Cascading effects on bacterial communities: cattle grazing causes a shift in the microbiome of a herbivorous caterpillar

Large mammalian herbivores greatly influence the functioning of grassland ecosystems. Through plant consumption, excreta, and trampling, they modify biodiversity, nutrient cycling, and soil properties. Grazing mammals can also alter soil and rhizosphere bacterial communities, but their effect on the microbiome of other animals in the habitat (i.e., insects) is unknown. Using an experimental field approach and Illumina MiSeq 16S rRNA gene sequencing, we analyzed the influence of cattle grazing on the microbial community of spring webworm caterpillars, Ocnogyna loewii. Our experimental setup included replicated grazed and non-grazed paddocks from which caterpillars were collected twice (first-second and fourth-fifth instar). The caterpillars' microbiome is composed mostly of Proteobacteria and Firmicutes, and contains a potential symbiont from the genus Carnobacterium (55% of reads). We found that grazing significantly altered the microbiome composition of late instar caterpillars, probably through changes in diet (plant) composition and availability. Furthermore, the microbiome composition of early instar caterpillars significantly differed from late instar caterpillars in 221 OTUs (58 genera). Pseudomonas and Acinetobacter were dominant in early instars, while Carnobacterium and Acinetobacter were dominant in late instars. This study provides new ecological perspectives on the cascading effects mammalian herbivores may have on the microbiome of other animals in their shared habitat.