Susceptibility of Red Mason Bee Larvae to Bacterial Threats Due to Microbiome Exchange with Imported Pollen Provisions

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.

Microbes make the meal: oligolectic bees require microbes within their host pollen to thrive

“…The microbial community within pollen provisions of solitary bees consist of symbiotic taxa that serve as conduits for nutrient transfer from pollen to larval biomass. This includes nutritional mutualists that are involved with pollen fermentation (Cohen et al ., 2020; Voulgari‐Kokota et al ., 2020), as well as other heterotrophic microbes that form a dominant source of dietary proteins and lipids for the developing bees (Dharampal et al ., 2019; Steffan et al ., 2019). Our results show that while pollen sterilization did not alter the net abundance of major dietary macromolecules, it would have eliminated the nutritional services derived from these pollen microbiota.…”

“…Given the high variability in the solitary bee microbiome (Keller et al ., 2013; McFrederick & Rehan, 2016; Cohen et al ., 2020), the pollen microbiota are more likely to experience frequent alterations to their community structure. Reshuffling of key members within this external microbial community can potentially disrupt the symbioses between microbes and oligolectic larvae, leading to unpredictable fitness outcomes for the developing bees (Voulgari‐Kokota et al ., 2019b, 2020).…”

“…consists of diverse environmentally acquired microbes, and is more likely to fluctuate based on location, foraging preferences, and nesting ecology (Keller et al ., 2013; Lozo et al ., 2015; McFrederick et al ., 2016; McFrederick & Rehan, 2016; Voulgari‐Kokota et al ., 2018, 2019a; Cohen et al ., 2020). Despite having lower taxonomic specificity, these microbes have a major influence in shaping the larval microbiome and determining bee fitness components (Voulgari‐Kokota et al ., 2019b, 2020). While comparing the microbial community associated with each pollen type fell beyond the scope of this study, further research is required to identify key microbial taxa responsible for ensuring the nutritional adequacy of host pollen types.…”

Linking bacterial diversity to floral identity in the bumble bee pollen basket

“…These genera include species that are known symbionts of the bee gut microbiome that are obtained during foraging and feeding (McFrederick et al., 2012, 2017). Acinetobacter is particularly well‐known from honey bee larvae in which it inhibits the growth of Paenibacillus larvae , the cause of American foulbrood in honey bees (Evans & Armstrong, 2006) and associated with brood mortality in the red mason bee, Osmia bicornis (Voulgari‐Kokota et al., 2020). Lactobacillus are ubiquitous members of the bee gut microbiome (Praet et al., 2018) having a functional role in fermentation and the production of lactic acid (McFrederick et al., 2018), which provides additional protection against the Paenibacillus larvae (Forsgren et al., 2010).…”

Pollen DNA metabarcoding and related methods in global change ecology: prospects, challenges, and progress

“…This accounts both for microbes with beneficial (Dharampal, Carlson, Currie, & Steffan, 2019;Vuong & McFrederick, 2019) and detrimental (A. Keller et al, 2018;Voulgari-Kokota, Steffan-Dewenter, & Keller, 2020) effects on host ecology, nutrition and health (Engel et al, 2016;Vannette, 2020;Voulgari-Kokota, Grimmer, Steffan-Dewenter, & Keller, 2018). Given these advantages of pollen-based networks and the high-throughput capabilities of genetic methods, a wide range of broad and fine-scale ecological questions may be answered.…”