Bumble bees in landscapes with abundant floral resources have lower pathogen loads

Abstract

The pollination services provided by bees are essential for supporting natural and agricultural ecosystems. However, bee population declines have been documented across the world. Many of the factors known to undermine bee health (e.g., poor nutrition) can decrease immunocompetence and, thereby, increase bees’ susceptibility to diseases. Given the myriad of stressors that can exacerbate disease in wild bee populations, assessments of the relative impact of landscape habitat conditions on bee pathogen prevalence are needed to effectively conserve pollinator populations. Herein, we assess how landscape-level conditions, including various metrics of floral/nesting resources, insecticides, weather, and honey bee (Apis mellifera) abundance, drive variation in wild bumble bee (Bombus impatiens) pathogen loads. Specifically, we screened 890 bumble bee workers from varied habitats in Pennsylvania, USA for three pathogens (deformed wing virus, black queen cell virus, and Vairimorpha (= Nosema) bombi), Defensin expression, and body size. Bumble bees collected within low-quality landscapes exhibited the highest pathogen loads, with spring floral resources and nesting habitat availability serving as the main drivers. We also found higher loads of pathogens where honey bee apiaries are more abundant, a positive relationship between Vairimorpha loads and rainfall, and differences in pathogens by geographic region. Collectively, our results highlight the need to support high-quality landscapes (i.e., those with abundant floral/nesting resources) to maintain healthy wild bee populations.

Functional traits linked to pathogen prevalence in wild bee communities

“…Additionally, smaller bumble bees often remain in the hive as brood-care workers while larger bees forage 73 , with the potential to further bias the sampling. A recent observational study similarly found no relationship between bee size and pathogen prevalence in wild bumble bee workers 74 . As such, determining whether landscape scale filtering of bees could increase susceptibility to infections is a necessary future research direction, and highlights the importance of experimental manipulations in the field with co-occurring stressors 75 .…”

Long‐term surveys support declines in early season forest plants used by bumblebees

“…Although novel pathogen exposure is thought to be the primary cause of B. affinis decline and threatens other species (Cameron et al, 2016;USFWS, 2019), poor resource availability can compound the effects of pathogen exposure. For example, higher pathogen loads were found to be present in B. impatiens in sites with lower spring resource availability (McNeil et al, 2020), suggesting additive stress from pathogen exposure, resource availability and nutrition, which are further compounded by other threats to bee populations like pesticides or climate change (Potts et al, 2010). In total, although declining resource availability, F I G U R E 5 Seasonal distribution of all bumblebee plants and bumblebee specimen records.…”

Mass-flowering monoculture attracts bees, amplifying parasite prevalence

“…Interestingly, non-crop floral abundance was not associated with significant increases to bee abundance or bee richness. Other studies also suggest that increases in floral abundance, without accompanying increases in bee abundance, dilute transmission of parasites and pathogens [19,26]. When floral abundance is high, bees may disperse across resources, and an individual bee may have a reduced likelihood of encountering an infected individual [18].…”

“…Alternatively, noncrop resources may dilute parasitism if the presence of many flowers decreases the likelihood of interactions between infected and healthy individuals [18]. Non-crop habitat, by providing diverse and abundant resources, can also increase host species richness or immunity, reducing infection [19,20].…”