Feature

Bee Hotels or Bates Motels?

The Need for Research in Guiding Pollinator Conservation Efforts

By Dr. Jennifer Tsuruda, Assistant Professor, UT Entomology and Plant Pathology Department, and Dr. Laura Russo, Assistant Professor, UT Ecology and Evolutionary Biology Department

Public awareness about the decline of bees and other pollinators in recent years has been impressive and ever-growing. Pollinators have a tremendous impact on the day-to-day lives of humans. About one-third of the food we eat has been directly influenced by the activities of animal pollinators, so the concerns of scientists and the public are understandable. Growing awareness about these problems is often accompanied by a desire to act and help conserve pollinators in our landscapes and production areas. The inputs from citizen action can outpace science-based predictions and recommendations and a good example involves bee hotels and native bee conservation.

What is a bee hotel?

Many people are familiar with social bees like honey bees and bumble bees. Less familiar are solitary bees that actually make up the majority of the ~3,500 native bee species found in North America. Rather than living within the same in colony structure, individual female solitary bees construct their own nest, sometimes adjacent to other solitary bee nests, yet each female provisions her own young without help from other bees, hence their “solitary” status. Approximately 70% of solitary bee species nest in the ground, while 30% nest above-ground in cavities such as stems, twigs, and abandoned tunnels in wood made by other insects. These cavities consist of a series of chambers, each provisioned with a ball of pollen mixed with nectar, and a single egg [Photo 1]. Chambers are sectioned off with partitioning material that varies with the type of bee – for example, leafcutter bees use chewed leaf tissue to separate chambers from one another [Photo 2]. After hatching from the egg, the larva consumes the founding female bee’s provisions and develops through multiple juvenile stages before pupating and emerging as an adult and leaving the nest.

Photo 1 – An inhabited tube or nesting cavity may contain one or a series of cells, each containing a single egg that is provisioned with a ball of pollen mixed with nectar. Cells can be sectioned off from each other with a partition made of mud, leaf or petal tissue, resin, chewed up wood pulp, or even ants (photo by C. D. Pless and G. Schweiger).

Photo 2 – By providing hollow stems with different diameters, and that are sealed at one end, a greater diversity of bees can occupy the bee hotel. Different species of bees and wasps require different materials to separate and seal their nesting tunnels. Here, chewed up green leaf tissue, dried grass, and mud seals can be seen, along with a returning leafcutter bee with leaf tissue in her mandibles (photo by Laura Russo).

A “Bee Hotel” is constructed with human-sourced or modified nesting aids that are designed for solitary stem-nesters such as leafcutter bees and mason bees [Photo 3]. These types of solitary bees are highly valuable pollinators in agricultural, urban, and natural landscapes [Photo 4]. Bee hotels [Photo 5] are commonly built as bundles of tubes or in the form of a wooden block with horizontal, drilled holes. Holes do not penetrate all the way through the back side of the block. When tubular stems are used, one end of each within a stem bundle is cut below the nodes so there is only one open end. When cardboard or paper straws are used, they are folded so that only one end is open. Stems, tubes, or paper straws are then tightly strapped or packed together in a protective “house” structure and positioned horizontal to the ground.

Nest designs and characteristics can influence the utilization by certain bees. For example, small cellophane bees will utilize nests with 1–4 mm diameter holes, whereas mason and leafcutter bees are more likely to use holes with 5–8 mm diameters 1 [Photo 3]. Additionally, a study on mason bees showed that more females were provisioned in a light blue nest box versus yellow or orange-colored boxes. Nest box placement can also influence colonization and success. Blocks and bundles are often housed within a structure or stand that has an overhang at the top, which helps protect from rain [Photo 5]. These larger structures should be placed in a location with limited wind exposure, facing south or southeast, and secured to prevent movement and risk of tipping over. Chicken wire or other wire mesh placed in front of the tunnels can help prevent predation by birds and small mammals.

Photo 3 – Bee hotels are visited by several types of insects. Three bee examples are shown here: Osmia bucephala (bufflehead mason bee, top), Anthidium manicatum (European wool carder bee, an introduced species, bottom left), Megachile sp. (leafcutter bee, bottom right) (photos by L. Russo).

