DISCUSSION
We observed that honeybees, bumblebees, and other pollinators differed
in the number of visits to flowers and the duration per visit to petals
and pollen but did not vary in the total length of time they spent on
flowers. Honeybees had fewer flower visits than bumblebees, and all
visiting pollinators spent similar amounts of time per visit interacting
with the pollen and nectar simultaneously (Figure 1). Yet, the sites
with more and longer honeybee visits to shared flowers had higherV. ceranae prevalence in bumblebees. Therefore, honeybee
visitation to flowers appears to have a disproportionate impact onV. ceranae prevalence in local bumblebee populations. Visitation
by bumblebees or other pollinators, in terms of the number of visits or
time spent on flowers, was not associated with V. ceranaeprevalence in either host species. These findings suggest honeybees may
play an important role in the spread of V. ceranae to bumblebees
through indirect contact via shared flowers in the natural environment.
Such pathogen spillover from honeybees to bumblebees is likely to have
negative consequences for bumblebee populations (Colla et al. 2006,
Furst et al. 2014).
The spillover from honeybees to bumblebees may occur differently
compared to transmission among honeybees. V. ceranae is easily
transmitted within honeybee hives when bees clean up fecal material, eat
contaminated food, or perform trophallaxis (Chen et al. 2008, Higes et
al. 2010). Further, drifting of honeybees among hives is known to occur
and is thought to play a role in the transmission of parasites,
including V. ceranae (Higes et al. 2010, Eberl and Muhammad
2022). As V. ceranae is a well-established concern for managed
honeybee populations (Higes et al. 2013) and is thought to spill over
from managed honeybee populations to native bumblebee populations (Furst
et al. 2014, Goulson and Hughes 2015, Alger et al. 2018), high V.
ceranae prevalence in honeybees may be driven by intraspecific
transmission occurring among and within honeybee hives. In contrast,V. ceranae prevalence in bumblebees may be driven in part by
parasite spillover from shared flowers with honeybees. Thus, spillover
from honeybees to bumblebees could explain why honeybee visitation
behavior was strongly correlated with V. ceranae prevalence in
bumblebees, but not with prevalence in honeybees.
Our results are consistent with prior small-scale lab experiments which
demonstrated that pollinator parasites, including V. ceranae , are
transmitted via contact with flowers (Durrer and Schmid-Hempel 1994,
Graystock et al. 2015, Purkiss and Lach 2019). Several recent studies
have further shown that pollinator parasites are commonly found on
flowers in the field, but their abundance varies based on flower
morphology, the environment, and pollinator visitation patterns (Alger
et al. 2019, Figueroa et al. 2019, Russell et al. 2019, Graystock et al.
2020). Furthermore, Graystock et al. (2015) experimentally showed that
23% of uninfected bumblebees that foraged on flowers recently visited
by infected honeybees became infected with V. ceranae . This
suggests that flowers can become hotspots for parasite dispersal once
contaminated (Graystock et al. 2015). However, few studies have examined
how differences in the pollinator community’s floral visitation
behaviors may impact parasite prevalence across multiple host species in
nature (but see Graystock et al. 2020), and V. ceranae in
particular has been neglected. As pathogens are a key driver of
pollinator population decline (Potts et al. 2010), it is crucial to
understand patterns of their transmission within and among pollinator
species in the natural environment. Our findings corroborate prior
experimental work and add that honeybee visitation to shared
flowers—especially in areas with generally high V. ceranaeinfection levels in honeybees—facilitates greater V. ceranaespillover from managed honeybees to wild bumblebees in the natural
environment.
While V. ceranae spillover via contaminated flowers seems likely,
little is known about how pollinator interactions with different parts
of inflorescences may affect the likelihood of V. ceranaetransmission. We examined the association between V. ceranaeprevalence and the duration per visit by honeybees, bumblebees, and
other pollinators to flower petals, nectaries, and pollen to explore
which parts of inflorescences may have the greatest impact on V.
ceranae spread. We found that higher V. ceranae prevalence in
bumblebees was associated with longer durations of honeybee interactions
per visit spent simultaneously contacting both the pollen and nectar of
inflorescences (pollen+nectar). These visits were characterized by
active foraging behavior for nectar and/or pollen while deeply embedded
within the corolla of the large squash flowers. Additionally, since
there was no difference among honeybees, bumblebees, and other
pollinators in time spent per visit on the pollen+nectar (Figure 1d),
our results suggest that time spent by honeybees on flowers
disproportionately increases the likelihood of parasite spillover to
bumblebees relative to time spend on flowers by bumblebees or other
pollinators.
The length of time that infected honeybees spend closely interacting
with both pollen and nectar—food resources that are consumed by many
pollinator species—likely contributes to V. ceranae spore
deposition on flower surfaces, which may be picked up and consumed by
subsequent floral visitors. V. ceranae is a fecal-orally
transmitted parasite (Chen et al. 2008, Smith 2012) and bees commonly
defecate on floral surfaces while foraging, with longer visits
increasing the likelihood of defecation (Bodden et al. 2019). V.
ceranae has been detected in honeybee salivary glands (Chen et al.
2009) and viable and infectious V. ceranae spores have been found
in the corbicular pollen of honeybees (Higes et al. 2008b), suggesting
that pollen can become contaminated during pollen collection. Therefore,
it is possible that the pollen on the stamen may be a key hot spot for
the deposition of V. ceranae by infected bees and the acquisition
of this contaminated pollen by susceptible bees. In contrast, nectar may
be a poor location for pathogen transmission because high sugar
concentrations can inhibit microbial growth and pathogen survival (Adler
et al. 2021). We observed that honeybees and bumblebees seemed to spend
more time on pollen+nectar interactions compared to pollen-only or
nectar-only interactions (Appendix S1: Table S3), likely owing to their
large size making it difficult to only contact one food source at a
time. Therefore, the long visits with high floral contact during which
honeybees and bumblebees foraged for pollen and nectar may have
increased the chances for transmission to occur.
We did not observe any relationships between V. ceranaeprevalence and the length of time pollinators spent interacting with
only the petals or pollen. Though many bees spent time on the petals,
bees were typically observed either resting or crawling on the petals
for very short periods of time. Though other pollinator parasites are
transmitted via floral petals (Figueroa et al. 2019), in our study
petal-only interactions were not linked with V. ceranaeprevalence. Pathogenic spores can often survive well on floral surfaces
(McArt et al. 2014), but their survival likely varies among different
plant species, flower parts, and the centrality of the plant in the
plant–pollinator network (Palmer-Young et al. 2016, Naughton et al.
2017, Adler et al. 2018, Figueroa et al. 2019, Piot et al. 2020). Since
this study only considered a single plant species and did not consider
the plant–pollinator network, future studies are needed to empirically
test how floral traits among different plant species affect
pollinator–flower interactions, explore the distribution of spores on
different floral surfaces in the natural environment, and determine the
consequences for V. ceranae parasite dispersal via different
parts of the inflorescences.
In contrast to V. ceranae prevalence in bumblebees, we
consistently found that V. ceranae prevalence in honeybees was
not correlated with flower visitation by any species. V. ceranaeprevalence in honeybees was high at all sites (57.1% to 81.3%;
Appendix S1 Figure S1, Table S9), indicating that honeybees experience
consistently high V. ceranae infection levels across the
landscape. The spillover of V. ceranae from managed honeybee
hosts to wild bumblebee populations would suggest that honeybees are a
highly competent host for V. ceranae that could be facilitating
transmission to other native bee species in pollinator communities
through indirect interactions on shared flowers.