Value of mixtures of vertically and horizontally transmitted variant mixtures of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) as the basis for biological insecticides

Abstract

The Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) is a highly pathogenic host-specific virus that can be used to control its natural host, the beet armyworm S. exigua (Lepidoptera: Noctuidae). Recent studies have demonstrated that distinct genotypes of SeMNPV differ in their insecticidal properties and could be associated with different routes of transmission. In the present study we evaluated the value of using mixed genotypes of SeMNPV that had favorable insecticidal properties (Se-G25), or the capability for vertical transmission (Se-Al1). Interestingly, mixed genotypes containing 25 and 75% of Se-G25 improved the pathogenicity of mixed genotype occlusion bodies (OBs) compared to Se-Al1 genotype OBs, although no differences were observed in speed of kill or OB production (OBs/larva). The capacity to produce covert infections was evaluated in adult survivors of a sublethal dose. The Se-Al1 genotype was the most efficient in producing sublethal infections (90% infection) and the mixture of 50% Al1 + 50% G25 (87%), which were significantly more efficient than the Se-G25 (51%) genotype alone. The mixture of 25% Al1 + 75% G25 and 75% Al1 + 25% G25 resulted in 72% and 80% of adults positive for covert infection by qPCR, respectively. A greenhouse experiment revealed that sublethal infections in adult survivors increased with the proportion of Se-Al1 genotype present in inoculum OBs. We conclude that the presence of vertically-transmitted variants is of limited interest for the development of biological insecticides based on SeMNPV.