Photo 4 – It is easy to see why solitary bees are such effective and valuable pollinators. This bee, in the family Megachilidae, was collected as she returned to a bee hotel. For these bees, pollen grains collect in a dense patch of stiff hairs on the underside of the abdomen rather than on their legs like honey bees or bumble bees (photo by J. Tsuruda).

Photo 5 – Example of a bee hotel with an overhang to protect from rain. The bound, wooden trays can be separated to allow for collection of cocoons and to facilitate cleaning, while the cardboard tubes in the “attic” can be replaced each year (photo by J. Tsuruda).

How are bee hotels used?

Artificial nests for solitary bees have been used for several decades. In agriculture, nesting structures have been used to increase solitary bee abundance for pollination services to crops such as alfalfa, almonds, and blueberries. Another use is to assess bee diversity and abundance across landscapes. By placing standardized nesting blocks in different areas, researchers can learn about local bee populations, as well as assess potential impacts introduced by environmental factors. Researchers also use nest boxes to acquire bees for experiments in other settings, and to better understand the biology, development, and behaviors of native bees. More recently, bee hotels have been used for educational outreach as a tool to increase interest and engagement in bee conservation.

Issues and challenges introduced by increased use of bee hotels

One fundamental concern about bee hotels is the mindset that merely setting up a nest box and leaving it untended will translate into providing true benefits to native bees. In reality, research studies have revealed complexities that are inherent with bee hotel attractiveness, utilization by solitary bees (and their parasites), and performance. These factors contribute to whether native bee abundance and diversity actually are enhanced. A few multi-year studies provide some guidance that help to inform likely impacts that can accrue with time. Below are some of the concerns that have been raised.

Unintended (or Unexpected) Guests: Residence by non-target arthropods

Native bees are not the only insects that utilize bee hotels [Photo 3]. MacIvor and Packer 2 conducted a three-year study of bee hotels in Canada with surprising results. They recorded use by native and introduced bee and wasp species and found that only 27.7% of all the bees and wasps reared were native bees. Roughly the same number of introduced bees were reared, and native wasps were the most abundant hotel residents. In a similar study from California 3 , native bees and wasps (combined) never occupied more than 25% of the nesting cavities – in their study, the most abundant inhabitant was the European earwig!

Would You Stay at an Unmaintained Hotel? Parasites and Parasitoids are Happy To

Parasites can be a particular challenge and significantly more native bees may become parasitized in bee hotels compared to introduced bees 2 . Mites, flies, and parasitoid wasps all are known to afflict bee hotels [Photos 6 and 7]. The close proximity of nesting sites is not natural for non-gregarious species, so that the likelihood increases of a parasite spreading across multiple nest sites. Successful bee hotels are not simply “set it and forget it” operations. Regular maintenance and cleaning of drilled wood blocks is necessary, but can be difficult. Tightly bound wooden trays can be opened for easier cleaning, yet gaps between trays can allow entry of parasites and parasitoids. Disposable cardboard tubes and stems/reeds are beneficial in that they can be regularly replaced, reducing risk.

Photo 6 – A highly occupied hotel provides a concentrated opportunity for parasitoids. Here, an ichneumonid wasp is laying eggs in a bee hotel (photo by Martin Cooper Ipswich, CC-BY-2.0).

Photo 7 – Here, a photo from Europe shows a red mason bee cocoon (Osmia bicornis) covered with mites that look like pale sandy particles. The cream-colored parasitic Houdini fly pupae (Cacoxenus indagator) can also be seen in this opened nesting tunnel from a bee hotel. The parasitic fly larvae do not directly attack the developing bee. Instead, the fly larvae compete with the bee larva for pollen provisions, which can result in the bee starving. Houdini flies occur on both coasts of the United States. For health of bee hotel residents, open and inspect nesting materials to help curb the spread of this parasite species (photo by orangeaurochs, CC-BY-2.0).

