocoons were predated in each group.
Percentage of Dinocampus coccinellae cocoons eaten by larval green lacewing, Chrysoperla carnea, when parasitoid cocoons were exposed alone, covered by a dead ladybird (Coleomegilla maculata), or attended by a living ladybird. Probabilities were obtained using the Fisher exact test, ***p , 0.0001. Numbers refer to sample sizes (from Maure et al 2011).

Parasitoid cocoons alone or sheltered with a dead ladybird suffered significantly more predation by lacewing larvae that did those protected by living ladybirds, supporting the hypothesis that the parasitoids manipulate the ladybird's behaviour to their own advantage, effectively converting them into their own bodyguards. The ladybird protective colours made little difference, although these are thought to protect against bird, not insect, predators.
  Presumably, there must be a cost to the larvae to manipulate the ladybird. She must left the ladybird alive and produce chemicals to make it into her bodyguard. Maure and coworkers also tested this, by measuring the relationship between the ladybird lifespan once the parasite emerged and the survival and fecundity of the parasite. Their results show a significant negative correlation between the ladybird lifespan - 25% survived the parasitoid emergence - and the number of mature eggs the parasitoids had in their bodies after emergence as adults. This suggest that indeed there is a cost to making your host into a bodyguard. Overall, though, it must compensate the parasite to shield itself with the live ladybird in terms of predator avoidance.

Maure F, Brodeur J, Ponlet N, Doyon J, Firlej A, Elguero E, & Thomas F (2011). The cost of a bodyguard. Biology letters PMID: 21697162


Nurturing nature said...

I wonder if this wasp parasitises the harlequin ladybird?

Africa Gómez said...

Good question! As you can imagine this has been tested in the lab and it appears - if I remember correctly - that the parasite does not survive as well inside the Harlequin as a host.