To date, genetically engineered baculoviruses introduce a promising research line to overcome the slow action of baculoviruses as biocontrol agents. On the other hand, searching for new natural baculovirus isolates with better insecticidal characteristics is still a developing subject of work (more safe and has not the risks of releasing genetically engineered product in nature). Although AcMNPV is considered as the type species in the genus nucleopolyhedrovirus, 15 species and 471 tentative species of NPV have been isolated . Abul-Nasr  isolated an Egyptian NPV from the cotton leaf worm S. littoralis. Cherry and Summers  isolated the two reference NPV types A and B. During 1986 – 1988, six natural isolates of SpliNPV were isolated from Giza, Menya, Kaha, Tokh, Kafr-Elsheikh and Gharbyia, Egypt (Khamiss, personal communication). Also, fifteen isolates of SpliNPV were isolated from the six above mentioned Egyptian localities in addition to Fayoum, Sakkara, Menofyia, Benisuef, Asyut, Sinnuris, Banha, Sharkyia and Elsaff-Giza, Egypt (Khamiss, personal communication).
Combining our data with that presented by Khamiss and by Seufi (personal communication), it can be concluded that 34 NPV isolates have been collected from Egypt between 1986 and 2000. This may reflect the suitability of the Egyptian environment for not only gathering and collecting new baculovirus isolates but also for using them as potential pesticides in integrated pest management (IPM) programs. Given that baculoviruses have been isolated from Upper as well as Lower Egypt, in which the cultivation was washed by chemical insecticides, it would be expected to find more and more isolates in virgin regions (where no insecticides were used).
The susceptibility of 2nd instar larvae of S. littoralis to the two polyhedrovirus species reported in this study was comparable to that observed by Lacey and coworkers . They reported that LC50s were 1.77 × 103 and 3.05 × 103 occlusion bodies (OB)/mm2 for Anagrapha falcifera (AfMNPV) and Autographa californica (AcMNPV) when applied to the neonate codling moth larvae. Our Egyptian isolate (SpliNPV) showed similar results to that obtained by Klein and Podoler against the Egyptian cotton leafworm . Abot and coworkers  clarified that LC50s of an NPV isolate against two A. gemmatalis populations varied from 129 to 316 OBs/ml diet. Their results fall within the range obtained with our isolate. On the other hand, our results showed lower LC50 when compared with that reported by Pawar and Ramakrishnan  and Komolpith and Ramakrishnan  whereas LC50 was 4.677 × 106 PIB/ml for 4-day old S. littoralis larvae. Similarly, Ashok and Ramakrishnan  reported higher LC50 (7.1 × 106 PIB/ml) for 3-day old S. litura larvae. Also, Stiles and Himmerich  introduced higher LC50 of AcMNPV against H. zea (3.46 × 104 – 6.38 × 105 PIB/ml). Finally, Abdel-Aziz (personal communication) presented higher LC50s (1.8 × 107 and 9.0 × 107 PIB/ml, respectively) for 2nd larval instar of S. littoralis. The variability of LC50s is probably due to the method of surface treatment, homogeneously treated diet, feeding habit of the insect species  or due to difference in larval age [36, 37]. It may also be due to difference in host susceptibility to NPV , number of virions per occlusion body, virulence of the virus strain and/or the difference in number of laboratory propagation cycles for the viral isolate. According to Van-Beek and Huges , the virulence of baculoviruses is best determined by the speed with which a given virus kills the insect pest.
LT values presented in Table (1) indicated that SpliNPV killed 2nd instar larvae of S. littoralis one day faster than AcMNPV. In comparing our results with that presented by other authors, many considerations have to be taken into account. Host range and LC95s of the viral isolates are the most important considerations. Although LC95 and LT95 that produced in this study are economically prohibitive, improving the insecticidal characteristics of such isolates (by formulation and synergistic additives) is a growing subject in many companies.
