Abstract
Ascoviruses are insect-specific large DNA viruses that mainly infect noctuid larvae, and are transmitted by parasitoids in the fields. Heliothis virescens ascovirus 3h (HvAV-3h) has been recently isolated from Spodoptera exigua, without parasitoid vector identified previously. Here we report that Microplitis similis, a solitary endoparasitoid wasp, could transmit HvAV-3h between S. exigua larvae in the laboratory. When the female parasitoid wasp acquired the virus and served as a vector, the period of virion viability on the ovipositor was 4.1 ± 1.4 days. Infected host larvae were still acceptable for egg laying by parasitoids, and the parasitoids thereafter transmitted virus to healthy hosts. Virus acquisition occurred only from donor hosts between 3 and 9 days post infection. The peak of virus acquisition (80.9 ± 6.3%) was found when M. similis wasps oviposited in larvae that had been inoculated with the virus 7 days previously. When virus infection of the host took place during the life cycle of the parasitoid wasp, it caused 1- to 4-day-old immature parasitoids death in the host, whilst a small proportion of 5- to 6-day-old and the majority of 7-day-old parasitoids larvae survived from the virus-infected hosts. Viral contamination did not reduce the life span or fecundity of female M. similis.
Highlights
Unlike other pathogens that have been successfully industrially produced for biocontrol agents, ascoviruses are rarely transmitted per os[15,19]; they can be efficiently transmitted by hymenopteran endoparasitoids, which introduce virus particles into susceptible hosts during the act of oviposition[17,20,21]
After a single sting on an infected host, M. similis could deliver the virus to recipient hosts, which could be confirmed by the milky-white color in the recipient S. exigua hemolymph
Given this evidence on the successful viral transmission via M. similis, in the system of Heliothis virescens ascovirus (HvAV)-3h, S. exigua and M. similis itself, M. similis served as a vector, transmitting virus among S. exigua individuals
Summary
Unlike other pathogens that have been successfully industrially produced for biocontrol agents, ascoviruses are rarely transmitted per os[15,19]; they can be efficiently transmitted by hymenopteran endoparasitoids, which introduce virus particles into susceptible hosts during the act of oviposition[17,20,21]. It is known that among hymenopteran species, some of the Braconidae and Ichneunmonidae species can efficiently transmit ascoviruses during the process of oviposition[21,22]. In the US, where Spodoptera frugiperda ascovirus (SfAV), Tricoplusia ni ascovirus (TnAV) and Heliothis virescens ascovirus (HvAV) were isolated, Cotesia marginiventris served as a vector to transmit Sf 82–126 (belongs to SfAV)[22]. We report Heliothis virescens ascovirus 3h (HvAV-3h), belonging to the HvAV complex[16,23], is transmissible among S. exigua hosts via Microplitis similis Lyle (Hymenoptera: Braconidae). Elucidating the parasitoid-wasp, which mediates viral transmission, is important for understanding ascoviruses biology, and for host – parasitoid wasp – pathogen system coevolution. We determined the transmission rates under different viral propagating stages, the virion viability on the ovipositor and the effects of HvAV-3h infection on M. similis development to understand the putative HvAV-3h transmission mechanism
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