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‘Bee Hotels’ as Tools for Native Pollinator Conservation: A Premature Verdict?

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Society is increasingly concerned with declining wild bee populations. Although most bees nest in the ground, considerable effort has centered on installing 'bee hotels'-also known as nest boxes or trap nests-which artificially aggregate nest sites of above ground nesting bees. Campaigns to 'save the bees' often promote these devices despite the absence of data indicating they have a positive effect. From a survey of almost 600 bee hotels set up over a period of three years in Toronto, Canada, introduced bees nested at 32.9% of sites and represented 24.6% of more than 27,000 total bees and wasps recorded (47.1% of all bees recorded). Native bees were parasitized more than introduced bees and females of introduced bee species provisioned nests with significantly more female larva each year. Native wasps were significantly more abundant than both native and introduced bees and occupied almost 3/4 of all bee hotels each year; further, introduced wasps were the only group to significantly increase in relative abundance year over year. More research is needed to elucidate the potential pitfalls and benefits of using bee hotels in the conservation and population dynamics of wild native bees.
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RESEARCH ARTICLE
Bee Hotelsas Tools for Native Pollinator
Conservation: A Premature Verdict?
J. Scott MacIvor*, Laurence Packer
Biology Department, York University, Toronto, Ontario, Canada
*jsmacivor@gmail.com
Abstract
Society is increasingly concerned with declining wild bee populations. Although most bees
nest in the ground, considerable effort has centered on installing bee hotels’—also known
as nest boxes or trap nestswhich artificially aggregate nest sites of above ground nesting
bees. Campaigns to save the beesoften promote these devices despite the absence of
data indicating they have a positive effect. From a survey of almost 600 bee hotels set up
over a period of three years in Toronto, Canada, introduced bees nested at 32.9% of sites
and represented 24.6% of more than 27,000 total bees and wasps recorded (47.1% of all
bees recorded). Native bees were parasitized more than introduced bees and females of in-
troduced bee species provisioned nests with significantly more female larva each year. Na-
tive wasps were significantly more abundant than both native and introduced bees and
occupied almost 3/4 of all bee hotels each year; further, introduced wasps were the only
group to significantly increase in relative abundance year over year. More research is need-
ed to elucidate the potential pitfalls and benefits of using bee hotels in the conservation and
population dynamics of wild native bees.
Introduction
Bees and the pollination services they provide are in decline as a result of various anthropogen-
ic activities that undermine bee foraging and nesting [1,2,3,4]. Concern for bees among the
general public has led to increases in the numbers of novice beekeepers in urban centers [5]
and augmentation of habitat for bees including the addition of both food (bee-friendly plants)
[6,7] and nest sites (bee hotels) [8]. The marketing of bee hotels to promote pollination and
wild pollinator conservation is widespread and expanding, at least in North America and Eu-
rope [9]. These structures, also known as trap-nests or nest boxes [10], use some bees prefer-
ences for nesting in above-ground cavities as arise naturally in a variety of settings such as
pithy stems and beetle burrows in wood [11,12]. Bee hotels are usually made from bundled
plant stems, paper-based tubes, or holes drilled in wood or molded in plastic; in all cases they
artificially aggregate nesting sites above densities naturally available for cavity-nesting bees
[10](Fig. 1A-C).
PLOS ONE | DOI:10.1371/journal.pone.0122126 March 18, 2015 1/13
a11111
OPEN ACCESS
Citation: 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
Academic Editor: Fabio S. Nascimento,
Universidade de São Paulo, Faculdade de Filosofia
Ciências e Letras de Ribeirão Preto, BRAZIL
Received: October 4, 2014
Accepted: February 7, 2015
Published: March 18, 2015
Copyright: © 2015 MacIvor, Packer. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are
credited.
Data Availability Statement: Data cannot be made
publicly available due to ethical restrictions that
protect the privacy of the bee hotel owners. Data are
available upon request by contacting
jsmacivor@gmail.com.
