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Venom from the Ectoparasitic Wasp Melittobia digitata (Hymenoptera: Eulophidae) Induces Paralysis and Developmental Delay in Natural and Factitious Hosts

Abstract Melittobia digitata Dahms (Hymenoptera: Eulophidae) is a gregarious ectoparasitoid that attacks the pupae of solitary bees and wasps. Although this wasp has been intensively studied in terms of behavior and life history, almost nothing is known about its venom. Moreover, the use of venom in the host–parasite relationship has been disputed despite observations that parasitized hosts seem to be halted in development and enter a state of paralysis. In this study, we examined the role of venom from M. digitata during parasitism by comparing the developmental fate of natural (alfalfa leafcutting bee, Megachile rotundata F.) and factitious [yellow mealworm, Tenebrio molitor L. and flesh fly, Sarcophaga bullata (Parker)] hosts after either natural envenomation or artificial venom injection. Isolated crude venom collected either by “milking” adult females or by dissections of venom glands was nearly equally effective in halting the development of M. rotundata, T. molitor, and S. bullata. Host responses to venom injection were dependent on the developmental stage of the host tested and concentration of venom injected. For all three hosts, venom seemed to induce paralysis, but the onset of paralysis was not the same in each host. T. molitor and M. rotundata entered a paralyzed state several hours (2–4) earlier than larvae of S. bullata. Likewise, the type of paralysis induced by venom from M. digitata was not the same for the three insects tested as evidenced by limited mobility and some progression of development in venom-injected M. rotundata and T. molitor, and a flaccid paralysis characterized by complete immobilization in larvae of S. bullata. Ultimately, all venom-injected hosts that became paralyzed eventually died. Our data suggest two potential modes of action for M. digitata venom.