Insufficient Polydnavirus Injection as a Physiological Factor Preventing Successful Parasitization by <i>Cotesia ruficrus</i> on Final Instar Host Larvae

Cotesia ruficrus (Cr; Braconidae) can parasitize young Mythimna separata (Ms) larvae, but not 6th (final) instar Ms larvae. In the late instar, parasitization fails because Cr eggs suffer melanization and encapsulation. However, when multiple Cr parasitized a single Ms larva through superparasitization, the successful parasitization rate increased with higher superparasitization frequency. To induce immunosuppression, Cr parasitized 6th instar Ms larvae that were artificially injected with Cr venom (V) and/or polydnavirus (PDV). Successful parasitization rate of Cr improved with an increased amount of PDV injection, but there was no change only with V. When Cr monoparasitized, 92% of the parasitoid eggs suffered melanization and encapsulation. However, when the amount of PDV from four additional wasps was increased, the number of Cr eggs suffered melanization and encapsulation decreased to less than 10%. The number of hyperspread cells (HSCs), necessary for melanization and encapsulation also decreased in Ms larvae. The C-type lectin Cky811 of Cotesia kariyai inhibits the function of Mys-IML, a C-type lectin required for adhesion of HSCs to foreign substances in Ms. This study suggests that Crf111, a Cr PDV-produced C-type lectin with high amino acid homology to Cky811, may inhibit Mys-IML function. Crf111 expression was significantly higher in hemocytes of 6th instar Ms larvae injected with 4 µL of PDV alongside Cr monoparasitization. These results suggest that when Cr parasitizes 6th instar Ms larvae, the amount of injected PDV is insufficient, resulting in low Crf111 expression, which fails to regulate the host's immune response, and thus, parasitization is unsuccessful.