Angela Noyes
Senior Thesis | 2025
A Survey of Honey Bees and Native Pollinators at the Arnold Arboretum of Harvard University
Angela Noyes
Abstract:
Concern that non-native honey bees (Apis mellifera) might outcompete native pollinators is causing worry among governments and conservation groups in the Northeast region of the United States. However, there is not enough data currently available to be considered in the decision-making process on whether it is necessary to place regulations on the honey bee population. To address the lack of information on a local scale, as well as the differing results of previous research on a larger scale, pollinators were observed at the Arnold Arboretum of Harvard University, a 281-acre preserve in Boston, Massachusetts (USA). It was found that while non-native honey bees were the most abundant pollinators, they coexist with native pollinators at the Arnold Arboretum. The preferences of pollinators for different types of plants were discovered, which will help future efforts to improve pollination.
Introduction:
Flower-visiting insects travel over a wide range of native and cultivated plants in search of food in the form of nectar and pollen. During these visits, they carry pollen from flower to flower; this movement of pollen is vital for plants to become fertilized and produce fruits, seeds, and young plants. It is estimated that one out of every three bites of food we eat is only able to exist because of animal pollinators (The importance of pollinators | USDA.). Unfortunately, there is clear evidence of a decline in the populations of flower-visiting insects including bumblebees, solitary bees, wasps, butterflies, flies, and beetles, as well as a decline in the plants that rely upon them (Biesmeijer et al., 2006; Gallai, Salles, Settele, & Vaissière, 2009; Goulson, Lye, & Darvill, 2008). Flower-visiting insects are threatened by many factors, including the destruction of their habitats, damage to nesting sites, decreased resource diversity, herbicide and pesticide use, leaf blowers, the spread of pathogens, and climate change (Potts et al., 2010).
In North America, another threat to native bees is nonnative honey bees (Apis mellifera). The Western Honey Bee (Apis mellifera) is native to Europe, Africa, and the Middle East but is commonly found in North America where it is used to produce honey and pollinate agricultural crops. Although honey bees do not behave aggressively towards other insects, they have the potential to harm native bees who are visiting the same flowers by competing for resources and spreading pathogens (Stout and Morales, 2009; Angelella et al., 2021; Iwasaki and Hogendoorn, 2021). Because honey bees deplete the amount of pollen and nectar available from native plants, it is possible that when honey bees are abundant, native bees are less abundant (Lindström, et al. 2016;
Mallinger et al., 2017; Ropars et al., 2019; Rasmussen et al., 2021; Renner et al., 2021; Iwasaki and Hogendoorn, 2022).
There is evidence supporting the argument that in some situations, honey bees outcompete native pollinators, although it is not known how or if this changes geographically (Hudewenz & Klein, 2013; Prendergast, Dixon, & Bateman, 2021; Iwasaki and Hogendoorn, 2022). It is also unclear how the observations made about honey bee behavior are affected by plants being in the same taxa, the abundance of plants, and the season.
This research can be useful to the ongoing conversations about regulating the honey bee population. Concerns that honey bees may outcompete and threaten native flower visitors have resulted in government regulations restricting the number and density of commercial honey bee hives and honey bee hives in conservation lands. These regulations can also affect people keeping a small number of hives on private lands. In the United States, many local governments and conservation groups are considering regulations.
Previous work has been carried out at the Arnold Arboretum of Harvard University. In the 1930s, Otto Plath observed that 13 bumblebee species were present at the Arnold Arboretum, and they visited 36 different plant taxa (Plath, 1934).
In our current study, we set out to conduct a survey of flower visitors at the Arnold Arboretum, since this has yet to be done for a large arboretum that contains a wide range of woody plants. The diversity of plants at the Arnold Arboretum provides an opportunity to see what plant size is preferred by each insect, and how their behavior correlates to plant habits, abundance of flowers, and more. The plants of study were primarily woody plants like trees, shrubs, and vines because although research has been done on flower-visiting insects at herbaceous plants, limited research has been done on woody plants.
