National Geographic

Wasps, ladybirds and the perils of hiring zombie bodyguards

Hiring zombie bodyguards to look after your children can have its drawbacks. They might end up with fewer children of their own.

The wasp Dinocampus coccinellae is a body-snatcher, or perhaps a “bodyguard-snatcher”. She’s on the hunt for a spotted ladybird. When she finds one, she stings it and lays an egg inside its body. Her grub hatches and starts eating the ladybird alive. Around three weeks later, it bursts out of its host.

But the ladybird doesn’t die. The grub hasn’t consumed all of its internal organs, and it leaves the ladybird partially paralysed but very much alive. Once out, it spins a silken cocoon between the ladybird’s legs and over the next week, it slowly transforms into an adult. Meanwhile, the ladybird stands guard over its own parasite. Its warning colours of red and black should deter would-be predators, and it twitches erratically if threats draw near. Its tour of duty only ends when the adult wasp eventually emerges from the cocoon and flies away.

The common assumption is that the ladybirds protect the developing wasps. Fanny Maure from the MIVEGEC research institute in France tested this by exposing wasp cocoons to hungry lacewings. If the cocoons were completely unprotected, the lacewings devoured all of them. If Maure placed a dead ladybird on top, around 85% of the cocoons were still eaten. But when the cocoons were protected by a living ladybird, the lacewings only ate around a third of them. The majority survived.

Another wasp, Glyptapanteles, also recruits a bodyguard – a zombie caterpillar that deters predators by head-banging. In this case, it’s possible that some of the wasp grubs stay behind in their host, manipulating its movements to protect their siblings. But D.coccinellae only lays one egg per ladybird, so that explanation doesn’t apply. For now, no one knows how the wasp manipulates the ladybird. Maure thinks that the larva probably leaves some sort of venom behind, which causes the ladybird to twitch and jerk.

Maure also found that the wasps pay a surprising price for their bodyguards. The longer the ladybirds stay alive, the less fertile the adult wasps are when they finally leave the cocoons. Their life spans stay the same, but their ability to raise the next generation suffers from the act of keeping their bodyguard.

If the venom idea is right, it could explain why the wasps take a hit to their fertility when they keep their hosts alive. Venom is a “costly” thing to make and any energy spent on doing so is energy that the grub isn’t using to grow and develop.

But Maure thinks that there is a simpler explanation. The grub grows by eating the caterpillar but it cannot consume the ladybird entirely or the would-be bodyguard would die. And a dead ladybird is hardly any use as a protector. So the grub has a trade-off to make: the more it eats, the stronger it will be when it finally becomes an adult, but the less likely it is to make it that far.

If this grisly tale of body-snatching and indentured servitude seems depressing, there is a silver lining. Maure found that around a quarter of the ladybirds survived their ordeal! A wasp grub had eaten many of their internal organs, forcibly pushed its way out of their body and forced them to stand guard for a week without any food, and yet, they survived.

These comebacks are virtually unheard of. Most body-snatching parasites kill their hosts and there are only a few examples of the hosts making a recovery. The secrets of the ladybirds’ resilience may be even more interesting than those of the wasp.

Reference: Maure, Brodeur, Ponlet, Doyon, Firlej, Elguero & Thomas. 2011. The cost of a bodyguard. Biology Letters http://dx.doi.org/10.1098/rsbl.2011.0415

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There are 15 Comments. Add Yours.

  1. Peter
    June 21, 2011

    “For now, no one knows how the wasp manipulates the caterpillar. ”

    Should it read “For now, no one knows how the wasp manipulates the ladybug.” ?

  2. Chris Lindsay
    June 21, 2011

    Maybe if the ladybug is very healthy, it can withstand the assault by the larvae, and ladybugs that aren’t healthy don’t survive. I guess I’m not seeing this as an example of an evolutionary trade-off necessarily. Maybe it could become one though?

    I think we need to put ladybugs on treadmills to determine their fitness level and then have another experiment to see if that’s related to their ability to survive the larvae.

  3. Barry
    June 21, 2011

    @Peter re-read the paragraph you sited. It should be caterpillar, as written.

  4. anne
    June 21, 2011

    Wow. Just… wow.

  5. joman
    June 22, 2011

    The more of these stories I read, the more I’m convinced that nature created ovipositors just to make sure humans had something we could have nightmares about. ugh….

  6. Noam
    June 22, 2011

    @Barry Peter is right. The sentence is about D. coccinellae so it should be ladybug and not caterpillar

    Oh, and great post, as usual.

  7. Johan Fruh
    June 22, 2011

    Could it be that there is no poison or manipulation?
    Maybe having something that large come out of your body after a certain amount of “pain”, could trigger a certain maternal instinct?
    Which makes me wonder, does the ladybug defend the cocoon as a female ladybug would defend her eggs? Or is it a completely different behaviour?

  8. Ed Yong
    June 22, 2011

    Fixed the ladybird/caterpillar error.

    @Johan – As far as I’m aware, ladybirds don’t show this sort of parental care for their own eggs.

  9. JMW
    June 22, 2011

    But if the ladybird is still alive, and functional, wouldn’t it naturally twitch or do something at the approach of a potentially predator?

    Perhaps if someone were to test the behaviour of ladybirds, who are not protecting a wasp cocoon, to the approach of a lacewing.

  10. Zombie
    June 22, 2011

    Presumably the ladybird has some kind of immune reaction to being parasitized; perhaps the fertility of the wasp is affected by its length of exposure to its bodyguard’s defenses.

    If some ladybirds survive, it could drive selection for defending themselves against the wasp larvae. Do the survivors tend to be those the wasps leave more quickly?

  11. Blackbird
    June 22, 2011

    A great piece as always Ed. Thanks so much for the link!

  12. MrO
    June 22, 2011

    Could it be that the ladybird, being alive but paralized, the only thing it can do to defend itself from a could be predator is to twitch since it can’t run away? if that’s the case, the parasitoid grub only benefits of that forced selfdefending behaviour of the bug

  13. bbleeker
    June 23, 2011

    @Joman Yeah, ovipositors are proof that there is no god – at least not one deserving of worship.

  14. Brian Too
    June 23, 2011

    The ladybug might survive the encounter, but I’m not sure they’d be exactly healthy and full functioning.

    The whole story gives me the heebie-jeebies.

  15. mfhawkes
    June 24, 2011

    I’d be interested in finding out if larvae are smaller or egress earlier when their bodyguards survive longer.

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