Determination of virulence and genetic variability of three Spodoptera littoralis nucleopolyhedrovirus isolates from Egypt

Abstract Background The genetic diversity is naturally occurring in baculovirus population, which may affect the potency of the bio-agent. The Spodoptera littoralis nucleopolyhedrovirus is a promising bio-control agent that can be used to control the cotton leafworm, S. littoralis in Egypt. This study described the virulence and genetic variability of 3 S. littoralis nucleopolyhedrovirus isolates named Cab2, P2 and P3 previously recovered from diseased S. littoralis larvae of cabbage, pepper and eggplant cultivation fields. Results The viral genomic DNA restriction analysis using PstI and ScaI endonucleases showed identical Pst1 restriction pattern of 10 fragments in both Cab2 and P3 isolates, while there are 7 extra fragments present in P2 isolate of about 25, 19.8, 15, 14, 11, 9 and 6.7 (kbp). On the other hand, the restriction pattern using ScaI revealed 10 common fragments in all three isolates, in addition to an extra marker fragment of about 25 kb in P3 isolate absent in both P2 and Cab3 isolates. The phylogenetic analysis of the partial nucleotide sequence of the polyhedrin gene confirmed an extensive degree of co-linearity among the genomes of the 3 tested isolates, with pairwise sequence identities of 97.04% with S. littoralis nucleopolyhedrovirus isolate A26-5 (Acc. No. AY706717). The median lethal concentration (LC50) value revealed that Cab2 had the best virulence toward the first instar of S. littoralis larvae with LC50 value of 1.59 × 104 Obs/ml compared to the virulence recorded by P3 and P2 which were 1.23 × 106 and 2.7 × 104 OBs/ml, respectively. The median survival time (LT50) value of Cab2 isolate was the highest speed of treated larval mortality with 120 hpi compared to P2 and P3, which recorded 132 hpi for both isolates. Conclusions Although all three S. littoralis nucleopolyhedrovirus isolates showed considered genotypic variation and insecticidal activity against S. littoralis larvae, the variant Cab2 showed the highest virulence and speed mortality against the first instar larvae of S. littoralis. The new variant proved to possess promising insecticidal properties for the development of virus-based biopesticide for the control of S. littoralis population in Egypt.