The goal of this study is to find evidence about whether honey bees and native bees coexist or compete. Additionally, the questions regarding honey bees excluding native pollinators, the ranges of plants that insects visit, and whether honey bees are visiting the same plants as native pollinators will be explored.
Methods:
Location
The Arnold Arboretum of Harvard University spans 281 acres in Boston, Massachusetts. The Arnold Arboretum has been home to a collection of woody plants dating back to its establishment in 1872, making it the oldest public arboretum in the United States and North America. The extensive collection of plants has been documented over the years including 16,215 woody plants identified, labeled, and listed as part of the collection as of July 2024. This location for study was chosen due to the collection being so well documented and historical. Utilizing historical data from Otto Plath’s observations in the 1930s, which recorded bumblebees and their favorite plants at the Arnold Arboretum (Plath, 1934), allows for a comparative analysis of changes over time.
Observation Protocol
The observation period took place from April 9, 2024, to July 30, 2024. Observations were made 1-3 times per week, depending on weather conditions. Observations were made between 8:00 am and 4:00 pm on days with clear skies, calm winds, and temperatures between 10 to 29.4 degrees Celsius. At each plant observed, the plant genus, species, habit (Wildflower Meadow, Ornamental Herb, Adventive Herb, Tree, Vine, Shrub), visitors, accession number along, date, time, observers,
temperature, wind speed, and cloud cover were noted. Also, an abundance score ranging from A1 (meaning there were few or scattered flowers) to A3 (meaning the plant was fully covered in flowers) was assigned. It should be noted that woody taxa include trees, vines, and shrubs while ornamental herbs are classified as herbaceous taxa. Adventive herbs and wildflower meadow plants are considered wild. Each plant was observed for five minutes, documenting the visitors and whether they were collecting nectar or pollen. For an insect to be deemed a visitor, it needed to land in or on a flower and forage for either pollen or nectar. This excludes insects feeding on leaves or using plants to perch or rest.
Flower Visitors
On the record sheet, flower visitors were separated into the following groups: bumblebees (Bombus sp.), honey bees (Apis mellifera), small native solitary bees (smaller than half the size of a honey bee), medium native solitary bees (half the size of honey bees), honey bee sized native solitary bees, cabbage butterflies (Pieris rapae), skipper butterflies (Hesperiidae), other butterflies (other Lepidoptera), hover flies (Syrphidae), other flies (other Diptera), beetles (Coleoptera), and other. Within the “other” category, two notable insects emerged and were highlighted in data entry: carpenter bees (Xylocopa virginica) and giant/sculpted resin bees (Megachile sculpturalis). Among bumblebees, three main species were identified: Common Eastern Bumblebee (Bombus impatiens), Brown-belted Bumblebee (Bombus griseocollis), and Two-spotted Bumblebee (Bombus bimaculatus). Additionally, a Confusing/Perplexing Bumblebee (Bombus perplexus) was also observed.
Data Management and Analysis
Data was recorded by hand; baseline analysis was then completed using Microsoft Excel.
Results:
There were 1392 observations altogether made over 41 days. In total, there were 704 unique taxa: 171 tree taxa, 353 shrub taxa, 48 ornamental herb taxa, 69 adventive herb taxa, 29 vine taxa, and 34 wildflower meadow taxa. There was an average of 34 observations per day and a range of 8 to 81 observations per day. Temperatures ranged from 5.5°C to 31.1°C; wind speeds ranged from 2 mph to 16 mph; and cloud cover ranged from 0% to 100%.
Over the research period, 10502 total insects were seen. There were 4127 honey bees, 2380 bumblebees, 1837 native bees, 349 carpenter bees, 343 resin bees, 701 beetles, 7 cabbage butterflies, 71 skipper butterflies, 57 other butterflies, 214 hoverflies, 130 other flies, 138 wasps, and 148 other visitors.
