Identification of Serratia marcescens isolated from Antheraea pernyi eggs and determination of bacterial pathogenicity and transmission pathway.

The Chinese oak silkworm, Antheraea pernyi, is an important alternate host for mass production of Trichogramma parasitoids, which play a vital role in the biological control of lepidopterous pests in China. The quality of A. pernyi eggs is particularly important for mass production of these parasitoids. In this study, a pathogenic bacterium, isolated and purified from A. pernyi eggs that had turned gray in color, was identified as Serratia marcescens. We used morphology, biochemistry and 16S rDNA analysis to characterize the strain, which was named "APE strain". Serratia marcescens APE strain was determined to be the causal bacterium associated with the disease in the eggs, verified by a test based on Koch's Postulates. We tested the pathogenicity of S. marcescens APE strain on A. pernyi eggs; the percentage of diseased (gray) eggs reached 57.78% when uninfected eggs were exposed to a concentration of 1 × 109 cfu/mL bacterial suspension for 7 h. S. marcescens was transmitted mechanically by Trichogramma parasitoids. The transmission rate was 25.56%. In a horizontal transmission test, the highest percentage of uninfected eggs that developed infections was 51.43% after being treated with contents of diseased eggs for 12 h. In a vertical transmission test, the number of infected eggs per treated adult female was 63.8-92.3 after treatment with different S. marcescens concentrations, significantly lower than the 304.3 eggs per female in the control group. Furthermore, the percentage of infected (gray) eggs produced by adult moths exposed to bacteria in the larval stage was 80.5-85.3%.