Bacillus thuringiensis and nucleopolyhedrovirus combination against Spodoptera litura alters the cigar tobacco leaves microbiome during air curing

Introduction Spodoptera litura continues feeding on cigar tobacco leaves during air curing, causing substantial economic losses. This study evaluated the synergistic efficacy of a Bacillus thuringiensis (Bt) and nucleopolyhedrovirus (NPV) combination for controlling Spodoptera litura and investigated its effects on the bacterial and fungal microbiomes of cigar tobacco leaves during air curing. Methods Laboratory bioassays were conducted to determine the optimal Bt:NPV ratio and evaluate the synergistic toxicity. The selected formulation was subsequently validated under field conditions. Cigar tobacco leaves treated with the optimal Bt–NPV mixture were air-cured for 23 days, after which bacterial (16s rRNA) and fungal (ITS) communities were characterized using Illumina NovaSeq high-throughput sequencing. Results The Bt:NPV mixture at a 4:1 ratio exhibited the strongest synergistic activity against second-instar Spodoptera litura , with an LC50 of 0.64 ml L −1 and a co-toxicity coefficient of 153.8. Bt–NPV treatment significantly altered the leaf microbiomes by reducing microbial richness while increasing microbial diversity and evenness. Compared with the control, treated leaves showed increased relative abundances of Pantoea , Pseudomonas , and Eurotiales, together with reduced abundances of Staphylococcus and Cladosporiales. These findings demonstrate that Bt–NPV treatment effectively controlled Spodoptera litura while reshaping bacterial and fungal community composition during air curing. Discussion The synergistic Bt–NPV formulation represents an effective and environmentally friendly strategy for controlling Spodoptera litura during air curing while promoting an ecological framework that may support leaf preservation. However, the direct effects of Bt–NPV-induced microbiome changes on cigar tobacco quality, aroma development, and chemical maturation were not evaluated and should be investigated through integrated chemical, metabolomic, and sensory analyses in future studies.