1. INTRODUCTION
Urbanization is growing rapidly worldwide leading to habitat loss and
modification, with detrimental consequences for biodiversity and
ecological functioning (Ayers & Rehan, 2021; Wenzel et al. 2020;
Fisogni et al., 2020). Wild pollinator insect communities are deeply
affected by this phenomenon, especially in the case of bees
(Hymenoptera: Anthophila). While some species are able to successfully
exploit these conditions and thrive in urban areas (Fitch et al., 2019),
others are facing challenges due to the lack of nesting and foraging
habitat (Cane et al., 2005), distance between green areas and hostile
climatic conditions (e.g. urban heat islands) (Biella et al., 2022),
pollution (Tommasi et al., 2022a) or parasite infections (Youngsteadt et
al., 2015). From a landscape perspective, urbanization significantly
shapes land-use features such as composition (the proportion of
different land use categories) and configuration (the spatial
arrangement of patches of land-use categories) (Fu & Weng, 2016), with
direct consequences for bees communities. In this context, green
habitats act as islands which are exploited by wild bees, and whose loss
has been associated to negative effects on bee species richness (Winfree
et al., 2009) and their foraging habits (Tommasi et al., 2022b, Andrieu
et al., 2009). Similarly, green habitat fragmentation may affect
pollinator species foraging behavior (Andrieu et al., 2009) and richness
with stronger consequences for the smaller and less mobile species
(Ayers & Rehan, 2021).
Urbanization may also have cascading effects on the health and fitness
of wild bees, for example by shaping the dynamics of their parasite
infections (Figueroa et al., 2020) contributing to population decline of
both managed and wild bee populations (Ivers et al., 2022). For
instance, urban green habitat availability and fragmentation may
indirectly affect parasite spread by shaping features of the host
community such as species richness and abundance (Tommasi et al., 2022).
Indeed, previous research focused on bumblebees suggested that
urbanization may promote the diffusion of these parasites (e.g.,Crithidia spp. and Nosema spp. ) in wild bees (Theodorou et
al., 2016) also due to changes in bumblebee community features (Ivers et
al., 2022), increasing the pressure experienced by these organisms in
urban habitat (but see Samuelson et al., 2020). However, despite the
increasingly higher research effort towards parasite spillover among
bees, the role of landscape and local features of green habitat in
shaping parasite dynamics is a topic largely neglected in literature.
Inter and intraspecific transmission of parasites usually occur through
direct contact between hosts or via contamination of flowers visited by
infected individuals (Singh et al., 2010). In this context, a major role
of beekeeping in contributing to the spread of parasites potentially
infecting wild species has also been reported. (Dolezal et al., 2016
Meeus et al., 2011; Cilia et al., 2023). Since apiculture exceptionally
increases the number of potential hosts in the area surrounding honeybee
hives, this anthropic activity could facilitate both the direct contact
between honeybees and wild species and the contamination of floral
resources. Beekeeping has gained growing importance worldwide, and
especially in urban areas mainly (Matsuzawa & Kohsaka, 2021), due to
the positive impact on community building and environmental education
(Skelton, 2006). Although several studies showed important effects of
apiculture on parasite distribution in the wild pollinators community
(Graystock et al., 2016a), nowadays dynamic and direct causes of this
impact are still unclear (Cilia et al., 2023).
To investigate the role of environmental features on parasite occurrence
different methodological approaches can be employed. For example
histopathological evaluation as well as species-specific fluorescence in
situ hybridization (FISH) can be employed for this purpose and have the
advantages of being able to clearly localize the infections and also
highlight the damages induced on tissues by parasites (Panek et al.,
2018). Other methods such as those based on PCR can be efficient
alternatives to more traditional approaches. Indeed, PCR-based
methodologies are rapid and can be widely performed. Even if PCR
detection does not allow discrimination between infection and
contamination by non germinated spores present in the digestive tract
(Gisder et al., 2020), it can reliably detect even low intensity or
latent infections (Graystock et al., 2015). Hence, PCR methods are
suitable for screening and could provide detailed insights on the role
of urbanization in pollinator epidemiology and on pollinator-parasite
interactions (Cohen el al., 2022.).
In this study we used a molecular approach to investigate the impact of
urban green areas in shaping the occurrence of parasites in two
bumblebee species, namely Bombus terrestris (Linnaeus, 1758) andB. pascuorum (Scopoli, 1763) both largely abundant also in cities
(Tommasi et al., 2022). We focused on the parasite richness, indicating
the number of different parasite taxa detected in each sample, and on
the probability of infection thus of detecting at least a single
parasite in a sample. Even if related to each other, these two variables
indicate different aspects: while one indicates the possibility of
co-infection and the parasite load, the other describes how probable it
is to be infected by any of the studied parasite species. We considered
several urban scales by focusing on the landscape structure, local
features and honeybee presence as potential intermediate vectors.
Specifically, we evaluated the impact of these features in shaping the
occurrence of the commonest bumblebees parasites, trypanosomatids
(Crithidia spp.), microsporidians (Nosema spp.) and
neogregarines (Apicystis spp.) (Ivers et al., 2022;
Theodorou et al., 2016; Youngstead et al., 2015). Since the reduced and
fragmented green habitats of the more urbanized landscape are expected
to concentrate bees to the remnant patches available for foraging
(Dylewski et al., 2019; Quistberg et al., 2016), we hypothesized to
observe an increase in the parasite richness and probability of
infection in the smaller and more fragmented green habitats of the more
urbanized areas. At the local scale, high availability and diversity of
floral resources are expected to increase pollinator community richness
and abundance (Tommasi et al., 2021, Hulsmann et al., 2015) thus
following disease ecology theory (Becker et al.,, 2015) we hypothesized
to observe also a positive correlation among flower abundance and
parasite richness and probability of infection. Furthermore, based on
previous research showing a positive correlation between beekeeping and
the spread of parasites to wild species (Graystock et al., 2016a), we
expected a higher parasite richness and probability of infection in
bumblebees collected from study sites with a higher abundance of
honeybee hives in the surrounding and/or located closer to the apiary.