Wasps slave spiders by turning them into zombies

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Credit Alex Hyde

Insects have a range of tactics they employ for their survival and growth and some have rather unpleasant habits of controlling others for their own good.

Reclinervellus nielseni, a wasp turns its targets – spider Cyclosa argenteoalba – into zombies by taking control of their nervous system and commanding them to build a tough ‘cocoon’ web from the original orb web to protect the developing wasp pupa after the spider’s death.

Discovered by Keizo Takasuka from Kobe University, Japan, R. nielseni incapacitate their hapless victims and once it has it in its clutches, it deposits its egg on or within the victim’s body, ready for the next generation to develop.

According to Takasuka, C. argenteoalba produce several different styles of web over the course of their lives – ‘orb’ webs when hunting and ‘resting’ webs for protection when moulting – each produced by a specific set of behaviours.

However, it wasn’t clear which of the spider’s behavioural patterns and web-types the wasp was adapting to its own ends until Takasuka investigated how the wasp manipulated its spider host and web. He reveals that the wasp larvae masters force their zombie spider hosts to build a modified and reinforced resting web before steering them back to the centre of the web to construct a cocoon. The study has been published in Journal of Experimental Biology at http://jeb.biologists.org.

Takasuka collected samples from two nearby cities (Tamba and Sasayama) from mid-April to mid-May to collect spiders complete with their webs and parasitic larvae. However, things didn’t go as planned because keeping the spiders alive in the lab before their zombie state was triggered was quite a challenge.

Some spiders didn’t build webs in captivity and he occasionally destroyed the delicate structures when he inadvertently snagged supporting frame threads attached to distant objects. However, after weeks of patiently nurturing the spiders, he was rewarded when 10 obediently constructed cocoon webs for their parasite masters.

Takasuka analysed the cocoon webs and found that they are quite similar to the resting webs, complete with fluffy decorative structures. Takasuka analysed the spiders’ behaviour as they constructed the cocoon webs over a 10 h period.

He found that the manipulated spiders (zombies) always constructed the new web on the site of the old orb web, painstakingly removing the sticky spiral first, then reinforcing the radial and frame threads and then adding the fibrous web decorations. And, when the web was complete and the wasp larva done with its spider slave, the larva directed the spider to return to the hub of the web before murdering it.

Takasuka and Kensuke Nakata photographed the webs in UV light and were impressed to see the fibrous decorations shining brightly, to deter other insects from inadvertently blundering into the pupa’s nursery. And when Tomoki Yasui, Toru Ishigami and Takasuka investigated the strength of the different types of silk that contribute to the web’s structure, they were amazed to see that the cocoon web was significantly tougher than the orb and resting webs: the breaking force of the cocoon radius and frame silks was 2.7-40 times greater than for the orb and resting webs.

However, the breaking stress of the silks was not increased significantly, leading Takasuka to suspect that instead of forcing the spiders to alter silk production, the wasp slave-masters were directing the spiders to lay down more fibres to strengthen the cocoon web, which is essentially a reinforced resting web.

Explaining that parasitized spiders transition into zombie web building even when the parasitic larva has been removed, Takasuka also suggests that resting web construction is triggered by the same hormones that control moulting and he suspects that the larva may inject a substance similar to a moulting hormone into the hapless spider during the later stages of its stay, ready to trigger cocoon web building when the larva is ready to pupate.