Abstract
Solitary bees are subject to a variety of pressures that cause severe population declines. Currently, habitat loss, temperature shifts, agrochemical exposure, and new parasites are identified as major threats. However, knowledge about detrimental bacteria is scarce, although they may disturb natural microbiomes, disturb nest environments, or harm the larvae directly. To address this gap, we investigated 12 Osmia bicornis nests with deceased larvae and 31 nests with healthy larvae from the same localities in a 16S ribosomal RNA (rRNA) gene metabarcoding study. We sampled larvae, pollen provisions, and nest material and then contrasted bacterial community composition and diversity in healthy and deceased nests. Microbiomes of pollen provisions and larvae showed similarities for healthy larvae, whilst this was not the case for deceased individuals. We identified three bacterial taxa assigned to Paenibacillus sp. (closely related to P. pabuli/amylolyticus/xylanexedens), Sporosarcina sp., and Bacillus sp. as indicative for bacterial communities of deceased larvae, as well as Lactobacillus for corresponding pollen provisions. Furthermore, we performed a provisioning experiment, where we fed larvae with untreated and sterilized pollens, as well as sterilized pollens inoculated with a Bacillus sp. isolate from a deceased larva. Untreated larval microbiomes were consistent with that of the pollen provided. Sterilized pollen alone did not lead to acute mortality, while no microbiome was recoverable from the larvae. In the inoculation treatment, we observed that larval microbiomes were dominated by the seeded bacterium, which resulted in enhanced mortality. These results support that larval microbiomes are strongly determined by the pollen provisions. Further, they underline the need for further investigation of the impact of detrimental bacterial acquired via pollens and potential buffering by a diverse pollen provision microbiome in solitary bees.
Highlights
Solitary bees are providers of invaluable ecological services [1,2], and increasing demand for pollination services results in their health being at the center of agro-economic research [3]
Pollen provisions, and soil from 43 O. bicornis nests by 16S ribosomal RNA (rRNA) gene metabarcoding
We identified strong changes in bacterial microbiome diversity, composition, and variability when comparing healthy with deceased red mason bee larvae and their pollen provisions
Summary
Solitary bees are providers of invaluable ecological services [1,2], and increasing demand for pollination services results in their health being at the center of agro-economic research [3]. Most studies on possible threats against bees focused on perils caused by increasing land-use intensity, Insects 2020, 11, 373; doi:10.3390/insects11060373 www.mdpi.com/journal/insects. Studies on honey bees revealed bacterial agents which can cause acute mortality in the hive [10]. Paenibacillus larvae [11] and Melissococcus plutonius [12] are widely accepted as the main causative pathogens for American and European. Studies on regional scales report that pathogens affect the abundance of bumble bees. Common reported pathogens of Bombus are trypanosomes and microsporidia, which are transmitted horizontally between bumble bee colonies and vertically within colonies [15,16].
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