Funding: JSM received funding from a Natural
Science and Engineering Council of Canada (CGS D
408565) (http://www.nserc-crsng.gc.ca/index_eng.
asp). LP received funding from a Natural Science and
Engineering Council of Canada Discovery Grant
(http://www.nserc-crsng.gc.ca/index_eng.asp). The
funders had no role in study design, data collection
Bee hotel development began in the 1950s when paper straws and wooden blocks with holes
drilled into them were experimentally set out to house the alfalfa leaf cutter bee [Megachile
rotundata (Fabricius)] in transportable and stackable containers [10]. At that time, farmers
from Utah to Saskatchewan were encouraging this exotic species to nest in holes they had
drilled into the sides of their own buildings [13]. Over the ensuing decades there has been an
increasing diversity of designs available for purchase as ready-mades or through DIY instruc-
tions (e.g. http://www.xerces.org/). In agricultural settings, a variety of mason bees, in addition
to the alfalfa leafcutter bee, have been managed successfully using bee hotels [13,14,15]. These
easily manipulated structures have also been used for ecological research [16,17,18,19]. Promo-
tion of bee hotels in urban gardening as a means of supporting native pollinators is a more re-
cent phenomenon. Here we investigate whether they do indeed support native pollinators
rather than introduced ones or other organisms entirely. Specifically, we test the
following hypotheses.
1. Compared to native bee species, introduced ones are more common in bee hotels. Intro-
duced species often exhibit greater flexibility in habitat requirements [20,21], allowing them
to colonize new environments; bee hotels may constitute such a novel environment.
2. Wasps (such as many solitary Vespidae) that seek out the same nesting cavities will be more
common than native bees in bee hotels because wasps use widely available nesting materials
Fig 1. Bee hotels. A. Bee hotel on a rooftop in London, UK (Photo: Thierry Spiess). B. Cartridge-style hotels made by bundlingwood (left) or plastic (right)
cartridges having drill holes along one edge for opening, inspecting and cleaning. C. Bee hotel having different nesting tube widths made of cardboard and
enclosed in a PVC pipe for protection (Photo: Ed Snodgrass). D. Ant colony (Tetramorium caespitum) that took over an unmaintained bee hotel. E. An
ichneumonid wasp parasitizing Osmia sp. through a cardboard nesting tube. F. Damage to the faceplate and nestingtubes in a bee hotel by an unknown bird.
doi:10.1371/journal.pone.0122126.g001
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and analysis, decision to publish, or preparation of
the manuscript.
Competing Interests: The authors have declared
that no competing interests exist.
to partition their nests (e.g. mud and grass) whereas bees use more site specific materials
(e.g. tree resins and leaves of certain plants) [10,11,22].
3. Introduced species will be more common in bee hotels located in areas that are most heavily
anthropogenically-modified. This is expected because recent studies that investigate urban
insect diversity find introduced species to be the dominant taxa [23,24].
4. Compared to native species, introduced ones will have decreased rates of parasitism. This is
a test of whether the enemy release hypothesis [25] applies to bees that nest in bee hotels. In
bee hotels, parasitism is greater compared with natural nesting sites [26] in part because ag-
gregated nests create an easier search target for parasites [27]. This may exacerbate the dif-
ferences in parasitism rates between native and exotic species.
If hypotheses 1, 3 and 4 were to be supported we could suggest the following two additional
hypotheses:
5. Introduced bees will show a greater increase in bee hotel use over time from year to year.
6. Introduced bees will exhibit greater population increase (expressed as number of females
per nest tube) than native species.
We test these hypotheses with 200 bee hotels set up annually for each of three years within
the city of Toronto, Canada. We test the first five hypotheses using all bees and wasps detected;
the fifth and sixth were explored using two congeneric pairs of the commonest species found,
in each case one member of the pair was introduced, the other native.
