# Serratia marcescens Is Associated with Larval Mortality and Gut Dysbiosis in the Hornet Vespa analis

## TL;DR

A red bacterium, Serratia marcescens, is linked to high larval mortality and gut imbalances in hornets raised in China, offering insights for better farming practices.

## Contribution

Identifies Serratia marcescens as a key pathogen in Vespa analis larvae disease outbreaks and reveals gut dysbiosis as a consequence.

## Key findings

- Serratia marcescens isolated from sick larvae caused similar symptoms when fed to healthy hornet colonies.
- Infected larvae showed gut dysbiosis with reduced microbial diversity and increased Enterobacterales.
- Healthy larvae had gut communities dominated by Proteobacteria.

## Abstract

Hornets are usually known as dangerous or invasive insects, but in some parts of China, they are also reared for food and traditional medicine and can provide important income for local farmers. In indoor farms in Dehong, Yunnan, outbreaks of disease in larvae of the hornet Vespa analis have caused yellow bodies, weakness, larvae falling from the nests, and high mortality, yet the cause was unknown. In this study, we used microbiological methods and feeding experiments to identify a candidate bacterial factor associated with outbreaks. We isolated a red bacterium from the guts of sick larvae, identified it as Serratia marcescens, and showed that feeding this bacterium to healthy colonies produced similar signs and deaths. We also found that the normal gut microbes became disturbed during infection. These results show that S. marcescens is a key factor in larval disease in V. analis and provide a scientific basis for better hygiene, pathogen monitoring, and health management in hornet rearing.

Social wasps, including hornets, are increasingly recognized not only as invasive pests but also as farmed insects; however, their gut microbiota and associated diseases remain poorly characterized. In indoor rearing facilities for the hornet Vespa analis in Dehong, Yunnan, China, we observed recurrent larval disease with weakness, larvae falling from the nests, and high mortality. To identify the causative agent and its effects on the gut community, we isolated bacteria from diseased larvae, characterized them by morphology, biochemical tests, and 16S rRNA gene sequencing, and then established an oral infection model. A red-pigmented isolate, designated YR2, was identified as Serratia marcescens. Oral inoculation with YR2 reproduced disease signs and significantly increased larval mortality, and a phenotypically consistent S. marcescens isolate was reisolated from infected larval guts. Amplicon sequencing showed that healthy larvae harbored gut communities dominated by Proteobacteria, whereas infection was associated with reduced diversity and a dysbiotic shift with enrichment of Enterobacterales. Our results support S. marcescens as a strong candidate pathogen associated with larval disease and mortality in Vespa analis under indoor-rearing conditions. Our findings provide a basis for pathogen surveillance and microbiota management in indoor hornet husbandry, and support improved biosecurity and health monitoring practices.