Synergistic Control of Blattella germanica by Beta-Cypermethrin and Metarhizium anisopliae: Disruption of Gut Microbiota, Histopathology, and Detoxification Systems

The Blattella germanica L. (Blattodea: Ectobiidae) is a major urban pest with widespread insecticide resistance. This study evaluated the synergistic effect of combining Metarhizium anisopliae with beta-cypermethrin (β-CYP) and investigated its mechanisms. Compatibility assays showed that β-CYP (1–10 μg/mL) was highly compatible with M. anisopliae (1 × 107–1 × 109 cfu/mL), with enhanced compatibility at lower concentrations. Bioassays confirmed significant synergy, increasing mortality and shortening median lethal time. Mechanistically, the combination accelerated histopathological damage to the midgut, hindgut, Malpighian tubules, and fat body, while promoting fungal proliferation in hemolymph and gut tissues. 16S rRNA sequencing revealed gut microbiota dysbiosis: Firmicutes replaced Bacteroidetes as the dominant phylum, opportunistic pathogens (Weissella, Alistipes) increased, and beneficial bacteria (Enterococcus, Fusobacterium) decreased. Weissella recolonization experiments confirmed its mortality-enhancing role. Additionally, the combination suppressed carboxylesterase activity and modulated immune gene expression (CYP4G19, BgPo), compromising host defense. Collectively, these findings demonstrate that β-CYP and M. anisopliae exert synergistic effects through multi-target mechanisms, offering a promising integrated pest management strategy that reduces chemical usage while enhancing biocontrol efficacy.