Methods
From May to October 20112013, 200 bee hotels were set up each year throughout the Toronto
area (each bee hotel representing one site) to survey above ground nesting bees [19]. The ma-
jority of sites were sampled all three years (73.7%), 16.9% were sampled over two years, and
9.4% were sampled in just one year. The bee hotels were made from 10cm-diameter, 28cm long
white PVC piping, with a circular faceplate made of insulation board into which 30 cardboard
nesting tubes (10 of each of three tube diameters; 3.4mm, 5.5mm, 7.6mm; all 15cm in length)
could be mounted (Fig. 1; see [19] for more detail). A bee that uses the hotel enters a suitable
cardboard tube (the one that best fits her body dimensions) and constructs brood cells in a se-
ries from the back of the tube to the front [10,11,28]. Bee hotels were set up individually at sites
at least 250m apart, in four different urban green space types: community gardens, residential
gardens, city parks, and building rooftops. Permission was granted to set up at each site after
meeting with individual site managers or homeowners to discuss the research.
At the end of each field season, the bee hotels were collected, each cardboard tube opened
and each brood cell removed, individually labeled and placed in storage to overwinter at 4°C.
In April of the following year brood cells were moved to a sealed incubation chamber kept at
26°C and 60% humidity until adult emergence. They were then sexed and identified to species,
permitting categorization of each individual as native or introduced to the study region
[29,30,31]. All bees and wasps are stored at the Packer Collection at York University (PCYU).
Over all sites and years, colonization (determined as presence in a bee hotel) and relative
abundance (the proportion of all brood cells that were of the focal species per bee hotel)
were compared between native and introduced bees (NB vs IB), native and introduced wasps
(NW vs. IW), as well as among all four groups using linear regression analysis (GLM) (α=
0.05) with a Tukey post hoc analysis in SPSS v21 (all analyses described hereafter used this
program). Colonization and relative abundance of native bees (NB) were also compared with
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all potential competitors of native bees for nesting opportunities in bee hotels (introduced
bees and introduced and native wasps grouped together; hereafter referred to as AO,asin
all others) using a paired t-test. The same GLM test was used to determine whether coloni-
zation or relative abundance between native and introduced bees and wasps differed by
site type.
The total number of parasites attacking bee and wasp brood were recorded by site, and the
parasites reared and identified as accurately as possible using standard morphological ap-
proaches combined with DNA barcoding [32]. The total parasitism rate combining all brood
cells over all three years were compared separately as before between native and introduced
bees and wasps using GLM analysis with Tukey post hoc testing to distinguish between differ-
ent bee and wasp groups. Although the bees and wasps we sampled were not released back to
the site from where they were collected, to examine patterns in use over time, the abundance of
each group per site were independently examined over the three years using a repeated mea-
sures ANOVA with data from the first year of sampling acting as a baseline for comparison.
This was completed only for the sites sampled in all three years (N = 147).
We compared the sex ratio, as well as an estimate of the rate of increase in population size
of the four most common bee species over the three-year study period. The most common bees
were two native [Osmia pumila Cresson, Megachile campanulae (Robertson)] and two intro-
duced species [O.caerulescens (L.) and M.rotundata](Table 1). The estimate of population in-
crease for each species was determined by comparing the number of female offspring
provisioned by nesting females in 30 individual nests of the same nesting tube width dimension
(1 per site; 10 sites selected randomly among those colonized by the species in all three years).
The sex ratio (recorded as the proportion of females per nest) and the estimate of population
increase over the study period were independently compared for two pairs of bee species using
GLM testing and post hoc analysis.
Results
Of 600 bee hotel/years set up, data were obtained from 574 (186 were recovered in 2011, 194 in
2012 and 194 in 2013). We found a total 27,275 individuals including 31 species of pollinating
bees (comprising 52% of all cavity-nesting bee species known from the area [29]) and an addi-
tional five cleptoparasitic bee species (36 bee species total) (Table 1). Ten of the species we
found were not native to the region, representing 76.9% of the known introduced cavity-
nesting bee fauna in southern Ontario (Table 1). The offspring generations of two introduced
species: O.caerulescens and M.rotundata were particularly common; representing 20.7% and
15.4% respectively of the total number of bees reared over the entire study period.
