Parasitization of Lacanobia oleracea (Lepidoptera : Noctuidae) by the ectoparasite wasp, Eulophus pennicornis - Effects of parasitization, venom and starvation on host haemocytes

In contrast to the situation with endoparasitic wasps, little is known abou t the effects of ectoparasitoids and their secretions on the haemocytes of their insect hosts. To address this deficit, a study has been made of the e ctoparasitic wasp, Eulophus pennicornis, and it's host, the tomato moth, La canobia oleracea. Using light microscopy, it was determined that L. olerace a has five main haemocyte types, namely, plasmatocytes, granular cells, sph erule cells, oenocytoids and pro-haemocytes, representing 56%, 30%, 10%, 2% and 2% of the population, respectively. Parasitization by E. pennicornis, resulted in an increase in the number of circulating haemocytes up to day t hree, followed by a decrease towards day eight; the latter being associated with changes to the morphology and viability of the cells. For example, on day five after parasitization, plasmatocytes and granular cells had become more rounded and put out pseudopods less readily compared with those from non-parasitized controls, whilst from day seven onwards there was a signifi cant decrease in haemocyte viability and by day nine, extensive haemocyte d amage and disintegration was evident. These changes were not observed when larvae were injected with E. pennicornis venom, or when haemocytes were exp osed directly to venom in vitro, neither did they occur in starved larvae. Thus, although the observed effects on L. oleracea haemocytes are definitel y associated with parasitization they are not due to wasp venom components, nor are they a non-specific effect resulting from nutritional deprivation. The possibility that the feeding wasp larvae produce factors which perturb host haemocytes in order to help condition the host to ensure that success ful parasitization occurs, is discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.