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 de fences 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. Fo r unparasitized insects, in vitro assays indicated that less than 3.0% oft. oleracea haemocytes on a monolayer formed rosettes with yeast cells or fre sh rabbit erythrocytes (rbc), and virtually no phagocytosis of these partic les occurred. In addition, although fixed rbc formed rosettes with 51.21% o f haemocytes, only about 3.0% of the haemocytes ingested one or more of the se particles. In contrast to this, B. cereus and E. coli were readily phago cytosed by 14.75% and 53.70% of haemocytes, respectively. These results ind icate that L. oleracea haemocytes can recognise different types of nonself particles and demonstrate that ingestion does not necessarily follow attach ment. When resetting and phagocytosis assays were performed with fixed rbc and FITC-labelled E. coli, and haemocytes from starved L. oleracea, PBS inj ected L. oleracea, and experimentally envenomated insects on day five of tr eatment, there was no significant difference in the percentage of resetting or phagocytosis occurring. When haemocytes from parasitized insects on day five of treatment were utilised, however, resetting and phagocytosis were reduced by 31.41% and 34.94%, respectively. Thus, the effects of parasitiza tion and experimental envenomation are not the same. In addition, suppressi on of host haemocyte-mediated recognition and phagocytosis was not a second ary effect of nutritional deprivation and was not due to ectoparasitoid ven om components, rather it was a direct result of parasitization of L. olerac ea by E. pennicornis. The putative nature and source of the immunosuppressi ve factor(s) involved is discussed with reference to those produced by endo parasitic wasps. (C) 1999 Elsevier Science Ltd. All rights reserved.