Spodoptera exigua nucleopolyhedrovirus is not infectious for Agrotis segetum larvae per os, but only after intrahemocoelic injection

Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) and Agrotis segetum NPV are close relatives, but distinct baculovirus species. Their genomic organization is remarkably similar. The cross infectivity of these two viruses for S. exigua and A. segetum larvae has been analyzed. AgseNPV was able to infect S. exigua orally, although the dose needed was high (LD50 of 8,3 x 105 OBs/larvae) and the yield of progeny polyhedra very low. In contrast, SeMNPV appeared to be non-infectious for A. segetum larvae when administered per os. The course of SeMNPV infection in A. segetum was monitored by real-time PCR in a time course experiment and showed that SeMNPV was able to infect and produce polyhedra in A. segetum larvae by intrahemocoelic injection. Infection of A. segetum with a mixture of AgseNPV and SeMNPV, only resulted in AgseNPV polyhedra. So, in the case of SeMNPV the midgut is the major barrier for infection. The possible mechanisms of A. segetum resistance for SeMNPV per os are discussed