## 1 Introduction  
Wild bees are an economically and ecologically important group of insects because of their ability to participate in plant pollination in a variety of ecosystems. In some cases, the pollination services of wild bee cannot be substituted by domesticated bees [1, 2, 3]. However, related research on honeybees has shown that the size of bee populations may be affected by microbiomes [4]. The symbiotic microorganisms have different effects on the health of the host, and have been extensively studied [5].  
Gut microbes in honeybees can promote growth, increase food consumption, and stimulate the immune system [6, 7]. Consequently, imbalance of bacterial communities may result in the occurrence of diseases [8]. Studies have demonstrated that the gut of social bees contains a highly conserved and well-defined core microbial community, usually composed of about 10 species [9, 10]. However, wild solitary bees, such as megachilid bees, have not conserved a bacterial community because their bacterial communities are influenced by the surroundings [11]. Nevertheless, a previous study showed that solitary bees had relatively stable and interrelated bacterial communities in their guts, such as Acetobacteraceae, Bacillaceae, Lactobacillusae, and others [12].  
The spread of symbiotic bacteria depends on bee contact among social bees, while that of solitary bees depends mainly on the environment, brood provisions and nestingmaterials introduced by their parents, such as _Osmia bicornis_ in the nest soil, _Megachile_ sp. on plant leaves, and _Osmia careulscens_ in the chewed plant tissues [13; 14]. Similar microbial communities have been identified in the gut and brood provision of _Osmia bicornis_, _Osmia caerulescens_, _Megachile rotundata_, and _Megachile versicolor_[15], which indicates that the microbes in the gut of bee larvae are strongly associated with flowers and brood provision. Therefore, environmental factors can cause a high diversity of bacteria in solitary bee nests [16].  
_Osmia excavata_ Alfken is a wild solitary bee. Males and females overwinter in the nest and mate in spring. Their new nests are built by females with dead and broken plant stems. The next generation completes its development from larva to adult in the nest tube [17]. The activities of _O. excavata_, such as nesting and visiting flowers, are mainly affected by various environmental factors, such as temperature, sunshine, wind speed. The higher the temperature, the higher the rate of bees visiting flowers and nesting. However, in cloudy or rainy and windy weather, the activity frequency and nesting rate of adult bees decreases significantly [18]. _Osmia excavation_ is widely distributed in northern China (including Liaoning, Shandong, Henan, Hebei, Shaanxi, Shanxi, Jiangsu) and are used as a pollinator for apples, pears, rape and other crops, and have been artificially released in China [19; 20]. Currently, studies on _O. excavata_ focused on their biological properties and pollination [21], whereas the bacterial communities related to its growth and development have not been studied.  
In this study, the structure and diversity of bacteria in the larval gut, brood provision, and nest soil of _O. excavata_ were analyzed through full-length 16S rRNA sequencing and correlations between different experimental groups were compared. This study showed that the bacterial communities in the gut of _O. excavata_ larvae were similar in different locations, which may be due to the similarity of their habitats. The purpose of this study was to preliminarily investigate the effects of brood provision and nesting soil on the intestinal bacterial community of larvae, in order to make the larvae grow and develop more healthily so that bees can better exert their pollination values, and to lay a scientific foundation for the subsequent exploration of the related microorganisms of wild pollinator bees such as _O. excavata_.