Butterfly Observations
While it was expected that butterflies would represent a significant amount of the 10502 observed insects, only 57 butterflies were observed over 44 plants. Of the butterflies observed, the species noted were Red Admiral (Vanessa atalanta), Monarch (Danaus plexippus), Tiger Swallowtail (Papilio glaucus), Black Swallowtail (Papilio polyxenes), Pearl Crescent (Phyciodes tharos), and Copper (Lycaena) Butterflies. The greatest number of butterflies seen in an observation was three and the least was one, showing how even on days where the most butterflies were seen, only a maximum of three butterflies were viewed on a single plant. Some of the favorite plants for butterflies were Malus sargentii, Malus sieboldii, Pycnanthemum tenuifolium, and Prunus x yedoensis.
Bee Observations
Over time, the number of honey bees, bumblebees, and native bees per observation increased (Figure 1). Honey bees had a minimum value of 0.12 honey bees per observation on May 10 and a maximum value of 17.71 honey bees per observation on July 28. Bumblebees had a minimum value of 0 bumblebees per observation on April 9 and April 29, as well as a maximum value of 11.39 bumblebees per observation on July 28. Native bees had a minimum value of 0.04 native bees per observation on May 11, and a maximum value of 12.64 native bees per observation on July 9. All three groups show an increase over time and hit a maximum number of visitors later in the season because their populations were increasing over the growing season and the weather was warmer.
Figure 1. The number of insects observed per plant for honey bees (HB), bumblebees (BB), and native bees (NB).
The number of carpenter bees per observation started increasing in late April and peaked on May 6 with a value of 1 carpenter bee per observation (Figure 2). The number of carpenter bees per observation decreased around June 16, a time during which carpenter bees were observed to be more lethargic because they reached the end of their lifetimes and were about to die. On June 24, the number of carpenter bees per observation was back to a low of 0 carpenter bees per observation. Around July 6, carpenter bee sightings increased again, and decreased around July 26. While the data shows a bimodal distribution, there is variability within short periods of time, which is likely due to changes in the plant species being observed. The bimodal distribution is as expected because carpenter bees usually appear in April and May to get food with the adults being active in May and June, and then proceed to mate and build their nests and die off. The second generation is active in July.
Figure 2. The number of insects observed per plant for resin bees (RB) and carpenter bees (CB). (Figure courtesy of Matty Carrozza)
The number of sculpted resin bees per observation increased over time, but not consistently. The number of sculpted resin bees per observation remained at 0 from the start of the observation period (April 9 to June 25) and went above 1 only 3 times. There were 3.48 resin bees per observation on July 8, 3.6 on July 15, and 1.5 on July 30. There is no consistent trend, but the spikes are likely caused by the flowering of the Golden Rain Tree (Koelreuteria paniculata) and the Japanese pagoda tree (Styphnolobium japonicum). The golden rain tree, which was observed on July 8 and 15, had the most visits by resin bees out of all the plants observed. The Japanese pagoda tree (Styphnolobium japonicum), which was observed on July 30, was second to the golden rain tree.
Visits Over Time
In the early spring, there was significant daily variation in plants having visitors compared to plants without visitors (Figure 3). These values range from 89.2 percent of observations having no visitors to 0 percent of observations having no visitors on any given day. As stated, the spring had more daily variation in these values compared to the fall, where the narrower distribution points to more consistent visitation patterns. Overall, the negative slope points to the gradual decrease in the percentage of observations that had no visits over time.
Figure 3. The percentage observations that had no visits from any insects.
The percentage of honey bees observed has remained consistent in comparison to bumblebees and other native bees. Throughout the observation period, honey bees were observed to be an average of 51.5% of the honey bee, bumblebee, and native bee observations. The percentage of honey bee observations had a minimum of 10.7% on May 10 and a maximum of 93% on May 13. The observations made on May 10 were made at 11:45 am when the temperature was 12.2°C. The observations made on May 13 were made at approximately the same time, with temperatures being only slightly higher than on May 10. Therefore, this variability seen within a short period of time is most likely caused by observing different plant species.
