Sunflower pollen reduces a gut pathogen in the model bee species, Bombus impatiens , but has weaker effects in three wild congeners

Abstract

Commercial bumblebees have become popular models to understand stressors and solutions for pollinator health, but few studies test whether results translate to other pollinators. Consuming sunflower pollen dramatically reduces infection by the gut parasite Crithidia bombi in commercially reared Bombus impatiens . We assessed the effect of sunflower pollen on infection in wild B. impatiens , Bombus griseocollis, Bombus bimaculatus and Bombus vagans . We also asked how pollen diet (50% sunflower pollen versus wildflower pollen) and infection (yes/no) affected performance in wild B. impatiens microcolonies. Compared to controls, sunflower pollen dramatically reduced Crithidia infection in commercial and wild B. impatiens, had similar but less dramatic effects in B. bimaculatus and B. vagans , and no effect in B. griseocollis . Bombus impatiens, B. bimaculatus and B. vagans are in the same subgenus, suggesting that responses to sunflower pollen may be phylogenetically conserved. In microcolonies, 50% sunflower pollen reduced infection compared to wildflower pollen, but also reduced reproduction. Sunflower pollen could control Crithidia infections in B. impatiens and potentially close relatives, but may hinder reproduction if other resources are scarce. We caution that research using managed bee species, such as B. impatiens , be interpreted carefully as findings may not relate to all bee species.

Sunflower plantings reduce a common gut pathogen and increase queen production in common eastern bumblebee colonies

“…We need to understand whether sunflower may have similar effects on pathogen loads in other bee species, particularly those experiencing population declines [85]. We recently found that consuming sunflower pollen had weaker effects on C. bombi infection in Bombus vagans and Bombus bimaculatus and no detectable effect in Bombus griseocollis [64], suggesting that benefits vary between bee species. That said, decreasing pathogen loads in B. impatiens might positively influence disease dynamics in congeners.…”

Understanding effects of floral products on bee parasites: Mechanisms, synergism, and ecological complexity

“…Recent work focusing on whole sunflower pollen, rather than constituent metabolites, found that B. impatiens individuals infected with C. bombi and fed sunflower pollen exhibited enhanced expression of AMP hymenoptaecin and Toll receptors in gut tissue, compared to infected bees fed wildflower pollen ( Giacomini et al, 2021a ). Interestingly, however, sunflower pollen consumption did not affect antibacterial activity in B. impatiens hemolymph ( Fowler et al, in press ), suggesting tissue-specific effects of floral products on immune response. These studies strongly indicate that consumption of a diversity of phytochemicals found in nectar and pollen can influence immune response.…”

“…Research has similarly focused on a very narrow subset of bees, specifically the western honey bee ( Apis mellifera ) and a few bumble bee species (particularly Bombus impatiens and B. terrestris ). Given the high degree of variability in antiparasitic effects found even among congeneric host species ( Fowler et al, 2022 ), there is a clear need to expand the scope of research to include a wider diversity of bee species. This need is particularly urgent in light of evidence that disease burden is associated with population declines, at least in bumble bees ( Averill et al, 2021 ; Cameron et al, 2011 ), and interest in using flowering species with demonstrated antiparasitic effects to reduce disease burden in wild bees ( Folly et al, 2021 ).…”

“…Several recent studies documenting antiparasitic effects of pollen or nectar of specific plant species advocate for increased planting of these species as a strategy for managing disease and promoting bee populations ( Folly et al, 2021 ; Giacomini et al, 2018 ). Setting aside broader questions regarding the utility of this approach ( Fowler et al, 2022 ; Palmer-Young et al, 2017a ), its impact will depend heavily on whether bees self-medicate. That is, a given increase in the availability of ‘medicinal’ plant species will have a much larger effect on parasite dynamics if infected bees preferentially forage on plants that reduce infection.…”

An evidence-based rationale for a North American commercial bumble bee clean stock certification program

“…These studies have also demonstrated that secondary metabolites can present some negative side effects to the bees, as well. Moreover, there is growing evidence that sunflower pollen consistently reduces C. bombi infections in B. impatiens, both in the lab and in the field (Fowler et al 2022a;Fowler et al 2022b;Fowler et al 2020;Giacomini et al 2018;LoCascio et al 2019), further highlighting the potential medicinal properties of different pollen types. Antiparasitic effects of natural plant compounds have attracted recent attention (Fitch et al 2022;Koch et al 2019), and further testing of these treatments is needed to determine their effectiveness as medicine for bumble bee colonies.…”