Environmentally acquired gut-associated bacteria are not critical for growth and survival in a solitary bee, Megachile rotundata

Abstract Background Social bees have been studied extensively for their gut microbial symbiosis and its beneficial functions to the host’s health. But the significance of the environmentally acquired, non-symbiotic gut microbiota in solitary bees remain inconclusive. Solitary bee, Megachile rotundata females provision their offspring with pollen from ​​​various ​plant species harboring diverse microbes that colonizes larval gut. The Apilactobacillus is the most abundant microbe, but evidence regarding the effects of Apilactobacillus and ​other provision microbes on growth and survival ​are ​lacking. We ​hypothesized that the presence of intact microbial communities especially, Apilactobacillus would enhance larval and prepupal development, weight, and survival. Conversely, the absence of intact microbial communities was expected to have a negative impact on bee fitness. We reared larvae on pollen provisions with natural microbial communities (Natural pollen) or without microbes (Sterile pollen). We also assessed the impact of Apilactobacillus micheneri by adding it to both types of pollen provisions. Results Feeding larvae with sterile pollen + A. micheneri led to the highest mortality. Larval development was significantly delayed in groups fed with sterile pollen and sterile pollen + A. micheneri compared to the control. Interestingly, larval and prepupal weights did not significantly differ when they were given sterile pollen, sterile pollen along with A. micheneri , and natural pollen along with A. micheneri , in comparison to those fed only natural pollen. 16S rRNA gene sequencing found a dominance of endosymbiont Sodalis when A. micheneri was introduced to natural pollen. Conclusion This study highlights that reliance on non-host specific, environmental bacteria do not shape the fitness of M. rotundata and may hold true for other solitary bees. Apilactobacillus micheneri is not necessarily a beneficial microbe, and is harmful if present in abundance without other microbes. The presence of Sodalis with abundant A. michene ri suggests potential crosstalk between both, shaping bee nutrition and health. This perspective will enable us to gain a deeper understanding of the significance of environmentally acquired microbiomes for the survival, growth, and development of other solitary bees.