One Size Does Not Fit All: Issues with Hotel Design

Bee hotels are often commercially advertised as a simple way to help bees; however, some design features may not align well with this goal. Tunnel dimensions can facilitate unintended results. Models providing large diameter cavity openings can attract non-target bees like non-native giant resin bees. Additionally, less desirable diameters and shorter tunnel lengths can result in increased production of male bees – males do not collect pollen. A recent comparison of commercial and custom bee nesting aids revealed that commercial models often are less occupied, may host fewer developing bees and wasps, and may support less species diversity 4 . The type of nesting material, diameter/length combinations, and smooth entrance surfaces all were factors in attracting more bees and wasps representing a greater diversity. Additional research is needed to help inform options for improving and optimizing commercial bee motel models.

Promoting conservation and native bees

While questions remain about whether bee hotels ultimately are helpful and provide lasting benefits for pollinator conservation purposes, these resources are undeniably enjoyable to watch and are a practical way to engage the public and increase awareness about solitary bees, other beneficial insects and parasites that share our environments, and the importance of habitat for conservation. An extended duration of seasonal floral species and abundance is needed to provide the pollen and nectar for provisioning the diverse assemblage of bee and wasp species that can be reared as bee hotel residents. Increased forage also can help alleviate competition and limit disease transmission between bees. A healthy and complex habitat can also provide resin, leaf tissue, and natural nesting sites for solitary bees. Simple examples can include leaving stems intact over the winter, followed by an early spring cutting of stems from otherwise healthy plants that have hollow or soft, pithy centers (like Joe Pye weed, cane berries, and sumac). These stems can be pruned down to 8–24 inches and dead (but healthy) twigs and stems can be left to remain in the landscape. These hollow or easily excavated stems left on the plant and in the landscape provide sites for stem-nesting bees in the spring, and the developing bees will emerge the following spring. Leaving patches of exposed and moistened soil can provide important resources used by ground-nesting bees. Together, with more research, we will learn how to better follow and supplement natural landscapes to attract, feed, and nurture native bee populations.

BEE HOTEL RESEARCH BEING CONDUCTED by the Russo Lab at the University of Tennessee

“Katie Wood is an undergraduate research student in my lab who is interested in comparing the bee hotels that she has seen in stores to a “professional” mason bee hotel. To evaluate these models, we set up four types of bee hotels that range in size and style at four locations in eastern Tennessee. We standardized the color of all the hotels and all are oriented to face south. This step is essential for limiting effects of these external variables. For all the hotels, we have measured nest depth, number of available nesting tubes, and uniformity versus variation in nest opening widths. We expect nest depth to be one of the primary predictors determining the preferences of nest cavity resources chosen by stem-nesting bees. Katie will monitor the nests throughout the spring and summer seasons in 2022 to assess and learn how the bees respond.”

– Dr. Laura Russo

REFERENCES and ADDITIONAL READING:

1. MacIvor JS (2017) Cavity-nest boxes for solitary bees: a century of design and research. Apidologie 48:31. doi: 10.1007/s13592-016- 0477-z

2. MacIvor JS, Packer L (2015) ‘Bee Hotels’ as Tools for Native Pollinator Conservation: A Premature Verdict? PLoS ONE 10(3): e0122126. doi: 10.1371/journal.pone.0122126

3. Barthell JF et al. (1998) Invader Effects in a Community of Cavity Nesting Megachilid Bees (Hymenoptera: Megachilidae). Environ Entomol 27:240. doi: 10.1093/ee/27.2.240

4. von Königslöw V et al. (2019) Benchmarking nesting aids for cavity-nesting bees and wasps. Biodivers Conserv 28: 3831. doi: 10.1007/ s10531-019-01853-1

5. Requier F, Leonhardt SD (2020) Beyond flowers: including non-floral resources in bee conservation schemes. J Insect Conserv 24:5. doi: 10.1007/s10841-019-00206-1

6. Turo KJ, Gardiner MM (2019) From potential to practical: Conserving bees in urban public greenspace. Front Ecol Environ 17:1. doi: 10.1002/ fee.2015

7. Washington State Department of Agriculture (Feb 2020) Houdini fly pest alert. https://cms. agr.wa.gov/WSDAKentico/Documents/PP/ PestProgram/PestAlert-HoudiniFlyFeb2020.pdf