A manipulative parasitic wasp

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William Eberhard (2000) describes an interesting relationship between a female wasp parasite, Hymenoepimecis sp., (Ichneumonidae) and its spider host, Plesiometa argyra (Araneidae). Although these species do not occur in southern Africa, they illustrate an interesting instance where an insect parasitoid is able to alter the behaviour of its spider host to the finest degree.

The orb spider is stung while on its web and is temporarily paralysed while the wasp lays her egg on it. The spider then recovers and goes about its life with the newly hatched wasp larva feeding on it by sucking its haemolymph (spider "blood"). 

For about 7 to 14 days, the spider continues building its usual orb webs for prey capture. However, in the evening of the night when it is to be killed by its wasp parasite, the spider weaves a different web, designed specifically to suit the purposes of the wasp. The wasp larva then moults, kills and consumes the spider and pupates, suspending itself safely from its custom-built cocoon web.

Normal web of the orb-weaving spider. Cocoon web and wasp cocoon from above.
Cocoon web and wasp cocoon from the side.
Images provided by W. Eberhard , used with permission.

The cocoon web is consistently made to the same pattern and deviations from that pattern would be disastrous for the wasp larva. The cocoon web is a simplified web  and the sticky spirals and multi-stranded cable and radial lines of the orb web are omitted. This simplified cocoon web suspends the wasp pupa safely protecting it from various adverse conditions. Vulnerability to heavy rains, for example, was observed in a related wasp species.

The spider's change in behaviour is thought to be induced chemically rather than by physical interference. The effect of the stimulus is both rapid and long-lasting. Observations were made where the wasp was removed earlier in the evening of the spider's final night and the spider did not spin the cocoon web. The wasp was left and only removed later in the evening and the spider was observed to proceed with the construction of the cocoon web.  When the spider was allowed to survive, it continued to make the cocoon web the following night and some spiders reverted to making more normal webs on subsequent nights.

Reference

  • Eberhard, W. G. 2000. Spider manipulation by a wasp larva. Nature Vol. 406. 20 July: 255 - 256.

Text by Margie Cochrane


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