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.