The Effect of Urbanization on Bumble Bee Communities in Greater Philadelphia

Bumble bees (Bombus spp.) are among the most important wild pollinators in temperate ecosystems in North America and Europe, and are believed to be vital to the functioning of the pollination networks in which they occur. Accordingly, evidence of their overall decline in Europe and more selective decline in the U.S. has raised concern about the long term persistence of many species. Human-induced changes in land use, including the loss of natural and semi-natural habitat and associated floral resources, are purported causes in several cases. Declines prompted an investigation of this potential trend in the urbanized landscape in and surrounding Philadelphia, PA. We surveyed the species richness and abundance of bumble bee communities in ten half-ha plots located in restored, managed meadows along a gradient of urban and suburban development in the Philadelphia metropolitan area from June 1 to August 15, 2006. In conjunction with collections, we measured floral density within each plot. We calculated the proportion of developed land at differing spatial scales ranging from 500 to 4000 m from survey sites. General linear models were used to test the effect of developed land, local meadow size, and floral resource density on overall bumble bee species richness and abundance. 
Development did not have any detectable effect on species richness at any tested spatial scale. Bee abundance was best explained by a model that included the proportion of developed land at the 2500 m scale. In contrast to our expectations, total bumble bee abundance was significantly higher in plots with a higher proportion of developed land surrounding the site (F1,5 = 8.13, P = 0.04). Local floral density did not significantly affect richness or abundance (F 1,5 = 0.93, P = 0.34), nor was local resource quality associated with development (r = 0.52, P = 0.12) . We hypothesize that gardens maintained in urban and suburban landscapes may provide an important and consistent food supply to bumble bees throughout the duration of their colony cycle. Forested habitats, which were the other dominate habitat type, may actually be relatively poor in bumble bee resources following closure of the canopy in late spring. This research indicates that restored meadows are excellent habitat for bumble bees and may promote pollination services in urbanized settings.
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Assessing chemical mechanisms underlying the effects of sunflower pollen on a gut pathogen in bumble bees

Many pollinator species are declining due to a variety of interacting stressors including pathogens, sparking interest in understanding factors that could mitigate these outcomes. Diet can affect host-pathogen interactions by changing nutritional reserves or providing bioactive secondary chemicals. Recent work found that sunflower pollen (Helianthus annuus) dramatically reduced cell counts of the gut pathogen Crithidia bombi in bumble bee workers (Bombus impatiens), but the mechanism underlying this effect is unknown. Here we analyzed methanolic extracts of sunflower pollen by LC-MS and identified triscoumaroyl spermidines as the major secondary metabolite components, along with a flavonoid quercetin-3-O-hexoside and a quercetin-3-O-(6-O-malonyl)-hexoside. We then tested the effect of triscoumaroyl spermidine and rutin (as a proxy for quercetin glycosides) on Crithidia infection in B. impatiens, compared to buckwheat pollen (Fagopyrum esculentum) as a negative control and sunflower pollen as a positive control. In addition, we tested the effect of nine fatty acids from sunflower pollen individually and in combination using similar methods. Although sunflower pollen consistently reduced Crithidia relative to control pollen, none of the compounds we tested had significant effects. In addition, diet treatments did not affect mortality, or sucrose or pollen consumption. Thus, the mechanisms underlying the medicinal effect of sunflower are still unknown; future work could use bioactivity-guided fractionation to more efficiently target compounds of interest, and explore non-chemical mechanisms. Ultimately, identifying the mechanism underlying the effect of sunflower pollen on pathogens will open up new avenues for managing bee health

Early resources lead to persistent benefits for bumble bee colony dynamics

Conditions experienced early in development can affect the future performance of individuals and populations. Demographic theories predict persistent population impacts of past resources, but few studies have experimentally tested such carry-over effects across generations or cohorts. We used bumble bees to test whether resource timing had persistent effects on within-colony dynamics over sequential cohorts of workers. We simulated a resource pulse for field colonies either early or late in their development and estimated colony growth rates during pulse- and non-pulse periods. During periods when resources were not supplemented, early-pulse colonies grew faster than late-pulse colonies; early-pulse colonies grew larger as a result. These results revealed persistent effects of past resources on current growth and support the importance of transient dynamics in natural ecological systems. Early-pulse colonies also produced more queen offspring, highlighting the critical nature of resource timing for the population, as well as colony, dynamics of a key pollinator