The percentage of observations that were native bees varied throughout the observation period, with no obvious pattern. Native bee observations were at a minimum of 3.07% on May 13 and a maximum of 82.1% on May 10. There was significant variation in
the percentage of each bee species out of total bee observations, which may be caused by observers visiting different plants and sections of the Arnold Arboretum on different days, with different weather, and at different times of day. Across all the flower visitors observed, honey bees made up 39.3%, bumblebees made up 22.7%, and native bees made up 17.5%. The percentage of honey bees is almost equal to the percentage of bumblebees and native bees combined, making them one of the most important pollinators.
Out of the total observations of honey bees, bumblebees, native bees, carpenter bees, and resin bees, 28.7% of those observations were on trees, 46% were on shrubs, 1.2% were on vines, 10.8% were on adventive herbs, 7.5% were on ornamental herbs, and 5.8% were in the wildflower meadow.
The distribution of honey bees among these different plant groups was expected to mirror the overall distribution of all visitors, as noted previously, and the observed data supports this expectation. Of all honey bee observations, 23.3% were made on trees, 39.9% on shrubs, 3.9% on vines, 12.7% on adventive herbs, 12.6% on ornamental herbs, and 7.5% in the wildflower meadow.
The distribution of bumblebees is also as expected, although it seems that bumblebees had a slight preference for shrubs. 58.3% of bumblebee observations were on shrubs, compared with the 46% expected. Native bees had a slight preference for shrubs, with 54% of native bee observations being on shrubs, in comparison to the 46% expected. Carpenter bees also preferred shrubs, with 58.2% of carpenter bee observations being on shrubs. There are more carpenter bee observations on vines than expected, with 8% of carpenter bee observations being on vines in comparison to the 1.2% expected. Resin bees preferred trees over all other plant habits, with 95% of resin bee observations being on trees. This is significant in comparison to the expected 28.6%.
There were no resin bee observations on vines, adventive herbs, or ornamental herbs. 0.3% of resin bee observations were in the wildflower meadow, and 4.2% on shrubs, but this percentage is small compared to the 46% expected on shrubs.
Honey bees made up the highest percentage of insects observed on adventive herbs, at 39.7%. Second to honey bees were beetles, which made up 24.3% of insect observations on adventive herbs. This high number of beetle observations is likely because of the high number of beetles found on Rose Mallow.
Of all the vine observations, 31.6% of flower visitors were honey bees, 2.3% were bumblebees, 25.6% were native bees, 0% were beetles, and all other visitors constituted 50%. This high number of “other visitors” is likely due to carpenter bees. These values are also affected by small sample size, as only 1.2% of all the observations were on vines.
Visit Difference Among Plant Species
Bumblebees were particularly abundant on several species of Tilia, Hydrangea, Clethra, and Hypericum. Hypericum species were served as significant pollen sources and Clethra as nectar sources. Bumblebees were also abundant on Helianthus tuberosus, Koelreuteria, and several other species. Many Rosa species were notable for being visited by bumblebees collecting pollen even though they did not have large numbers of bumblebee species.
Honey bees were most abundant on Helianthus tuberosus, Tetradium daniellii, and Fluegga, where they formed large concentrations. They were also commonly observed on herbaceous species such as Pycnanthemum, Calamintha nepeta, and early spring Prunus species as well as single species of Hydrangea and Tilia.
While there was some overlap between the favorite plants of bumblebees and honey bees, such as Helianthus tuberosus,
Koelreuteria, Tilia tomentosa, and Pycnanthemum incanum, both groups had distinct preferences. Bumblees had a much greater affinity for Hydrangea, Tilia, and Hypericum species, and honey bees had their own favorites.
Native bees, encompassing a diverse range of species, were grouped together in this study, and visited a large variety of plants. The most visited plant was Aesculus parviflora, which had large numbers of small metallic green bees visiting flowers. The native bees showed a strong tendency to visit members of the Rosaceae family, including the general Sorbaria, Physocarpus, Spiraea, Stephanandra, and Pyrus species.
