Insect Science (2011) 00, 1–13, DOI 10.1111/j.1744-7917.2011.01454.x ORIGINAL ARTICLE Parasitism of Pieris rapae (Lepidoptera: Pieridae) by the endoparasitic wasp Pteromalus puparum (Hymenoptera: Pteromalidae): Effects of parasitism on differential hemocyte counts, micro- and ultra-structures of host hemocytes Qian-Qian Zhang, Jia Huang, Jia-Ying Zhu and Gong-Yin Ye State Key Laboratory of Rice Biology & Key laboratory of Molecular Biology of Crop Pathology and Insects of Ministry of Agriculture, Institute of Insect Sciences, Zhejiang University, Hangzhou, China Abstract Parasitism by the endoparasitic wasp Pteromalus puparum (Hymenoptera: Pteromalidae) by using only its associated venom, can suppress the immunal responses of Pieris rapae (Lepidoptera: Pieridae). However, up to now, current knowledge of the mech- anisms has been limited. The response of host hemocytes to parasitism was investigated using a combination of light and transmission electron microscopy (TEM). Five hemocyte types, prohemocytes (PRs), granulocytes (GRs), plasmatocytes (PLs), oenocytoids (OEs) and coagulocytes (COs), were observed and characterized from both unparasitized and parasitized Pieris rapae pupae. Light microscopy showed that both GRs and PLs became more round and spread abnormally after parasitism, whereas the shape of other types of hemocytes remained unaffected. In addition, the size of PRs and PLs became larger while OEs became smaller. The proportion of PRs significantly increased after parasitism and that of PLs decreased by 43.9%, but there was no significant increase of GRs and OEs. TEM showed that all types of hemocytes except COs were damaged to various degrees after parasitism, especially resulting in electron opaque cytoplasm and nucleus, fewer cell organelles of rough endoplasmic reticulum, mitochondria and vesicles. Our results indicate that parasitism by P. puparum affects differential hemocyte counts and structures of host hemocytes, particularly for GRs and PLs, which may be the main cause of the parasitoid suppressing host cellular immune responses. Key words hemocytes, light microscopy, parasitism, Pieris rapae, Pteromalus puparum, transmission electron microscopy, ultrastructure Introduction It is well known that insect hosts have evolved highly effi- cient hemocyte-mediated encapsulation reactions against larger foreign substances like parasitoid eggs (Lavine & Correspondence: Gong-Yin Ye, State Key Laboratory of Rice Biology & Key laboratory of Molecular Biology of Crop Pathol- ogy and Insects of Ministry of Agriculture, Institute of In- sect Sciences, Zhejiang University, Hangzhou 310029, China. Tel: +86 571 8697 1696; fax: +86 571 8604 9815; email: chu@zju.edu.cn Strand, 2002; Carton et al., 2008; Strand, 2008). For suc- cessful parasitism, parasitoid wasps have developed var- ious strategies to overcome their host’s immune defenses and to regulate the host’s physiological conditions to ben- efit their own survival and development by injecting ma- ternal factors with their eggs at the time of oviposition (Vinson, 1990; Brodeur & Boivin, 2004). These fac- tors include the venom, polydanviruses polydnaviruses (PDVs), virus-like particles (VLPs) and ovarian pro- teins, which have extensively been frequently described in many host-parasitoid systems (Beckage & Gelman, 2004; Pennacchio & Strand, 2006; Suzuki & Tanaka, 2006; Suzuki et al., 2008, 2009; Mabiala-Moundoungou C  2011 The Authors Journal compilation C  Institute of Zoology, Chinese Academy of Sciences 1