Pollinators are of great importance in ecosystems and contribute about $136 billion annually to agricultural systems in China (Ouyang et al., 2019). Due to the intensification and expansion of agriculture, the diversity of pollinators has been declining rapidly for decades, which has led to the loss, fragmentation, and degradation of pollinator habitats and a severe decline in global pollination (Foley et al., 2011; Féon et al., 2013; Belsky and Joshi, 2019). In the past 50 years, the dependence of global agriculture on pollinators has increased by more than 300%, while the abundance of pollinators worldwide has continued to decline (Potts et al., 2016). Previous studies have shown that planting of pollen plants in pollinator habitats can significantly increase the number and species of pollinators (Jönsson et al., 2015; M’Gonigle et al., 2016; Warzecha et al., 2018; Williams and Lonsdorf, 2018; Nichols et al., 2019). Therefore, it is necessary to clarify how to choose the best plants for different pollinators.

The internal transcribed spacer II (ITS2) barcode based on metabarcoding method can identify pollen plant species from mixed pollen and shows great potential in the analysis of beebread ingredients (Pornon et al., 2016; De-Vere et al., 2017; Lucas et al., 2018). Compared with traditional palynology, the molecular method not only has a higher sensitivity for the identification of mixed pollen but also effectively reduces time and cost (Lang et al., 2018; Tremblay et al., 2019). The ITS2 region has proved to be a core barcode for pollen identification (Richardson et al., 2015; Gous et al., 2019; Tremblay et al., 2019). The barcode combination of rbcL, trnH-psbA, and ITS2 successfully identified the main pollen plants, Vitex negundo and Vitex quinata, in the beebread of Megachile strupigera by He et al. (2020). Therefore, DNA metabarcode not only helps to identify species and determine the relative abundance of pollen plants in beebread but is also used to construct pollination networks, monitor plant pathogens, assess species diversity, and predict biodiversity patterns (Huang et al., 2020).

To meet the pollination demand of economic crops, pollinators with commercial potential have been found and developed (Ryder et al., 2020). Bumblebees have been successfully used as pollinators for eggplant, tomato, peach, and pear (Wu et al., 2006). Megachile rotundata was used for the pollination of alfalfa (Xu et al., 2009). In orchards, the bee of genus Osmia was considered as efficient pollinators of some Rosaceae plants, such as apricot, plum, cherry, peach, pear, and apple (Parker, 1981; Torchio et al., 1987; Wei et al., 1991; Xu et al., 1994). Importantly, Osmia bee can be artificially stocked due to its advantages such as easy management, low cost, and high pollination efficiency (Wei and Zhao, 1995; Wei et al., 2000b; Dai, 2004). In north and northwest of China, there are mainly five species of Osmia (O. excavata, O. cornifrons, O. taurus, O. jacoti, and O. pedicornis) that play important roles in enhancing pollination, increasing fruit diameter and the number of seeds per fruit, and decreasing the percentage of asymmetrical fruit (Wei et al., 2002). For O. excavata, it has many advantages, such as low-temperature tolerance, long daily activity time, high flower-visiting efficiency, centralized pollination range, and easy management (Wei and Yuan, 1997). Therefore, it was considered as an excellent and large-scale stocking pollinator. The pollination of O. excavata can create more than $27.8, $9.4, and $3.0 billion for cherries, apples, and rapes planting, respectively, each year (Liu et al., 2018, 2019a,b). Pollination can significantly improve the fruit set rate and fruit quality, and it has broad application prospects in the future of fruit planting industry (He and Zhou, 2009).

However, the short flowering period of fruit trees limits the nutritional requirements of O. excavata during the foraging period (Yan et al., 2018). Because of this limitation, survival and large-scale stocking of O. excavata face great challenges. A previous study showed that the diversity of pollen plants is the main factor affecting the diversity of wild bee species (Kovács-Hostyánszki et al., 2017). Therefore, the effective solution is to grow other pollen plants in the orchard. This will not only improve the biodiversity of plantations but also provide adequate food sources for bees (Boyle et al., 2020). So, it is necessary to understand the foraging preferences of bees. In this study, beebread of O. excavata was collected by trap-nesting from 20 experimental plots in China. The species and abundance of plants were determined by ITS2 barcode based on metabarcoding method, and the dominant plants that meet the nutrition needs of bees were further analyzed.

Wild bees are important pollinators for economic crops and wild plants. They rely on different landscapes to obtain flowering resources and nesting habitats (Kratschmer et al., 2019). In this study, 20 experimental plots in China were set up to collect the beebread of O. excavata. A total of 26 pollen plant species were identified, belonging to 14 genera of nine families and including 17 economic crops and nine wild plant species (Robinia pseudoacacia, Deutzia discolor, J. interior, P. rhoeas, P. sinensis, Rosa canina, R. gymnocarpa, R. laevigata, and R. primula). Based on the composition and relative abundance of pollens (Figures 1, 2), O. excavata is an effective pollinator for economic crops (B. juncea, Brassica sp., M. pumila, Malus sp., P. brachypoda, and P. spinosa).

