Ecological Drivers of Microbiota Diversity in the Pharmacophagous Turnip Sawfly

Animals encounter diverse ecological factors that shape their microbiota. While plants often provide nutrition, some animals also utilise them for non-nutritional purposes, that is, pharmacophagy. Although both uses may influence herbivore microbiomes, the effects of pharmacophagy remain underexplored. We studied such effects in the holometabolous insect Athalia rosae, whose larvae feed on Brassicaceae plants, while adults take up nectar from Apiaceae and, additionally, clerodanoids (plant specialised metabolites) through pharmacophagy from Lamiaceae species. We examined how nutritional and non-nutritional plant use affect microbiota diversity, composition and predicted function. We also tested whether the microbiota of larval faeces can serve as a non-invasive proxy for larval microbiota and compared microbiota profiles across rearing environments (laboratory vs. wild) and specific life stages (i.e., larvae vs. adults). Microbiota composition was highly context-dependent, with indicator species analyses revealing treatment-specific amplicon sequence variants. Diet, starvation and clerodanoid access each resulted in distinct communities; faeces reliably reflected larval microbiota; and wild-caught adults harboured the most diverse communities. Predicted microbial functions varied with diet and pharmacophagy, particularly in pathways putatively linked to metabolite degradation, energy metabolism and detoxification. Together, our findings suggest that both nutritional and non-nutritional plant use shape intraspecific microbiota variation in insects.