A novel tank-mix formulation increases the efficacy of alphabaculoviruses on different phylloplanes

Abstract

Spodoptera littoralis Boisduval (Lepidoptera: Noctuidae) and Spodoptera exigua Hübner (Lepidoptera: Noctuidae) pose substantial threats to many crops, necessitating the exploration of biopesticides as potential chemical alternatives. One alternative is baculoviruses; however, their instability in the field has hindered their widespread use. Host plant phylloplane affects baculovirus activity at varying levels in different host plants. Formulation contributes significantly to optimizing the baculoviral stability on different phylloplanes against environmental conditions; however, it is expensive and difficult to make in developing or nondeveloped countries. In the current study, we developed a simple tank-mix application (MBF-Tm5) for immediate use, resembling the characteristics of a suspension concentrate formulation for Spodoptera littoralis nucleopolyhedrovirus (SpliNPV) and Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV). We examined their biological activity against 2nd instar larvae first on an artificial diet under laboratory conditions and on eggplant and pepper phylloplane in greenhouse conditions compared to plain viruses. This formulation exhibited no significant improvement in the biological activity of both viruses on an artificial diet under laboratory conditions but significantly improved the biological activity of both viruses on both plants under greenhouse conditions. The original activity remaining (OAR%) of both unformulated and formulated viruses decreased over time under greenhouse conditions; however, the OAR value of both viruses on eggplants was significantly higher than on pepper plants. Overall, the tank-mix simple formulation of baculoviruses might be a great alternative for improved stability in nature, providing better control.

Keywords: Spodoptera; baculovirus; biological activity; phylloplane; tank-mix formulation.

Fig. 1.

Eggplant and pepper leaves at adaxial and abaxial surfaces.

Fig. 2.

The original activity remaining (OAR%) of SpliNPV and SeMNPV both at a concentration of 1 × 10⁷ OB/ml as unformulated suspension (UFS) or in combination with tank mix (MBF-Tm5) formulation against second-instar larvae of S. littoralis and S. exigua, respectively, over the course of exposure days to the phylloplane of eggplant (Ep) and pepper (P) plants. Values bearing different uppercase letters were significantly different among exposure days for a given treatment (SpliNPV MBF-Tm5 on eggplant: F_{(5, 12)} = 1008.9, P < 0.01; on pepper: F_{(5, 12)} = 1755.9, P < 0.01; UFS on eggplant: F_{(5, 12)} = 2101.9, P < 0.01; on pepper: F_{(5, 12)} = 2781.0, P < 0.01; SeMNPV MBF-Tm5 on eggplant: F_{(5, 12)} = 1462.9, P < 0.01; on pepper: F_{(5, 12)} = 2052.5, P < 0.01; UFS on eggplant: F_{(5, 12)} = 2679.6, P < 0.01; on pepper: F_{(5, 12)} = 3079.2, P < 0.01) and values bearing different lowercase letters were significantly different between treatments for a given exposure day (1 DAEX SpliNPV: F_{(3, 8)} = 55.6, P < 0.01; SeMNPV: F_{(3, 8)} = 56.6, P < 0.01; 2 DAEX SpliNPV: F_{(3, 8)} = 374.2, P < 0.01; SeMNPV: F_{(3, 8)} = 331.6, P < 0.01; 3 DAEX SpliNPV: F_{(3, 8)} = 450.7, P < 0.01; SeMNPV: F_{(3, 8)} = 484.6, P < 0.01; 4 DAEX SpliNPV: F_{(3, 8)} = 966.8, P < 0.01; SeMNPV: F_{(3, 8)} = 820.5, P < 0.01; 5 DAEX SpliNPV: F_{(3, 8)} = 1342.1, P < 0.01; SeMNPV: F_{(3, 8)} = 1531.2, P < 0.01; 6 DAEX SpliNPV: F_{(3, 8)} = 1744.1, P < 0.01; SeMNPV: F_{(3, 8)} = 1254.0, P < 0.01) using Tukey’s HSD test (P < 0.05). The OAR value, remaining consistently at 100% on day 0, was therefore not included.