3. RESULTS
The three parasite groups were detected in both the bumblebee species with evident differences in terms of infection rate. Infection ofMicrosporidia occurred in similar rates in B. pascuorumand B. terrestris (42.71 % in B. pascuorum and 46.88% inB. terrestris ), while different patterns of infections were detected in the case of trypanosomatid (8.33 % in B. pascuorumand 46.88% in B. terrestris ) and neogregarines (1.04 % inB. pascuorum and 15.63% in B. terrestris ).
The combination of coinfection rates highlighted different coinfection patterns in the two bumblebee species (Table 2), with B. terrestris hosting a higher parasite richness compared to B. pascuorum . In particular, B, pascuorum showed a 51.04% rate of carrying 0 infections, while B. terrestris demonstrated a much lower probability (21.88%). Furthermore, 78.13% of B. terrestris carried at least one parasite, while in B. pascuorumit was 48.96%.
[ TABLE 2]
The two bumblebee species responded differently to the factors describing the urbanization phenomenon. The probability of infection significantly increased in response to floral abundance (Figure 2a, Table 3) and decreased in response to ENN (Figure 2b, Table 3) inB. pascuorum. All the other predictor variables (i.e. , percentage of green habitat, distance from hives, floral diversity and the number of hives in the surrounding of sites) did not show significant effects on parasite richness (Table 3). However, none influenced B.terrestris’ probability of infection, as it was not influenced by the evaluated predictor variables.
[ FIGURE 2]
Parasite richness was lower in B. pascuorum, with a maximum of 2 target parasites per sample, compared to B. terrestris whose maximum 3 target parasites. Nevertheless, local variables such as the floral abundance and the distance from honeybee hives significantly shaped parasite richness. Specifically, the parasite richness increased in response to floral abundance in B. pascuorum (Figure 3a, Table 3) and distance from honeybee hives in B. terrestris (Figure 3b, Table3). All the other predictor variables (i.e percentage of green habitat, ENN, floral diversity and the number of hives in the surrounding of sites) did not show significant effects on parasite richness (Table 3).
[ FIGURE 3]
[ TABLE 3]
4. DISCUSSION
In this study we explored urban pollinator-parasite interactions focusing on the relationships between parasites’ incidence and coinfection rate with the green habitat availability and fragmentation, floral resources availability, and the proximity to beehives.
Our results showed cleardifferences in the infection and co-infection rate in the two bumblebee species, with B. pascuorum emerging as less prone to host parasites compared to B. terrestris. These differences are largely due to trypanosomatids (Crithidia spp.) and neogregarines (Apicystis spp.) that were detected with higher rates in B.terrestris. Moreover, coinfections due to two or more target parasites were extremely rare in B. pascuorum but relatively common in B. terrestris. A higher parasite prevalence in B. terrestris compared to B. pascuorum and other congenerics has been previously reported (Cameron et al., 2011; Goulson et al., 2012; Jabal-Uriel et al., 2017) but a reliable explanation of these interspecific differences has not been provided yet. In this context, a number of factors could be involved and constitute valid research questions to be further addressed. The first factor could be that the lower occurrence of infected individuals in B. pascuorummay suggest a lack of tolerance of this species towards infections that could significantly reduce the survival of infected individuals and thus the possibility to collect and analyze infected specimens (as we collected only living individuals in this study). On the other hand,B. pascuorum may be particularly resistant towards parasite contamination due to morphological, physiological or ecological aspects (e.g., its nesting and foraging habits). Large body size and foraging breath are bee traits supposed to increase exposure to parasites (Cohen et al., 2021), and this could explain the observed idiosyncratic pattern of infection between the two investigated bumblebee species, withB. pascuorum being smaller and with a slightly narrower foraging breath (Harder 1985). Furthermore, the two bumblebees also differ in colony size, where B. terrestris and B. pascuorum could reach 1000 and 150 individuals per colony, respectively (Von Hagen & Aichhorn, 2014) and this might mediate their epidemiology via intra-colony transmission.
Based on our results, the landscape green habitat fragmentation significantly shapes parasite occurrence, and in particular it seems to reduce the probability of infection in B. pascuorum. This finding disproves our expectation of having higher parasite occurrence in the more fragmented habitats due to aggregation of bumblebee hosts in the few green remnants available (amplification effect, Becker et al., 2015). In an urban context, the dispersion of green areas and the presence of inhospitable surfaces (concrete) affects the incidence of bumblebees as they could hardly arrive and forage in progressively isolated green areas. In this case, a lower hosts availability could explain the observed reduction in parasite occurrence. Therefore, future investigation will benefit from a clear determination of the community of flower visitors in terms of species richness and abundance. This will allow a deeper comprehension of the indirect effects of land-use features mediated by dilution, thus lower parasite prevalence due to higher species richness and diversity (Civitello et al., 2015) , or amplification effects.
