Abstract: The activation of prophenoloxidase (ProPO) in the hemolymph of the diamondback moth Plutella xylostella larvae against infection of Zoophthora radicans isolates from different hosts was investigated. The ProPO was found primarily on the membrane or inside host hemocytes, but less in plasma. When laminarin as an immune activator was present, phenoloxidase (PO) activities in cell debris, hemocyte lysate solution and plasma were measured as 26.80, 16.68, and 2.53 U, respectively. The ProPO was significantly activated by laminarin present in the plasma due to a much higher PO activity than that when the laminarin or the plasma was tested alone. The ProPO activity increased by 2.9－3.4-fold when the lysate of Z. radicans mycelia was present in host plasma. Among the four isolates tested, the PO activity was highest in ARSEF1342, medium in ARSEF2699 or F99101, but lowest in ARSEF1100. Furthermore, the activated PO in the mycelia of Z. radicans caused melanism, an evidence of the immune defending response of hosts to fungal infection. The PO activity was triggered most conspicuously by ARSEF1342 than by ARSEF2699 or F99101, but very weakly in ARSEF1100. The fact of the more attachment of the PO to the laminarin than to Z. radicans isolates revealed a trend that the fungal pathogen may to some extent evade from the host immune response. This was further revealed by the correlation between the virulence of different isolates against P. xylostella and the ProPO activation by the isolates or the following PO activities in the lysates of their mycelia.

Key words: Zoophthora radicans, Plutella xylostella, prophenoloxidase, phenoloxidase, insect immunity