There was no significant difference between native and introduced bees in the number of
sites occupied (Fig. 2A), with introduced bees nesting in bee hotels at an average of 32.9% of
sites per year (native bees: 39.8%). Native bees colonized significantly fewer sites than did all
other groups combined (AO: 70.5%) (Fig. 2B).
There was no significant difference in the relative abundance of introduced and native bees
reared from bee hotels (Fig. 2C); introduced bees represented 47.1% of the total number of
bees reared (56.9% for native bees), and 24.6% of all bees and wasps reared (27.6% for native
bees). However, the relative abundance of native bees was significantly less (t = 9.239,
p<0.001) than that of all competing groups combined (AO: 72.4%) (Fig. 2D). Native wasps
were significantly more abundant than any other group (Fig. 2A;F
3
= 20.46, p<0.001) and
comprised 37.8% of all bees and wasps reared from bee hotels.
The type of urban green space was a significant determinant of the abundance of native bees
(F
3
= 5.369, p = 0.001, greatest in residential gardens), introduced bees (F
3
= 4.511, p = 0.004,
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greatest on rooftops, and to a lesser extent community gardens) and native wasps (F
3
= 5.880,
p = 0.006, greatest in urban parks), but not of introduced wasps (Fig. 3). Significantly more na-
tive bees were parasitized compared to introduced bees (t = 13.904, p<0.001) (Fig. 4), although
parasitism rates did not differ between introduced and native wasps.
Table 1. List of all bee species recorded in the study area per year (Y: yes; N: no).
Family Genus Species 2011 2012 2013
Apidae Anthophora terminalis Cresson N N Y
Megachildae Megachile brevis Say N Y N
campanulae (Robertson) Y Y Y
centuncularis (Linnaeus)*YYY
frigida Smith N Y Y
inermis Provancher Y N N
mendica Cresson Y Y N
pugnata Say Y Y Y
relativa Cresson Y Y Y
rotundata Fabricius Y Y Y
sculpturalis Smith N N Y
Heriades carinata Cresson Y Y Y
variolosa (Cresson) Y N N
Chelostoma campanularum (Kirby) N Y Y
rapunculi (Lepeletier) Y N Y
Hoplitis producta (Cresson) Y Y Y
spoliata (Provancher) Y Y N
truncata (Cresson) N N Y
Osmia pumila Cresson Y Y Y
caerulescens (Linnaeus) Y Y Y
lignaria Say Y Y Y
atriventris Cresson N Y N
Anthidium manicatum (Linnaeus) Y N Y
Coelioxys alternata Say
+
YNN
moesta Cresson
+
NYN
sayi Robertson
+
YYY
Stelis lateralis Cresson
+
YYN
vernalis Mitchell
+
NYN
Colletidae Hylaeus afnis Smith Y Y Y
annulatus (Linnaeus) Y Y Y
hyalinatus Smith N Y Y
leptocephalus Morawitz N Y Y
mesillae (Cockerell) Y N Y
modestus Say Y Y Y
punctatus Brullé Y Y N
verticalis (Cresson) N Y N
Bolded species are introduced to the study region. Missing introduced cavity-nesting bees known from the region included: Anthidium oblongatum (Illiger),
Hoplitis anthocopoides (Schenck), and Megachile ericetorum Mitchell.
*M.centuncularis status is not clear, with Giles and Ascher (2006) denoting the species as introduced.
+
denotes species that are cleptoparasites.
doi:10.1371/journal.pone.0122126.t001
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Repeated measures analysis showed that there was no significant change in relative abun-
dance of native or introduced bees or native wasps year-over-year; however, there was a signifi-
cant increase in introduced wasps (F
3
= 6.555, p<0.001) (Fig. 2C).
Finally, the sex ratio as determined by the number of females provisioned per preferred
nesting tube width in the two pairs of native and introduced bees species was significantly
more skewed towards females in introduced than native bees (F
3
= 28.683, p = 0.033) (Fig. 5A).
