Intestinal microbiome interactions influence Metarhizium-based biocontrol efficacy against the sugar beet weevil.

The sugar beet weevil is considered one of the most economically important insect pests in sugar beet cultivation. A promising biological control strategy involves the natural interaction between entomopathogenic fungi and arthropods. The successful application of M. brunneum as part of integrated biological control strategies against the sugar beet weevil has already been demonstrated resulting in lethal mycosis. However, the efficacy of this strain is affected by multiple factors. The intestinal microbiome of insects harbours beneficial microbes that possess various functions, such as defence mechanisms against insect-pathogens. Thus, investigating intestinal microbial interactions in combination with Metarhizium-application could reveal microbes that modulate susceptibility to pathogens. This study investigated whether intestinal microbial interactions influence mycosis caused by M. brunneum and M. robertsii. We analysed the intestinal microbiome of both treated and untreated sugar beet weevils, distinguishing between mycotic and non-mycotic individuals at the time of death. Notably, Pantoea and Enterobacter were significantly associated with mycotic individuals and may act as a potential antagonist to Metarhizium. In contrast, healthy individuals harboured diverse microbial communities that may provide a protective barrier against entomopathogens. However, the intestinal microbiome of non-mycotic specimens also comprised genera with presumed insecticidal properties, including Serratia, Penicillium and Cladosporium. The last two were also observed in the intestines of male individuals, which were generally at a higher risk of mortality. Further investigation is needed to confirm their insecticidal potential in the sugar beet weevil. A combined application could improve the efficacy of Metarhizium-based biocontrol, contributing to more sustainable pest management strategies.