Synergism between Hydramethylnon and Metarhizium anisopliae and Their Influence on the Gut Microbiome of Blattella germanica (L.)

SIMPLE SUMMARY: The widespread use of insecticides has cause extensive resistance in German cockroach (Blattella germanica) populations globally. Biological control has the potential to mitigate insecticide resistance, and Metarhizium anisopliae, an entomopathogenic fungus, alone and in combination with various insecticides has shown good effects against cockroaches. This experiment compared the cumulative mortality after infecting B. germanica with M. anisopliae conidia by per os infection and topical dorsal infection. To probe the mechanisms that underlie the synergism between M. anisopliae and hydramethylnon, we conducted dose-response assays with cockroaches fed combinations of them and characterized the gut microbiome of treated cockroaches. The results showed that the mortality of per os infection was lower than that of topical dorsal infection. In addition, the combination of M. anisopliae and hydramethylnon had a synergistic effect. The gut microbiome was also altered by hydramethylnon treatment. Therefore, we speculate that one of the mechanism underlying this synergism is that hydramethylnon promotes the survival of M. anisopliae in the harsh gut environment and enhances its virulence on German cockroaches by altering the gut microbiome. This may help to develop new types of bio-control glue baits for the control of cockroaches. ABSTRACT: (1) Background: The widespread use of insecticides has cause extensive resistance in German cockroach (Blattella germanica) populations globally. Biological control has the potential to mitigate insecticide resistance, and Metarhizium anisopliae (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 B. germanica with M. anisopliae conidia by two routes, per os and topical application. To probe the mechanisms that underlie the synergism between M. anisopliae and hydramethylnon, we conducted dose–response assays with cockroaches fed combinations of M. anisopliae 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 M. anisopliae and hydramethylnon had a synergistic effect in 16 treatments. The gut microbiome was also altered by hydramethylnon treatment. The abundance of Parabacteroides and Enterococcus declined with the hydramethylnon and combination treatments, which are known to have anti-inflammatory and antifungal activities. The abundance of Alistipes, 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 M. anisopliae 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 B. germanica and lay the foundation for the development of new baits.