Assessing the efficacy of combined nucleopolyhedrovirus and granulovirus isolates against Spodoptera littoralis (Boisd.) (lepidoptera: Noctuidae): a laboratory-based investigation

The Egyptian cotton leafworm, Spodoptera littoralis , is a major pest of cotton and various crops worldwide. Baculoviruses, highly specific insect viruses, are significant from both an evolutionary and practical perspective when utilized as biocontrol agents. This study compares the virulence of two baculoviruses isolates: Spodoptera littoralis nucleopolyhedrovirus isolate AN1956 (SpliNPV-AN1956) and Spodoptera littoralis granulovirus isolate EG24 (SpliGV-EG24) in a single and mixed-infection toward first-instar larvae of S. littoralis . Here, we test the hypothesis that mixed infection with SpliNPV-AN1956 and SpliGV-EG24 may results in enhanced virulence, indicating a synergistic interaction compared to single-virus infections. Larval bioassays were conducted using insect diet mixed with specific virus concentration ranged from 10 3 to 5 × 10 5 OBs/mL, where neonate larvae were exposed to virus-treated diet for a defined period, with each treatment replicated three times to ensure statistical robustness. Mortality data were corrected using Abbott’s formula and subjected to probit regression analysis to estimate LC₅₀ values with corresponding 95% confidence intervals. The results showed that the LC₅₀ values were 2.03 × 10⁵ OBs/mL for SpliGV and 7.47 × 10⁴ OBs/mL for SpliNPV, demonstrating that SpliNPV was approximately 2.7-fold more potent than SpliGV in a single infection experiment. Upon mixed infection, the highest larval mortality (84.0%) was recorded in the GV + NPV H (high concentration) treatment and was significantly greater than that observed in the individual-virus treatments. This was followed by NPV H alone (70.1%), SpliNPV L (low concentration) mixed with SpliGV (51.4%), and then SpliGV alone (46.6%). While the lowest mean larval mortality occurred with the single application of NPV L (20.8%). However, comparison of observed and expected mortality indicated that the effect of mixed-virus treatments were additive interaction rather than synergistic effect (GV + NPV H : χ² = 0.297, P = 0.586; GV + NPVL: χ² = 0.385, P = 0.535). The combined application of SpliGV and SpliNPV increased larval mortality relative to single-virus treatments. However, this interaction was additive interaction rather than synergistic effect under the tested conditions. Hence, these findings may support the potential integration of mixed baculovirus formulations into pest management strategies targeting S. littoralis.