Post-mortem incubation influences occlusion body production in nucleopolyhedrovirus-infected larvae of Spodoptera frugiperda.
The inefficient production of nucleopolyhedrovirus occlusion bodies (OBs) can limit the commercialization of virus-based insecticides. The production of OBs in SfMNPV-infected Spodoptera frugiperda fourth instars was compared among groups of larvae that were frozen (-20°C) immediately following death, or subjected to a 9-day period of post-mortem incubation at 5°C or 15°C. Incubation at 15°C resulted in a ∼40% increase in total OB production and OBs per mg larval weight, compared to the -20°C and 5°C treatments. OBs from the 5°C treatment were ∼10% smaller in cross sectional area and small OBs (<1 µm2) were more abundant than in other treatments, possibly due to reduced post-mortem OB maturation in this treatment. SfMNPV genomic DNA in OB samples was 3.6-fold higher in the -20°C treatment than the 5°C treatment and 1.7-fold higher than the 15°C treatment, possibly due to differences in the exposure of viral genomes to degradative enzymes. However, these differences did not affect the concentration-mortality relationship or speed of kill of OBs from the different treatments. The abundance of aerobic microbes increased from ∼3×107 CFU/ml in the -20°C treatment, to approximately 2 × 108 and 3×108 CFU/ml in the 5°C and 15°C incubation treatments, respectively, similar to levels seen in other nucleopolyhedroviruses produced in insects. We conclude that post-mortem incubation at 15°C likely involves continuing processes of virion occlusion and OB maturation that increase overall OB production without loss of insecticidal activity, although the value of this step in commercial virus insecticide production will depend on the cost of the incubation step and the value of the additional OBs produced.