RNA sequencing (RNA-Seq) is a powerful tool that has been used to discover novel RNA viruses and to detect pathogenic viruses [19, 46, 47]. Many studies focus on the viromics of honey bee using sequencing methods, and this has greatly improved our understanding of bee viruses [16, 22, 48–50]. Compared to honey bee, there are few virus studies on non-Apis insect pollinators, although their survival is under similar threat as honey bee [6, 31, 51, 52]. By using RNA-Seq technology, we preliminarily explored the viral composition of non-Apis pollinators and their predators.
Honey bee pathogenic viruses spread to non-Apis wild insect pollinators, and at least seven viruses (DWV, BQCV, SBV, IAPV, ABPV, SBPV, and CBPV) were detected in non-Apis insect pollinators. DWV and BQCV were able to replicate their genomes in bumble bees [29, 31, 45]. DWV, ABPV, and CBPV were detected in our sequencing data, and our samples did not include A. cerana, A. mellifera, or other Apis species, indicating that the three honey bee pathogenic viruses were detected in non-Apis wild insect pollinators. Our results were consistent with previous reports. Spread of honey bee viruses to other pollinators should be noticed, especially when bees are used to provide commercial pollination services . There is no doubt that commercial pollination services have created great value for agricultural production [54–56], but improving virus detection is an effective means to prevent the transmission of viruses from commercial insect pollinators to wild insect pollinators.
Five previously reported insect viruses (host in parentheses) were found in our samples. Mayfield virus 1 (B. terrestris), V. velutina associated acypi-like virus (V. velutina nigrithorax), Scaldis River bee virus (O. cornuta), Hubei diptera virus 6 (Diptera), and Arboretum almendravirus (P. albigenu) have been previously reported in other places and times [20, 57–59], and these viruses were also detected in our samples, indicating that they may be prevalent in the corresponding host and may exhibit pathogenicity.
We detected 26 novel RNA viruses by assembling sequencing data. Bee-infecting viruses are primarily positive-sense ssRNA (+ ssRNA) viruses of the order Picornavirales [27, 60]. Seven of these novel viruses were classified in the order Picornavirales, suggesting that viruses of Picornavirales also infect non-Apis insect pollinators. Viruses in the families Rhabdoviridae, Flaviviridae, Orthomyxoviridae, and Sinhaliviridae can infect honey bee [16, 21]. Novel viruses belonging to these families were detected in this study, suggesting that non-Apis insect pollinators and their predators have similar virus classes as honey bee. But the difference is that five rhabdoviruses were detected in this study and only two rhabdoviruses have been reported in honey bee. This clearly infers that rhabdoviruses are more common in non-Apis pollinators and predators. Virgaviridae consists of plant viruses , and three novel viruses from this family were detected in this study. We think that these three viruses may have come from the contamination of plants. Due to the complex living environment of insect pollinators, we cannot exclude novel plant, fungal, and bacterial viruses. Seven novel viruses (i.e., XPLV2, XPLV4, XPLV5, XPLV1, XNLV, XIV, and XOLV) showed amino acid similarity to honey bee pathogenic viruses (i.e., DWV, SBPV, SBV, LSV, CBPV, and VOV-1). Five of these could replicate their genomes in the corresponding host, suggesting that they are pathogenic to the corresponding host. In addition, the hosts of these viruses closely interact with honey bee in the same ecosystem, suggesting that they are a potential threat to bees, just as host honey bees may also spread the virus to non-Apis insect pollinators .
The decline in insect pollinators is simultaneously driven by several factors and could act synergistically [5, 62]. The interaction of multiple factors makes it more difficult to understand the reasons for the decline in insect pollinators. Some insecticides can exert additive or synergistic effects on virus-induced mortality and replication in honey bees by affecting their immune system . Diet and virus infection are related in honey bee. High-quality diet has the potential to reduce mortality in the face of infection with IAPV . Base on this study, we can see that the threat of the virus to wild pollinator insects may exceed our expectations. Not only the spread of bee virus threatens them, but also the virus carried by them may pose a greater threaten. virus research on insect pollinators mainly focuses on bees, but research on non-Apis virus host species and have same habitat as them is scarce.