This trend was driven by one native, O.pumila, which provisioned, on average, half as many fe-
males as the other native (M.campanulae) per brood cell. Osmia pumila was the only bee
among the four to prefer the 3.4mm nesting tube width (73.1% of all nest tubes occupied and
77.1% of all brood produced). The average number of female offspring provisioned per female
did not change significantly for any of the four species over the three years of study (F
3
= 0.738,
Fig 2. Presence and abundance of bees and wasps over all sampling years. A. The number of sites occupied by native bees (NB), introduced bees (IB),
native wasps (NW) and introduced wasps (IW) over three years at over 600 bee hotels set up through out the city of Toronto and the surrounding region. B.
Comparison of the number of sites occupied by NB and the other groups competing for nesting space combined (AO). C. The total number of brood cells
produced in bee hotels per year by native and introduced bees and wasps, and D. shows a comparison between native bees and the remaining groups
combined. Lower-case lettering indicates significant differences and in all graphs hereafter.
doi:10.1371/journal.pone.0122126.g002
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p = 0.481). However the estimate of the rate of population increase differed significantly
among species with both introduced bees (M.rotundata,O.caerulescens) and one native
(M.campanulae) provisioning significantly more female offspring per nesting female com-
pared to native O.pumila (F
3
= 25.636, p<0.001) (Fig. 5B).
Discussion
We investigated the relative use of bee hotels by native and introduced bees and wasps to assess
the potential of these novel habitat augmentation schemes for increasing populations of native
bees. Several lines of evidence suggested that native bees performed comparatively poorly.
First, although there was no difference in the abundance or colonization of bee hotels by in-
troduced and native bees, native bees were in the minority, representing 27.7% of all bees and
wasps reared (AO + NB). Thus, our hypothesis that introduced bees would use the hotels more
often than native bees was rejected. This result was similar to that found in a study in California
where native bees or wasps never amounted to more than 25% of bee hotel occupants over two
years [21]. Grouping all potential competitors of native bees for nesting opportunities in bee
hotels (AO), we found that their colonization rate and abundance was greater than that of na-
tive bees. Native wasps were significantly more abundant than native and introduced bees, and
so our second hypothesis, that wasps could outcompete bees for these nesting structures was
supported. Our third hypothesisthat site type, as determined by the type of urban green
space where the bee hotel was installedwould influence the relative abundance of native bees
was supported. Bee hotels in residential gardens had significantly more native bees (e.g. [33])
while more anthropogenically-modified sites (e.g. vegetated rooftops) supported significantly
higher numbers of introduced bees (Fig. 3).
Our fourth hypothesis was that introduced bees would be parasitized less often than native
bees. This pattern was evident when all years were combined (Fig. 4) and so we accepted our
Fig 3. Mean relative abundance of bees and wasps at all site types over all years. Lower-case lettering indicates significant differences.
doi:10.1371/journal.pone.0122126.g003
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fourth hypothesis. From the repeated measures analysis and using the first year of abundance
data per sites as a baseline for comparison, no significant difference in changes in abundance
was evident from year to year for native or introduced bees, and we rejected our
fifth hypothesis.
Our sixth hypothesis that introduced bees would exhibit greater population increase than
native bees was partially supported. Our most common native bee, O.pumila, provisioned sig-
nificantly fewer females per nest than did either of our two introduced bees (Fig. 5). Osmia
pumila preferred smaller diameter nesting tubes than did our other three species and because
males are smaller than females use of smaller diameter tubes is expected to result in a more
male-biased sex ratio. For example, an increasingly male biased sex ratio was reported for
Osmia lignaria in smaller sized nesting tube diameters [34]. To check whether reduced female
Fig 4. The proportion of parasitized brood cells from all sites and years combined.
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Fig 5. Number of females provisioned in nests by the most common native and introduced bees. A. Differences in the mean number of females in
nests of four bee species [two introduced: Osmia caerulescens (OC), Megachile rotundata (MR); and two natives: Osmia pumila (OP), Megachile
campanulae (MC)]. B. Estimates of the rate of population increase in those same bees as determined by the number of female offspring provisioned per
individual nesting female over three years. Lower-case lettering reflects significant differences.
doi:10.1371/journal.pone.0122126.g005
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production by O.pumila might have been an artefact of tunnel width preferences we looked at
its sex ratio in tubes of both diameters. In 3.4mm tubes, 7.9% of the brood was female whereas
in 5.5mm tubes 58.6% were females giving a total of 19.5% female overall in the population of
O.pumila. The other three species preferred to nest in the 5.5mm nesting tubes (M.campanu-
lae = 81.3% of all brood reared, M.rotundata = 60.2%, O.caerulescens = 77.8%).
