Wasps, ladybirds and the perils of hiring zombie bodyguards

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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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