Bt Exposure-Induced Death of Dioryctria abietella (Lepidoptera: Pyralidae) Involvement in Alterations of Gene Expression and Enzyme Activity

<i>Dioryctria abietella</i> (Lepidoptera: Pyralidae) is a destructive forest pest for coniferous trees. <i>Bacillus thuringiensis</i> has been widely applied in forestry as a biological control agent to control it. However, the mechanisms of Bt-induced mortality in <i>D. abietella</i>, particularly its effects on gene expression and enzyme activities, remain unclear. Here, bioassay, enzyme assay, transcriptome sequencing, and gene expression profiling were employed to explore the relationship between the toxin-receptor, defense, and lethal mechanisms of <i>D. abietella</i> after Bt exposure. In a toxicity bioassay, <i>Bacillus thuringiensis galleriae 05041</i> strain (<i>Bt05041</i>) was the most toxic insecticide to the larvae of <i>D. abietella,</i> with LC₅₀ values of 3.15 × 10⁸ Colony-Forming Units (CFUs) mL^-1 at 72 h after treatment. Transcriptome analysis revealed that the gene expression patterns of <i>D. abietella</i> after 8 h of <i>Bt05041</i> exposure (Bt8) varied considerably from the <i>Bt05041</i>-treated for 2 h group (Bt2). In the Bt2 group, differentially expressed genes were significantly enriched in cellular and bioenergy pathways of lysosome, insulin signaling, cGMP-PKG signaling, etc. Immune-related pathways were activated, namely cAMP, AMPK, MAPK, Rap1, IMD, and Toll pathways. Meanwhile, Bt8 treatment caused metabolic changes in basic substances such as amino acids, glucose, nucleic acids, and fatty acids. <i>Bt05041</i> exposure activated the activities of defense enzymes and induced gene expression changes in <i>D. abietella</i> larvae. Among them, most Bt-receptor genes had higher expression levels than defense enzyme genes. Overall, these findings reveal a possible mechanism underlying Bt-mediated death in <i>D. abietella</i> larvae. This work provides valuable information in terms of biological control strategies.