The dependency between parasite abundance and host distribution in the urban landscape may result from a major importance of floral resources availability in increasing parasite occurrence, as resulted from our analyses. When flowers are highly available and a rich pollinator community is present there, it is expected to observe a dilution effect of parasites in several hosts, especially over large surfaces (Piot et al., 2019). However, our results may support the alternative hypothesis that floral resources improve parasite transmission due to the higher attractiveness of floral spots and the consequent higher bee aggregation. In this context, both the probability of infection and the parasite richness shown by B. pascuorum were higher where more flowers were available for foraging. This result confirms the well-known role of flowers as hubs for parasites spread among individuals (Pinilla‐Gallego et al., 2022). Indeed, about 10% of flowers were found to host one or more parasites of bees (Graystock et al., 2020) and shorter and wider flowers promote higher transmillability (Pinilla-Gallego et al., 2022). This could be a peculiarity of “poor-quality”, fragmented landscapes and of disconnected urban green areas, where flowers occur in aggregated ways or cover specific areas, thus aggregating pollinators too and promoting infection, as higher bee pathogens are found in flower strips near isolated semi-natural patches (Piot et al., 2019). This highlights potential risks associated with those interventions that aim at safeguarding pollinators through floral strip planting since these could facilitate the spread of infections in wild bee communities. While in B. pascuorum the probability of infection and parasite richness were shaped by landscape and local features a similar trend was not highlighted in B.terrestris that resulted more susceptible to parasite infection independently from landscape and local features. Interestingly, B. terrestris parasite richness was affected by the proximity to beekeeping sites. Other studies highlighted a major role of apiculture for the spread of parasites (Nanetti et al., 2021; Martínez-López et al., 2021) but without focusing on beekeeping activities within the urban environment. This will require further insights since the worry of pollinator decline is pushing many citizens and cooperative companies to adopt honeybee hives in cities (Sponsler & Bratman, 2021). In this study, parasite richness decreased in proximity to honeybee hives, likely because apiculture disproportionally increases the abundance of Apis mellifera in the city and thus favoring the spread of parasites on the most abundant host rather than on alternative bumblebee hosts. However, here we evaluated the impact of apiculture using proxy variables of honeybee presence, that do not exhaustively describe the local abundance of Apis mellifera and do not consider beekeeping practices that are known to shape parasite transmission to wild species (Piot et al., 2022). A detailed estimation of these variables would improve the comprehension of the impact of apiculture on the spread of parasites to urban and non urban wild bees.
5. CONCLUSIONS
Landscape and local features of urban green habitats shaped the occurrence of parasites with marked differences among the two investigated bumblebee species. This novel finding highlights the importance of designing proper target conservation efforts based on pollinator species-specific knowledge. The planting of flower strips is gaining importance among the efforts to safeguard pollinators; however, our findings also shed light on the potential detrimental effects of these practices that must be considered by public administrators. Further investigation related to the spatial arrangement of flower patches, as well as an evaluation of the most suitable flower species in terms of morphological traits and shapes with lower potential for parasite transmission will be useful to refine these conservation measures. Moreover, we recommend to consider the configuration of green areas at the landscape scale, here confirmed as a driver able to shape parasite dynamics, in implementing these conservation measures. The results obtained here will contribute to the fine tuning of the interventions aimed at improving pollinator health and wellbeing also in urban areas, indirectly contributing to our well-being since as resumed by the one-health concept human and ecosystem health are inextricably linked.
ACKNOWLEDGEMENTS
Project funded under the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4 - Call for tender No. 3138 of 16 December 2021, rectified by Decree n.3175 of 18 December 2021 of Italian Ministry of University and Research funded by the European Union – NextGenerationEU,Project code CN_00000033, Concession Decree No. 1034 of 17 June 2022 adopted by the Italian Ministry of University and Research, CUP, H43C22000530001 Project title “National Biodiversity Future Center - NBFC” .
Also funded by PROGETTO LIFE20 PRE/BE/000008 - UrbanGreeningPlans - CUP E42H21000060002 within the research agreement with Parco Nord Milano.
We are grateful to the staff of Parco Nord Milano for their support in the field activities.
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DATA ACCESSIBILITY AND BENEFIT-SHARING
All relevant data are within the paper or stored in figshare at the following linkhttps://figshare.com/s/c58df945039de5747072. Benefits from this research arise from the sharing of our data and results on public databases as described above.
AUTHOR CONTRIBUTIONS
Conceptualization, NT ; Investigation, NT, BC; Formal Analysis, NT, BC; Writing - Original Draft, NT, BC; Writing - Review and Editing, NT, BC, PB, EP, AG, MC ; Funding acquisition, PB, AG.
TABLES AND FIGURES