Suppression of the prophenoloxidase cascade in the larval haemolymph of the turnip moth, Agrotis segetum by an entomopathogenic nematode, Steinernema carpocapsae and its symbiotic bacterium

To study the suppression of the prophenoloxidase cascade in the larval haemolymph of the turnip moth, Agrotis segetum by Steinernema carpocapsae and its symbiotic bacterium Xenorhabdus nematophilus, living S. carpocapsae, heat-killed S. carpocapsae, living X. nematophilus, and heat-killed X. nematophilus were injected in four different treatments into final-instar larvae of A. segetum. In all of the treatments, the activation of prophenoloxidase cascade (initiated by laminarin in the insect haemolymph which was drawn 1 h after injection) was inhibited. This inhibition occurred in a dose-dependent manner. On the other hand, injection of non-pathogenic and fungus-feeding nematodes, Aphelenchus avenae did not induce a similar inhibition, and the nematodes were found to be entrapped in nodules within a melanized sheath. Co-injection of Aph. avenae with S. carpocapsae or X. nematophilus suppressed the prophenoloxidase cascade, while Aph. avenae survived in the haemolymph without being trapped by haemocytes. In all haemolymph of larvae into which foreign-bodies were injected, the levels of prophenoloxidase were essentially the same as those from control larvae, when determined by α-chymotrypsin treatment. These results indicate that S. carpocapsae and/or X. nematophilus suppress the prophenoloxidase cascade at an early step and also interfere with the role of haemocytes in self-defence.