Altogether, our study findings show that bee hotels appear to differentially augment popula-
tions of wasps rather than those of native bees, and introduced bees outperform at least some
native bee species in some population parameters in bee hotels and in some urban green space
types. These results highlight a need for increased study of bee hotels and their associated im-
pact upon bee biodiversity and pollination in the urban setting.
One reason bee hotels are promoted is their potential for augmenting pollination of native
plants [35] and/or crops [36]. However, introduced pollinators, which in this study represented
almost half of all bees reared, are often the dominant or sole pollinator(s) of introduced plants
[37,38,39], whereas, native bees prefer native plants to alien ones [40].
At their worst, bee hotels may act as population sinks for bees through facilitating the in-
crease of parasites, predators (e.g. Fig. 1D-F), and diseases as a result of functional responses to
unnaturally high nest densities and nesting site entrances set up in two-dimensions rather than
in the more three dimensional arrangement found in nature (e.g. erect plant stems, decaying
logs) [26]. Bee hotels may be designed to encourage different bee species by varying nesting
tube/hole width or length, but encouraging different bee species to co-aggregate in a bee hotel
might inadvertently increase opportunity for parasites to attack related species: developing
novel hosts or affect more susceptible species [41]. Although there has been little discussion of
parasite loads obtained with different bee hotel designs [11] in all cases where nesting sites and
nesting bees are aggregated, the chance of parasites finding and attacking nests is increased
[27]. Some bee hotels have thin-walled nest tubes that facilitate parasite transfer within the
hotel, even by parasitic insects with short ovipositors such as generalist Monodontomerus
wasps [42]. This can result in mortality of entire hotel contents [11]. The relative influence of
host aggregation was not examined in this study, however we did find a significant increase in
the total number of parasites attacking native bees compared to introduced ones. These find-
ings might have resulted from enemy release among introduced bees, which were free of spe-
cialist parasites that attack them in their native ranges [25].
Given bee hotels could have a negative or a positive impact on their target organisms, an ob-
vious question is: How can designs be modified to promote the desired outcomes? Finding an-
swers to this would involve increased research on the parameters that vary among different bee
hotel designs and their relative success at promoting native bees. This could include studies
that manipulate the number, positioning/location, and materials used in bee hotel construc-
tion. The impact of maintenance might include replacement of completed nesting tubes with
new unoccupied ones to reduce within-season competition for nest sites. Matching the length
or especially the width [11,13] of nesting tubes in bee hotels to that of preferred plant stems
and beetle-bored holes in wood could reveal parameters that increase attractiveness to specific
native bees, reduce rates of parasitism, and/or increase the number of females provisioned per
nest (e.g. [34,43]). For example, a nest tube diameter between 3.4mm and 5.5mm would seem
to be necessary for population increase in O.pumila.
Bee-washing: A Call for Research
We advocate for due diligence on the part of retailers and promoters of bee hotels to avoid
bee-washing; that is, green-washing [44] as applied to potentially misleading claims for aug-
mentation of native and wild bee populations. To ensure bee-washingis minimized, it is
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imperative that more research be performed on the design and effectiveness of bee hotels. Bee
hotels are useful for ecological and behavioral studies, outreach in citizen science and pollinator
education campaigns. Sampling with them can even reflect the diversity of the larger bee com-
munity (e.g. including bees that nest in the ground [45]). However the magnitude of potential
pitfalls noted above needs to be assessed through continued study, especially of the impact of
hotels on native bee population dynamics. Such work would also provide detailed data on the
pollen and nesting resources used, parasite associations, sex ratios, and behaviors.
