Abstract: There were virus-like filaments (VLFs) in the calyx regions of oviduct in the female wasp, Microplitis mediator Haliday. When an egg passed through the calyx region, the VLF attached to its surface and entered into the hemocoel of the host along with the egg. Transmission electron microscopy showed that an electron dense core of 35 nm in diameter was wrapped by one-layer membrane in the center of the VLF. The VLF exhibited a very long levogyrate structure with a diameter of 25 nm under negative staining. All of the wasp's eggs without VLFs were encapsulated in the hosts, early 3rd instar larvae of Mythimna separata while 88.2% of eggs were protected from encapsulation when they were covered by VLFs in the same host. The ability of VLF in protecting the wasp's eggs from encapsulation was different in the hosts at different stages. 64.7% and 9.5% newly-laid eggs was unencapsulated in the early 3rd and 4th instar host larvae respectively. These results suggested that the VLF only provided partial protection for the wasp's eggs and other factors (calyx fluid and venom, etc.) should make certain contribution to the immune system of the host, too.

Key words: Microplitis mediator, Mythimna separata, viruslike filaments, VLF, encapsulation