The function of a trypsin-like enzyme in the saliva of Euplectrus separatae larvae

Larvae of the gregarious ectoparasitoid, Euplectrus separatae, a species that parasitizes Pseudaletia separata, migrate from the dorsal to the ventral side of the host larva for pupation 7 days after parasitization. The parasitized host larvae die after the migration. The body mass of the parasitoid larvae increases while that of the host larva drastically decreases. Most of the tissue in the dead host larvae completely collapses. In this study, we examined the cause of host death and how the tissues collapse. Artificial removal of all parasitoid larvae before their migration on day 7 rescued the host larvae, but removal after parasitoid migration did not rescue the hosts. Tissues of the dead host larvae were completely liquefied. Injection of saliva from day 7 parasitoid larvae into host larvae killed the host larvae. High activity of a trypsin-like enzyme was detected in the saliva of day 7 parasitoids. Though phospholipase B and hyaluronidase were also detected in the saliva, commercial phospholipase B and hyaluronidase did not kill the hosts, whereas an injection of commercial trypsin was lethal. The trypsin-injected hosts showed the same tissue collapse as noted in parasitized and saliva-injected hosts. Leupeptin, a trypsin inhibitor, reduced mortality when injected into day 7 hosts (parasitoids were removal following migration). These observations suggest that the day 7 parasitoid larvae release saliva containing a trypsin-like enzyme to digest the host tissues following migration.