Evaluation of manipulative effects by an ichneumonid spider-ectoparasitoid larva upon an orb-weaving spider host (Araneidae: Cyclosa argenteoalba) by means of surgical removal and transplantation

Abstract

Surgical removal and larval transplantation experiments were carried out to evaluate behavioral manipulative effects caused by larvae of the polysphinctine ichneumonid, Reclinervellus nielseni (Roman, 1923), on its orb-weaving spider host, Cyclosa argenteoalba Bösenberg & Strand, 1906 (Araneidae). Residual behavioral effects on spiders from which parasitoid larvae had been removed were recognized and are described. Some transplanted parasitoid larvae also successfully consumed alternative spider hosts provided to them, and completed metamorphosis. The removal experiment supports previous hypotheses proposed by Eberhard, that larval effects involve chemicals, which are still unidentified and are dosage-dependent, because (1) spiders with parasitoid larvae removed continued to show manipulated behavior, and (2) the later in their development that the parasitoid larvae were removed, the more acute remaining effects were (e.g., production of V radii, new hub loops and silk decorations on webs). Further, the results support the idea that the spider's web-building behavior is composed of independent units (modules), because whether or not specific web features were produced depended on when the larva was removed. Transplantation experiments provide a new technique for rearing parasitoid larvae whose host spider dies: penultimate instar parasitoid larvae were successfully reared following their removal from their original host by providing them with another host spider held by a mesh. Transplantation of a small young larva by gluing it onto another host that was released after successful transplantation, caused the new host to produce a modified web identical to the “cocoon” webs typically produced by parasitized spiders. This shows that venom injected by the adult parasitoid prior to oviposition is not necessary to induce behavioral changes in the host, which can be caused entirely by wasp larval secretions.