Carpenter bees primarily visited plants that other species visited less, such as Kolkwitzia, Cercis, Wisteria, Styrax, Sophora, Deutzia, Syringa, Buddleia, and Penstemon, many of which had large flowers. Carpenter bees also visit some species favored by other species such as Hydrangea, Aesculus, and Clethra.
Resin bees are less abundant in the data set, as they only appear in July. They were found commonly on Styphnolobium, Oxydendrum arboreum, and Aesculus parviflora. They often visited Koelreuteria and Tetradium, where honey bees are also abundant.
Insect visitation varied significantly across plant species. Helianthus tuberosus had the highest number and diversity of flower visitors. Koelreuteria was visited by large numbers of honey bees and resin bees while aesculus parviflora was visited by large numbers of small native bees, resin bees, and other insects. Hibiscus moscheutos was frequently visited by small pollen-eating beetles, and some honey bees. Certain plants had large numbers of honey bees, such as Tetradium, Calamintha, and Flueggia.
When comparing data between bumblebees and honey bees, with honey bees on the x-axis and bumblebees on the y-axis on a logarithmic scale, it can be noted that there was no noticeable
correlation (Figure 5). Although there were observations with only honey bee visitors or solely bumblebee visitors, there were also many that had a mix of the two, showing no evidence of exclusion between the two insects. If the data was grouped near either the xaxis or the y-axis it would show that there is a relationship between bumblebees and honey bees.
Figure 5. Each point is an observation where there are a certain number of honey bees and a certain number of bumblebees. Points lying on the x-axis represent plants observed where there were 0 bumblebee visits. Points lying on the y-axis represent plants observed where there were 0 honey bee visits. All the values are +1, so the point (1,1) is where there were 0 observations of honey bees and 0 observations of bumblebees.
Examples of plants that honey bees preferred and bumblebees did not were Tetradium danielii, Fluegga suffruticosa, and Hydrangea paniculata. Tetradium danielii had 60 honey bees and 0 bumblebees in one observation, Flueggea suffruticosa had 100 honey bees and 0 bumblebees and Hydrangea paniculata had 75 honey bee visits and 0 bumblebee visits. Notable plants that bumblebees liked and honey bees did not prefer were Hypericum prolificum and Hypericum kalmianum. Hypericum prolificum had 80 bumblebees and 1 honey bee in one observation, while Hypericum kalmianum had 50 bumblebees and 1 honey bee. Plants that both honey bees and bumblebees often visited were Helianthus tuberosus and Pycnanthemum incanum. Helianthus tuberosus had 200 honey bees and 30 bumblebees in one observation and Pycnanthemum incanum had 100 honey bees and 20 bumblebees. There are also plants that neither honey bees nor bumblebees visited. Physocarpus opulifolius had 0 honey bees and bumblebees, but 40 other visitors. Physocarpus opulifolius was only observed once. Daucus carota had 0 honey bees and bumblebees, but 32 other visitors. Daucus carota was observed multiple times and consistently had no honey bee or bumblebee visitors.
iNaturalist
Observers gathered a collection of photos of flower visitors during observation. These photos were then saved for reference and/or were entered into iNaturalist. Inaturalist is a social network for sharing biodiversity information. During this research, iNaturalist was used lightly to identify uncommon insects and test the reliability of the network. From those involved in the project, there are currently more than 300 observations of pollinators at the Arnold Arboretum and 89 different species logged in iNaturalist.
Around 150 of these observations are research grade. The total number of observations made on plants and animals at the Arnold Arboretum, from all iNaturalist users, is over 11 thousand. Almost 6.5 thousand of these observations are research grade.
