Composition and function of a bacterial community shed light on the pest status of Rhizoglyphus robini

The bulb mite Rhizoglyphus robini, a pest on Liliacean cultures, has previously been observed to occur mainly in fungus infected plant cultures and in choice tests preferred infected plants, when offered infected and uninfected onion seedlings. We tested the hypotheses that a) the mites feed on the fungus more than the host plant itself and are able to use fungal carbohydrates, and that b) the bacterial community associated with the mite plays the important role of chitin digestion to make the fungal carbohydrate and protein accessible to the mite. We could show that mite fecundity was indeed higher after fungal nutrition and that chitin was digested by mite homogenate, bacteria being responsible for chitinase production. We also described the bacterial community using Tab-encoded sequencing of 16S rRNA gene fragents (454 pyrosequencing)from the V4-V6 regions. Genomic DNA was extracted from pools (~250 individuals)of laboratorty-reared mites, originating from three field populations. Those were originally collected from mite-infested Alium, Lilium and Ruscus.The core-bacterial community of R. robini consisted of three dominant genera: Myroides (41.4%), Serratia (11.4%) and Alcaligenes (4.5%), of which Serratia and Alcaligenes are known chitinase producers.