Larger Workers Outperform Smaller Workers Across Resource Environments: An Evaluation of Demographic Data Using Functional Linear Models

Behavior and organization of social groups is thought to be vital to the functioning of societies, yet the contributions of various roles within social groups toward population growth and dynamics have been difficult to quantify. A common approach to quantifying these role-based contributions is evaluating the number of individuals conducting certain roles, which ignores how behavior might scale up to effects at the population-level. Manipulative experiments are another common approach to determine population-level effects, but they often ignore potential feedbacks associated with these various roles. Here, we evaluate the effects of worker size distribution in bumblebee colonies on worker production in 24 observational colonies across three environments, using functional linear models. Functional linear models are an underused correlative technique that has been used to assess lag effects of environmental drivers on plant performance. We demonstrate potential applications of this technique for exploring high-dimensional ecological systems, such as the contributions of individuals with different traits to colony dynamics. We found that more larger workers had mostly positive effects and more smaller workers had negative effects on worker production. Most of these effects were only detected under low or fluctuating resource environments suggesting that the advantage of colonies with larger-bodied workers becomes more apparent under stressful conditions. We also demonstrate the wider ecological application of functional linear models. We highlight the advantages and limitations when considering these models, and how they are a valuable complement to many of these performance-based and manipulative experiments

Sunflower plantings reduce a common gut pathogen and increase queen production in bumble bee colonies

We evaluated whether plantings of sunflower (Helianthus annuus), whose pollen reduces infection by some pathogens when fed to bees in captivity, lowered pathogen levels and increased reproduction in free-foraging bumble bee colonies (Bombus impatiens). We placed pairs of commercial colonies of B. impatiens at 20 mixed vegetable farms in western Massachusetts between Jul-23 and Oct-6 2019. Flowering resources typically visited by bumble bees were quantified at each farm twice to characterize abundance and diversity. We also visited each farm 3-4 times and at each visit, we (a) recorded colony weights to track growth, (b) collected ~10 corbicular loads from returning foragers (per site) to assess usage of sunflower and other Asteraceae, and (c) collected 10 returning foraging workers from each colony entrance for later pathogen analysis. Visual assessment of pathogen samples and pollen composition occurred at UMass Amherst during the 2019-20 academic year. Molecular assessment of pathogen samples occurred at UC Riverside. An associated manuscript with this title and these authors is being submitted for publication.https://scholarworks.umass.edu/data/1158/thumbnail.jp

Flower plantings support wild bee reproduction and may also mitigate pesticide exposure effects

Sustainable agriculture relies on pollinators, and wild bees benefit yield of multiple crops. However, the combined exposure to pesticides and loss of flower resources, driven by agricultural intensification, contribute to declining diversity and abundance of many bee taxa. Flower plantings along the margins of agricultural fields offer diverse food resources not directly treated with pesticides. To investigate the potential of flower plantings to mitigate bee pesticide exposure effects and support bee reproduction, we selected replicated sites in intensively farmed landscapes where half contained flower plantings. We assessed solitary bee Osmia lignaria and bumble bee Bombus vosnesenskii nesting and reproduction throughout the season in these landscapes. We also quantified local and landscape flower resources and used bee-collected pollen to determine forage resource use and pesticide exposure and risk. Flower plantings, and their local flower resources, increased O. lignaria nesting probability. Bombus vosnesenskii reproduction was more strongly related to landscape than local flower resources. Bees at sites with and without flower plantings experienced similar pesticide risk, and the local flowers, alongside flowers in the landscape, were sources of pesticide exposure particularly for O. lignaria. However, local flower resources mitigated negative pesticide effects on B. vosnesenskii reproduction. Synthesis and applications. Bees in agricultural landscapes are threatened by pesticide exposure and loss of flower resources through agricultural intensification. Therefore, finding solutions to mitigate negative effects of pesticide use and flower deficiency is urgent. Our findings point towards flower plantings as a potential solution to support bee populations by mitigating pesticide exposure effects and providing key forage. Further investigation of the balance between forage benefits and added pesticide risk is needed to reveal contexts where net benefits occur