![]() Nutritional deprivation is likely to act through altering immune function, as immune genes are more highly expressed when forage is abundant 24. terrestris) exhibited increased mortality from viruses only if starved 23. Given the myriad of threats to pollinator populations, it can be difficult to tease apart the relative importance of each of these factors to bee health.Äetermining the leading threats to bee populations is further complicated by interactions among these stressors in the landscape, specifically many of the factors known to undermine bee health (such as poor nutrition or exposure to pesticides) can increase susceptibility to disease 19. Bees are more likely to be nutritionally deprived in landscapes with fewer and less diverse flowering plants, and nutritional deprivation can reduce immunocompetence and increase pathogen and parasite load 19, 20. For example, honey bees fed diets lacking pollen had higher loads of viruses 21 and diets with low pollen species diversity increased honey bee mortality when bees were infected with Vairimorpha 22. More recently, climate change has been added to the list of major threats to global bee populations, as large-scale geographic analyses identified climate shifts as a contributing factor to local bumble bee extirpation 18. Exposure to insecticides, particularly neonicotinoids, can lead to a variety of negative effects on native bees and managed honey bees alike, including compromised learning and foraging capabilities and reduced reproductive output 2, 15, 16, 17. In the US, the toxic load to bees of insecticides applied in agricultural landscapes has increased significantly over the last few decades 14. These honey bee viruses often spill over into native bee populations 10, 11, 12, where they impose negative effects on wild bee health 7, 13. Several bee viruses also contribute to declining honey bee survival 7. For example, the microsporidian Vairimorpha (= Nosema) bombi (hereafter, Vairimorpha) has been identified as a major contributor to bumble bee population losses, leading to the extirpation and near extinction of several bumble bee species in North America 8, 9. Bee losses, especially those of honey bees ( Apis mellifera) and bumble bees ( Bombus spp.), have more recently been ascribed to rising levels of novel bee pathogens 3, 7. Extensive habitat loss and degradation results in a dearth of floral resources and nest sites which has contributed to loss of wild bee abundance and diversity 5, 6. Declines in bee populations have been attributed to several factors 4. The pollination services provided by bees are critical for supporting healthy and diverse natural and agricultural ecosystems 1. Continuing declines documented in populations of wild and managed bees across the world thus pose significant threats to the stability of these systems 2, 3. 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. 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. 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. 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. 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. ![]() 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. Many of the factors known to undermine bee health (e.g., poor nutrition) can decrease immunocompetence and, thereby, increase bees’ susceptibility to diseases. However, bee population declines have been documented across the world. The pollination services provided by bees are essential for supporting natural and agricultural ecosystems. ![]()
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