Historical Data from Otto Plath
In 1934, Otto Plath published his book “Bumblebees and Their Ways”. For the 13 bumblebee species he noted at the Arnold Arboretum, he recorded them visiting 36 plant taxa (species and genera) on the grounds. During our survey of flower visitors in 2024, four species of bumblebees were observed, and these species were visiting a total of around 219 taxa. Of the 36 plant taxa being visited during Plath’s time, 28 of them were observed in this study; the other species either did not flower this year or were in bloom before or after this study. Of these 28 plant species, 23 of them had bumblebee visitors this year, and five species had no bumblebee visitors this year: Arctium minus, Pieris, Ribes aureum, Ribes sp., and Salix sp. Most of these plant species that were reported by Plath to be visited by species are still present at the Arboretum, so the absence of bumblebees on these six taxa cannot be attributed to the extinction of certain bumblebee species. It is possible that plants were not observed at the right time of day or weather conditions for bumblebee activity. The fact that bumblebees were observed visiting about six times more taxa than Plath is likely attributed to the fact that more plants are growing on the grounds of the Arnold Arboretum than in Plath’s time. Recording flower visitors was the focus of this study’s efforts whereas for Plath this was not a priority, and this study recorded the species name for species within large genera such as Tilia and Rosa, whereas Plath just recorded them by their genus name.
Discussion
Our observations showed that on many plants, there were few insect visitors. We also often saw honey bees and native pollinators on the same plants, as well as different plants. Therefore, it can be concluded that there are abundant resources at the Arnold Arboretum and there is minimal competition among pollinators. There are many flower visitors at the Arnold Arboretum, which have different feeding preferences. There are many different situations at the Arnold Arboretum: there are a lot of flower resources available at the Arnold Arboretum with different flower densities, sizes, and colors for many flower visitors. There was extreme variability in the number of species observed day to day likely due to the weather or the plants being observed on a particular day. There was a surprisingly low number of butterfly observations.
The results suggest that it is not necessary for governments to place regulations on honey bee populations, at least not in the area around the Arnold Arboretum, where honey bees coexist with native pollinators.
The Arnold Arboretum has active public outreach, which is an opportunity to introduce the public to unfamiliar insects they might find interesting. There is potential for the creation of public activities where people are shown and taught about insects such as sculpted resin bees, carpenter bees, and carter bees which have interesting stories and are easy to identify.
There are also things to change and improve when future research is conducted. It would be valuable to track whether plants are being observed during their peak flowering, before, or after. The extent to which the plants are flowering can affect its attractiveness to visitors and tracking this can give more insight and understanding into the number of visitors observed on the plant. Another valuable piece of information to take note of during
future research is the amount of sunlight the plant is in. The amount of sunlight can be recorded as in sun, in shade, or partial shade. Typically, plants that are in the sun have more visitors, making sunlight a valuable factor in pollinator research. During this study, the types of bumblebees and the abundance of flowers were not recorded from the beginning but rather later on during the research period. It is important to record the types of bumblebees and the abundance of flowers from the beginning of the observation period. Future research also presents the opportunity to be more in-depth in identifying insects. Rather than putting insects into groups, as was done with native bees, identifying more species within these groups would be a valuable next step. This study also has the potential to extend to more places and can serve as a model for future research. Working with other botanical gardens, especially those where honey bees are native, would provide deeper insight into the relationships between insects in different regions. It would also be valuable to determine whether local people are keeping honey bee hives and where they are keeping them, to know where the honey bees are coming from. Protocol as to how to estimate the number of insects on varied plants, especially trees, could also be improved.
References
Angelella, G.M., McCullough, C., O’Rourke, M.E., 2021. Honey bee hives decrease wild bee abundance, species richness, and fruit count on farms regardless of wildflower strips. Scientific Reports, 11 (1). https://doi.org/10.1038/s41598021-81967-1.
