Can mixtures of horizontally and vertically transmitted nucleopolyhedrovirus genotypes be effective for biological control of Spodoptera exigua ?

Previous studies identified distinct genotypes of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) that were associated with horizontal transmission (named HT-SeG25) or vertical transmission (named VT-SeAl1) in the host insect, S. exigua (Lepidoptera: Noctuidae). We examined the use of mixtures of occlusion bodies (OBs) of these genotypes as the basis for a virus preparation that could provide immediate pest control and establish a persistent sublethal infection in the survivors of an OB application for transgenerational pest suppression. Mixtures of HT-SeG25 + VT-SeAl1 comprising 25:75 or 75:25 % of each genotype, respectively, resulted in improved OB pathogenicity in terms of concentration-mortality metrics compared to OBs of VT-SeAl1 alone or similar values compared to OBs of the HT-SeG25 genotype alone. In contrast, no significant differences were observed in speed of kill or mean OB production per larva. Laboratory and greenhouse trials revealed that the prevalence of sublethal infection in adults that survived OB treatments in the larval stage increased with the proportion of VT-SeAl1 present in the inoculum, as determined by qPCR. Greenhouse trials indicated that the 75 % VT-SeAl1 + 25 % HT-SeG25 mixture was as effective as methoxyfenozide in preventing pest damage to pepper fruits. The potential contribution of vertically transmitted genotypes to transgenerational suppression of pest populations is discussed.