Parasitization of Lacanobia oleracea (Lepidoptera) by the ectoparasitic wasp, Eulophus pennicornis, suppresses haemocyte-mediated recognition of non-self and phagocytosis

Although many endoparasitic wasps suppress the haemocyte-mediated immune defences of their insect hosts, the effects of ectoparasitoids are virtually unknown. In view of this, a study has been made of the ectoparasitic wasp, Eulophus pennicornis, and its host, the tomato moth, Lacanobia oleracea. For unparasitized insects, in vitro assays indicated that less than 3.0% of L. oleracea haemocytes on a monolayer formed rosettes with yeast cells or fresh rabbit erythrocytes (rbc), and virtually no phagocytosis of these particles occurred. In addition, although fixed rbc formed rosettes with 51.21% of haemocytes, only about 3.0% of the haemocytes ingested one or more of these particles. In contrast to this, B. cereus and E. coli were readily phagocytosed by 14.75% and 53.70% of haemocytes, respectively. These results indicate that L. oleracea haemocytes can recognise different types of non-self particles and demonstrate that ingestion does not necessarily follow attachment. When rosetting and phagocytosis assays were performed with fixed rbc and FITC-labelled E. coli, and haemocytes from starved L. oleracea, PBS injected L. oleracea, and experimentally envenomated insects on day five of treatment, there was no significant difference in the percentage of rosetting or phagocytosis occurring. When haemocytes from parasitized insects on day five of treatment were utilised, however, rosetting and phagocytosis were reduced by 31.41% and 34.94%, respectively. Thus, the effects of parasitization and experimental envenomation are not the same. In addition, suppression of host haemocyte-mediated recognition and phagocytosis was not a secondary effect of nutritional deprivation and was not due to ectoparasitoid venom components, rather it was a direct result of parasitization of L. oleracea by E. pennicornis. The putative nature and source of the immunosuppressive factor(s) involved is discussed with reference to those produced by endoparasitic wasps.