Combined Analysis of Metabolomics and Transcriptome Revealed the Effect of Bacillus thuringiensis on the 5th Instar Larvae of Dendrolimus kikuchii Matsumura

<i>Dendrolimus kikuchii Matsumura</i> (<i>D. kikuchii</i>) is a serious pest of coniferous trees. <i>Bacillus thuringiensis</i> (<i>Bt</i>) has been widely studied and applied as a biological control agent for a variety of pests. Here, we found that the mortality rate of <i>D. kikuchii</i> larvae after being fed <i>Bt</i> reached 95.33% at 24 h; the midgut membrane tissue was ulcerated and liquefied, the MDA content in the midgut tissue decreased and the SOD, CAT and GPx enzyme activities increased, indicating that <i>Bt</i> has toxic effects on <i>D. kikuchii</i> larvae. In addition, transmission electron microscopy showed that <i>Bt</i> infection caused severe deformation of the nucleus of the midgut tissue of <i>D. kikuchii</i> larvae, vacuoles in the nucleolus, swelling and shedding of microvilli, severe degradation of mitochondria and endoplasmic reticulum and decreased number. Surprisingly, metabolomics and transcriptome association analysis revealed that four metabolic-related signaling pathways, Nicotinate and nicotinamide metabolism, Longevity regulating pathway-worm, Vitamin digestion and absorption and Lysine degradation, were co-annotated in larvae. More surprisingly, Niacinamide was a common differential metabolite in the first three signaling pathways, and both Niacinamide and L-2-Aminoadipic acid were reduced. The differentially expressed genes involved in the four signaling pathways, including <i>NNT</i>, <i>ALDH</i>, <i>PNLIP</i>, <i>SETMAR</i>, <i>GST</i> and <i>RNASEK</i>, were significantly down-regulated, but only <i>SLC23A1</i> gene expression was up-regulated. Our results illustrate the effects of <i>Bt</i> on the 5th instar larvae of <i>D. kikuchii</i> at the tissue, cell and molecular levels, and provide theoretical support for the study of <i>Bt</i> as a new biological control agent for <i>D. kikuchii</i>.