Parasitism by Cotesia flavipes alters the haemocyte population and phenoloxidase activity of the sugarcane borer, Diatraea saccharalis

Parasitism of Diatraea saccharalis (Fabricius) (Lepidoptera: Crambidae) larvae by Cotesia flavipes Cameron (Hymenoptera: Braconidae) or injection of C. flavipes polydnavirus causes numerous alterations in host physiology including developmental arrest, abrogation of host immunity, and biochemical changes in proteins, carbohydrates, glycogen, and lipids. This study focused on changes in haemocyte composition occurring in the cellular immune system of parasitised D. saccharalis larvae. We also analysed the effects of parasitisation by C. flavipes on humoral immunity in terms of phenoloxidase (PO) activity in the host. In nonparasitised D. saccharalis larvae, granular cells represented the main haemocyte type (39%) and plasmatocytes were also present at around 35% among the total haemocytes. The percentages of these two major haemocytes decreased significantly in parasitised larvae after 6 days of parasitoid oviposition and the total haemocyte counts exhibited a significant reduction in parasitised larvae 3 and 6 days following parasitisation. The parasitised larvae also showed a significant decrease in humoral immune capacity as evidenced by reduction of PO activity. Moreover, the plasma had more PO activity than the haemocytes and the parasitised larvae showed less PO activity than the control larvae. This research demonstrates that the parasitism of C. flavipes adversely affects the total haemocyte populations and PO activity of D. saccharalis larvae, which would contribute to host immunosuppression.