Biesmeijer, J. C., Roberts, S. P. M., Reemer, M., Ohlemüller, R., Edwards, M., Peeters, T., . . . Kunin, W. E. (2006). Parallel declines in pollinators and insect-pollinated plants in Britain and the Netherlands. Science, 313(5785), 351–354. doi:10.1126/science.1127863
Gallai, N., Salles, J., Settele, J., & Vaissière, B. E. (2009). Economic valuation of the vulnerability of world agriculture confronted with pollinator decline. Ecological Economics, 68(3), 810–821.
doi:10.1016/j.ecolecon.2008.06.014
Goulson, D., Lye, G. C., & Darvill, B. (2008). Decline and conservation of bumble bees. Annual Review of Entomology, 53(Volume 53, 2008), 191–208.
doi:10.1146/annurev.ento.53.103106.093454
Hudewenz, A., & Klein, A. (2013). Competition between honey bees and wild bees and the role of nesting resources in a nature reserve. Journal of Insect Conservation, 17(6), 1275–1283. doi:10.1007/s10841-013-9609-1
Iwasaki, J.M., Hogendoorn, K., 2021. How protection of honey bees can help and hinder bee conservation. Current Opinion in Insect Science 46, 112-118.
Iwasaki, J.M., Hogendoorn, K., 2022. Mounting evidence that managed and introduced bees have negative impacts on wild bees: an updated review. Curr. Res. Insect Sci. 2: 100043.
Lindström, Herbertsson, L., Rundlöf, M., Bommarco, R., Smith, H.
G., 2016. Experimental evidence that honey bees depress wild insect densities in a flowering crop. Proceedings of the Royal Society. B, Biological Sciences 283 (1843), 20161641–20161641.https://doi.org/10.1098/rspb.2016.1641.
Mallinger, R.E., Gaines-Day, H.R., Gratton, C., 2017. Do managed bees have negative effects on wild bees?: a systematic review of the literature. PLOS ONE 12 (12), e0189268–e0189268. https://doi.org/10.1371/journal.pone.0189268.
Plath, O. E. (1934). Bumblebees and their ways. New York: Macmillan Co. Retrieved from https://bu.primo.exlibrisgroup.com/discovery/fulldisplay?d ocid=alma99195137080001161&context=L&vid=01BOSU _INST:BU&lang=en&search_scope=MyInst_and_CI_NoL ondon&adaptor=Local%20Search%20Engine&tab=Everyt hing&query=any,contains,%E2%80%9CBumblebees%20a nd%20Their%20Ways%E2%80%9D&sortby=rank&mode =Basic
Potts, S. G., Biesmeijer, J. C., Kremen, C., Neumann, P., Schweiger, O., & Kunin, W. E. (2010). Global pollinator declines: Trends, impacts and drivers. Trends in Ecology & Evolution, 25(6), 345–353. doi:10.1016/j.tree.2010.01.007
Prendergast, K. S., Dixon, K. W., & Bateman, P. W. (2021).
Interactions between the introduced European honey bee and native bees in urban areas varies by year, habitat type and native bee guild. Biological Journal of the Linnean Society, 133(3), 725–743. doi:10.1093/biolinnean/blab024
Rasmussen, C., Dupont, Y.L., Madsen, H.B., Bogusch, P., Goulson, D., Herbertsson, L., Maia, K. P., Nielsen, A., Olesen, J.M., Potts, S.G., Roberts, S.P.M., Sydenham, M.A.K., Kryger, P., 2021. Evaluating competition for forage plants between honey bees and wild bees in
Denmark. PloS One 16 (4), e0250056–e0250056. https://doi.org/10.1371/journal.pone.0250056.
Renner, S., Graf, M.S., Hentschel, Z., Krause, H., Fleischmann, A., 2021. High honey bee abundances reduce wild bee abundances on flowers in the city of Munich. Oecologia 195 (3), 825–831. https://doi.org/10.1007/s00442-02104862-6.
Ropars, Dajoz, I., Fontaine, C., Muratet, A., Geslin, B., 2019. Wild pollinator activity negatively related to honey bee colony densities in urban context. PloS One 14 (9), e0222316–e0222316. https://doi.org/10.1371/journal.pone.0222316.
Stout, J.C., Morales, C.L., 2009. Ecological impacts of invasive alien species on bees. Apidologie 40 (3), 388–409. https://doi.org/10.1051/apido/2009023 DE.
The importance of pollinators | USDA. Retrieved from https://www.usda.gov/peoples-garden/pollinators