Previous studies indicated that the larvae of O. excavata released at the early flowering stages of economic crops will grow into effective pollinators, especially Rosaceae and Brassicaceae plants (Yang et al., 1997; Wei et al., 2000a; Hawkins et al., 2015; Gresty et al., 2018). However, the flowering period (7–15 days; Supplementary Table 5) (Lech et al., 2008; Liu et al., 2008, 2010; Wang et al., 2012; Lu et al., 2013; Lin et al., 2019; Zhang et al., 2019, 2020; Li et al., 2020) of most economic crops is short and cannot cover the entire adult stage of O. excavata (40–55 days; Wei et al., 2002; Men et al., 2018). Through the observation of professional beekeepers and our field work on the stocking of O. excavata, when O. excavata had no diverse food sources except economic crops, this gap seriously affects the bees to reserve beebread for their offspring, which causes a low recovery rate after artificial stocking. The number of reduced populations needs to be supplemented every year, excluding the situation of unnatural death, human activities, and population migration.

Although when the beebread from H5/H6 experimental plots were collected, the economic crops (pear and rape trees) in the plantation were in the late flowering stage, which led to the lack of pollen resources, O. excavata could still obtain pollens from wild plants (J. interior, R. gymnocarpa, R. laevigata, and R. primula; Figure 1). This means that in order to reduce the impact of the lack of pollen resources of economic crops, O. excavata can use the pollen of wild plants as their main source of nutrition for meeting survival needs. Plantations (orchards and farmlands) were dominated by a single type of economic crops and lack of flowering resources of wild plants (Figure 2A). A further analysis revealed that more wild plants in beebread were from SHNs (Figure 2A). Papanikolaou et al. (2017) found that increasing the richness of plants in the habitat would significantly affect the abundance and diversity of bees. When the pollen resources of economic crops cannot meet the needs of the entire adult stage of O. excavata, wild flowering plants can be planted in plantations or introduced into SNHs. The transplanting of wild flowering plants into the plantation helps to attract O. excavata and increase food sources. Ideal type of wild flowering plants can be selected from nine wild plants identified in beebread, especially J. interior, R. laevigata, and R. gymnocarpa.

Piko et al. (2021) concluded that the mixed flower field model can effectively provide attractive food resources, thereby greatly increasing the abundance and diversity of bumblebee. Bihaly et al. (2020) also emphasized that planting a variety of flowering herbaceous plants on a small scale could significantly increase the number of bee nests in the orchard. In response to concerns about the food source of the O. excavata, combing the previous results, a conjecture is proposed: a mixed flower field model suitable for O. excavata (MFF-OE model). We can plant a variety of favorite wild flowering plants around the plantation to meet the needs of pollen resources during the entire adult stage of O. excavata. However, the model needs to be further explored in terms of variety allocation, optimum planting area selection, and planting scale, etc. The comprehensive analysis of Chao1 and Shannon indexes reflected lower species richness and diversity in Liaoning. The Liaoning region can be used as a pilot for the exploration and implementation of the MFF-OE model. Taken together, we look forward that the implementation of the MFF-OE model can effectively solve the shortage of pollen resources of economic crops.

Osmia excavata has been successfully used for pollination in commercial orchards because it can significantly improve fruit setting rate and fruit quality. In this study, beebread of O. excavata was collected from various habitats by trap-nesting and analyzed by DNA metabarcoding technology. A total 26 pollen plants in 14 genera and nine families were identified, including 17 economic crops and nine wild plant species. The abundances of wild flowering plants in the plantation were lower than those in SNHs. According to the composition and abundance of pollen plants, O. excavata is regarded as an effective pollinator for economic crops in Rosaceae and Brassicaceae from the molecular level.

The sequence data that support the findings of this study are openly available in GenBank of NCBI at https://www.ncbi.nlm.nih.gov/. The associated BioProject, SRA, and BioSample numbers are PRJNA742556, SRR15011441, and SAMN19967841, respectively.

All experiments procedures for this study complied with the current animal ethics guidelines and did not involve any protected animals.

AL and DH were responsible for the conceptualization and supervision of the study. HL, FD, and YH were responsible for the methodology and writing – original draft preparation. HL, FD, YL, KZ, and HZ were responsible for the software. ZZ, CZ, AL, and DH performed the validation. HL and FD conducted the formal analysis and visualization. FD, YL, KZ, HZ, and DH conducted the investigation. FD and DH were responsible for the resources. HL, FD, YL, KZ, HZ, and DH were responsible for data curation. YH, ZZ, CZ, AL, and DH were responsible for writing – review and editing. DH was responsible for project administration. HL, ZZ, AL, and DH were responsible for funding acquisition. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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