Abstract:

Changes in the fat body structure of the host Plutella xylostella (Lepidoptera:  Plutellidae) larvae induced by different parasitism factors of the parasitoid Cotesia plutellae (Kurdjumov) (Hymenoptera:  Braconidae) in different parasitized states were observed with light microscopy and transmission electron microscopy. The results revealed that the fat body cytology of parasitized larvae was different from those of pseudo-parasitized host and non-parasitized control. The polydnavirus (PDV) and venom had no obvious effects on the fat body structure in the pseudo-parasitism condition, where the fat body maintained the integrality with normal stacks of endoplasmic reticulum and mitochondria, glycogens accumulated in some cells, and more amounts of total lipids present but smaller as compared with those of the non-parasitized host larvae. The most striking features of fat bodies in the parasitized larvae were the cytoplasmic organelles decreased and mitochondria with ill-defined cristae, and especially during the final parasitism stage (when parasitoid larvae completed their development),  the fat body structure was everely destroyed. In contrast, when the non-parasitized larvae were in the final phase of the 4th instar, the fat body cells developed normally and were undergoing transformation from the larval into prepupal stage, with large lipid bodies and rich mitochondria and glycogen present，some of them existing in free cell rosettes. These results suggest that different parasitism factors, i.e.PDV, venom, teratocytes and parasitoid larvae, have different effects on the host fat bodies.

Key words: Cotesia plutellae, Plutella xylostella, parasitism factors, polydnavirus, teratocyte, venom, fat body structure