Recovery of spiders from the effects of parasitic wasps: implications for fine-tuned mechanisms of manipulation

The larva of the ichneumonid wasp Polysphincta gutfreundi induces its host, the orb-weaving spider Allocyclosa bifurca, to build a highly modified, physically stable orb web, to which the larva then attaches its pupal cocoon, and to add an otherwise unusual linear silk stabilimentum to this web that may camouflage the cocoon. The effects of the larva are apparently due to a chemical product or products that it introduces into the spider. Behavioural modification is gradual, and various behavioural effects arise in a consistent order. If the wasp larva is experimentally removed just before it kills the spider, the spiderʹs behaviour recovers gradually in the reverse order. In addition, a greater delay in removing the larva leads to more pronounced and enduring behavioural changes, so the larval effects may depend on a cumulative or dose-dependent process. Changes in numbers and lengths of radii and numbers of sticky spiral loops could result from correlated larval effects on the reduction in the amount of silk in the spiderʹs sticky spiral silk glands (or a signal thereof), but several other types of behavioural change are probably under separate controls; multiple larval products may be involved. The larvae may affect higher levels of behavioural decisions by spiders that determine overall web ‘design’, rather than lower levels, such as control of particular behaviour patterns, as may be affected by related wasps. The larvaʹs effects are fine-tuned to details of the hostʹs natural history.