The full length of polyhedrin gene from lepidopteran NPVs ranged from 483 bp to 747 bp . In case of polyhedrin gene from Spodoptera sp. NPVs, its full length ranges from 510 bp to 747 bp in comparison to that from Autographa sp. NPVs which ranges from 507 bp to 738 bp . Therefore, it could be said that Polh-cr represented about 65% of the full length of polyhedrin gene. On comparing nucleotide sequence of Polh-cr to all available sequences in the GenBank, it created a significant homology with 100 NPV and 11 GV genes. It showed 99% identity with S. littoralis polyhedrin gene (Acc# D01017), 95% with S. littura polyhedrin gene (Acc# AY552474) and 93% with S. littura polyhedrin genes (Acc# AY549963, AF325155, AF037262 and AF068189). In addition, it was 90% similar to Lymantria dispar polyhedrin genes (Acc# AF499687, AF081810 and M23176), 88% to Malacosoma neustria polyhedrin gene (Acc# X55658) and 87% to B. mori polyhedrin genes (Acc# M10043 and X63614). Furthermore, it showed 86% homology with S. litura and Amsacta albistriga polyhedrin genes (Acc# X94437 and AF118850, respectively) and 85% with S. exigua and Malacosoma neustria polyhedrin gene (Acc# AF169823; AY127899 and AJ277555, respectively). These results ensured that Polh-cr is a highly conserved region within polyhedrin gene of about one sixth of the known NPV species. Consequently, it could be used in many molecular techniques concerned with baculoviruses.
Knowing that the full length of polyhedrin protein from lepidopteran NPVs ranges from 161 a.a. (e.g. Hyphanteria cunea NPV, Acc# AAW49190 and Bombyx mori, Acc# ABB16300) to 249 a.a. (e.g. Spodoptera litura NPVs, Acc# AAZ78353, NP_258269 and AAS90121). In case of Spodoptera sp. NPV, full length of polyhedrin protein ranges from 170 a.a. (Acc# AAW49204) to 249 a.a. (Acc# AAZ78353) in comparison to Autographa sp. NPV which ranges from 169 a.a. (Acc# AAW63393) to 246 a.a. (Acc# AAA46736). The deduced amino acid sequence of Polh-cr was compared to other polyhedrins. Alignment results revealed that Polh-cr was 100% and 99% identical to S. littoralis polyhedrins (Acc# AAC33752, AAR04375; P24646, JU0382 and BAA00824, respectively). It was also 99% similar to S. litura polyhedrins (Acc# NP258269, AAC09246, AAL01689 and AAS58468). In addition, it has 98% identity with polyhedrin of S. littoralis (Acc# AA590121). Furthermore, it showed 89% similarity with polyhedrins of S. exigua and Ecotropis obligua polyhedrins (Acc# AAF33532, JQ1868, NP037761, 001586; AAB53632, AAQ88174, P07388 and AAA46739).
Using AcMNPV nucleotide sequence (Acc# M25054) and amino acid sequence (Acc# AAA46736) as references for comparison with our sequences, it was found that 57 different nucleotides and 12 gaps in nucleotide sequence resulted in 15 different amino acids in the putative polypeptide (I, N, V, V, E, D, S, T, A, T, S, D, K, Y and I a.a. from our putative polypeptide were replaced with V, S, I, I, D, E, V, N, S, H, E, N, R, E and V a.a. from AcMNPV putative polypeptide, respectively). Surprisingly, Polh-cr putative polypeptide was identical to SpliNPV polypeptides (Acc# AAW49208, BAA00824, AAT10182 and AAW49207) with one amino acid replacement (I with V at the position 133). These results suggested the possible difference in codon usage among the compared isolates. It might also give a specific property to our putative polypeptide.
Phylogenetic analyses of the Polh-cr nucleotide seuquence and its deduced polypeptide revealed that Polh-cr is genetically related to a large number of published nucleotide sequences and to a larger number of published amino acid sequences. This finding made it is preferred to develop kits that use viral protein (polyhedrin) in detecting NPVs because it will be wider-used than DNA method.