Nucleopolyhedrovirus (SfMNPV) causes the liquefaction of dead larvae integument (isolate 19) in Spodoptera frugiperda

The Spodoptera frugiperda nucleopolyhedrovirus (SfMNPV) causes the liquefaction of the dead larvae integument (isolate 19) and makes large-scale production laborious and expensive, because larvae must be frozen before being harvested. One dead larva was found with the integument not disrupted (isolate 6) and was multiplied during 5 generations in laboratory. Detection and sequencing of chitinase and cathepsin genes were performed as well as LC50, LT50 and field experiment using a wettable powder formulation. The new Brazilian isolate 6 of S. frugiperda was confirmed to harbour cathepsin and chitinase genes. Restriction fragment analysis with BamHI and HindIII did not show differences between isolate 19 and 6. PCR amplification of the regions encompassing the chitinase and the cathepsin genes produced an amplicon whose size was the same for the two isolates. Alignment (isolate 6) obtained with the sequence of isolate 19 revealed a deletion of one base located within the chitinase gene. The frameshift resulted in appearance of a stop codon 15 base pairs downstream the mutation. LC50 was similar to both isolates (2.6x105 and 3.6x105 PIB/mL to isolate 6 and 19, respectively) but LT50 was around three days longer to isolate 6 than 19. Field experiments using 1x107 PIBs/mL showed mortality up to 90% when larvae were collected 24 and 48 hours after the bioinsecticide was sprayed. Using isolate 6, the larval equivalent/ha could be lowered to around 80 to 120 larvae/ha, which is equivalent to 10.75 and 13.86 g of dead larvae/ha, respectively.