Cd exposure-triggered susceptibility to Bacillus thuringiensis in Lymantria dispar involves in gut microbiota dysbiosis and hemolymph metabolic disorder.

The immunotoxicity induced by heavy metals on herbivorous insects reflect the alterations of the susceptibility to entomopathogenic agents in herbivorous insects exposed to heavy metal. In the present study, the susceptibility of gypsy moth larvae to Bacillus thuringiensis under Cd treatment at low and high dosages was investigated, and the gut microbiome-hemolymph metabolome responses that affected larval disease susceptibility caused by Cd exposure were examined. Our results showed that mortality of gypsy moth larvae caused by B. thuringiensis was significantly higher in larvae pre-exposed to Cd stress, and there was a synergistic effect between Cd pre-exposure and bacterial infection. Exposure to Cd significantly decreased the abundance of several probiotics (e.g., Serratia for the low Cd dosage and Weissella, Aeroonas, and Serratia for the high Cd dosage) and increased the abundances of several pathogenic bacteria (e.g., Stenotrophomonas, Gardnerella, and Cutibacterium for the low Cd dosage and Pluralibacter and Tsukamurella for the high Cd dosage) compared to the controls. Moreover, metabolomics analysis indicated that amino acid biosynthesis and metabolism were significantly perturbed in larval hemolymph under Cd exposure at both the low and high dosages. Correlation analysis demonstrated that several altered metabolites in larval hemolymph were significantly correlated with changes in the gut microbial community. The results demonstrate that prior exposure to Cd increases the susceptibility of gypsy moth larvae to B. thuringiensis in a synergistic fashion due to gut microbiota dysbiosis and hemolymph metabolic disorder, and thus microbial-based biological control may be the best pest control strategy in heavy metal-polluted areas.