| Summary: | (1) Background: The widespread use of insecticides has cause extensive resistance in German cockroach (<i>Blattella germanica</i>) populations globally. Biological control has the potential to mitigate insecticide resistance, and <i>Metarhizium anisopliae</i> (Meschn.) Sorokin, an entomopathogenic fungus, alone and in combination with various insecticides, has shown good effects against cockroaches. (2) Methods: This experiment compared the cumulative mortality after infecting <i>B. germanica</i> with <i>M. anisopliae</i> conidia by two routes, per os and topical application. To probe the mechanisms that underlie the synergism between <i>M. anisopliae</i> and hydramethylnon, we conducted dose–response assays with cockroaches fed combinations of <i>M. anisopliae</i> and hydramethylnon and characterized the gut microbiomes of the treated cockroaches. (3) Results: The study showed that the mortality with per os infection was lower than that with topical application. In addition, the combination of <i>M. anisopliae</i> and hydramethylnon had a synergistic effect in 16 treatments. The gut microbiome was also altered by hydramethylnon treatment. The abundance of <i>Parabacteroides</i> and <i>Enterococcus</i> declined with the hydramethylnon and combination treatments, which are known to have anti-inflammatory and antifungal activities. The abundance of <i>Alistipes</i>, which is a fungal cell wall component, significantly increased in these treatments. (4) Conclusions: Therefore, we speculate that the major mechanism underlying this synergism is hydramethylnon promoting the survival of <i>M. anisopliae</i> in the harsh gut environment and enhancing its virulence for German cockroaches by altering the gut microbiome. This may provide a method for the fight against <i>B. germanica</i> and lay the foundation for the development of new baits.
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