Comparing nesting success in bee hotels with that at naturally occurring nesting sites
[46,47] could improve the effectiveness of this management tool and permit its integration into
landscape planning practices targeting conservation. Specifically, through better designs mod-
eled after natural conditions both in materials used and details of positioning in the environ-
ment. At present, some bee hotels marketed for consumer use may act as sinks for their target
organisms through provision of entirely inappropriate edaphic conditions as a result of the ma-
terials used. For example, some designs are simply holes drilled (or molded) into solid plastic
blocks. It seems highly improbable that these designs will provide the same moisture balance as
occurs in nature and increased moisture retention likely leads to increased brood mortality due
to mold, which can represent a large proportion of total brood mortality [48]. Set up and orien-
tation could also be linked to attractiveness to native bees, especially versus wasps (e.g. [49]);
for example, solitary wasps can be more prevalent in bee hotels placed in shaded conditions
[22]. These wasps may provide important services to urban gardeners as predators of pests
[50], but compete with bees for nesting space in bee hotels. This suggests the need for a deeper
understanding of the relative importance of the pollination augmentation versus pest control
potential of bee hotels. In sum, we advocate for more research and increased responsibility on
the part of retailers and advocates of bee hotels so that these structures are designed and man-
aged to minimize negative effects and become truly useful tools for conservation biologists and
conservation-minded citizens.
Acknowledgments
We thank all of the homeowners and site managers for allowing us to set up and monitor bee
hotels. We also thank Alana Pindar for commenting on the manuscript, Baharak Salehi, Jenni-
fer Albert, Jennifer Cabral, Veronica Ladico, Armando Ponte, and Sheila Dumesh for help sort-
ing bees and wasps reared, and Ally Ruttan for contributing to the data on Megachile
rotundata.
Author Contributions
Conceived and designed the experiments: JSM LP. Performed the experiments: JSM. Analyzed
the data: JSM. Contributed reagents/materials/analysis tools: JSM. Wrote the paper: JSM LP.
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Green-Washing Pollinator Habitat
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... Furthermore, understanding nesting requirements is crucial for conservation initiatives within these highly altered systems, given the extent to which native vegetation is often removed and the floral landscape simplified (Landis, 2017). One way to augment wild pollinator nesting habitat, which has recently grown in popularity, is the installation of trap-nests, otherwise known as bee hotels (MacIvor & Packer, 2015). ...
... Trap-nests are typically artificial nesting structures made from wood, bamboo, reeds, paper straws, rammed earth, sand, and cement mix or plastic, designed to attract cavity-nesting semi-social or solitary bees. Studies have shown that trap-nests can also provide a nesting or habitat resource for various other types of insects, including wasps (MacIvor & Packer, 2015;Staab et al., 2018) although knowledge of the degree to which crop pollinators or insects more broadly use trap-nests in agroecosystems is limited. The utilization of trap-nests by invertebrates is influenced by many factors, including their innate nesting or habitat preferences, for example, wood, stems, or compact sand or soil. ...
... The capacity for trap-nests to support a diverse invertebrate assemblage has also been shown in a study of 600 trap-nests over 3 years in Toronto, Canada, by MacIvor and Packer (2015). These authors found that native wasps were the most abundant insect group using the trap-nests, occupying 75% of these each year and far outnumbering native and introduced bees. ...
Article
Supporting and promoting invertebrate diversity within agricultural ecosystems has numerous benefits, including the provision of pollination services. Many insects, including wild pollinators, require floral resources for food and structural habitat for nesting. To support pollinators, research studies and agri‐environment schemes have sought to supplement floral resources, but little is known about the value of different types of nesting habitat enhancements (e.g., trap‐nests or bee hotels). We deployed eight replicates of each of three types (bamboo reed, hardwood block and sand/cement brick) of trap‐nests at five orchards in two apple and cherry growing regions (Bilpin and Orange) in Australia. Both reed and hardwood block trap‐nests attracted a diverse array of invertebrates, such as ants, wasps, spiders and bees, including a cleptoparasitic bee species (Thyreus sp.) not previously recorded in the region. Interestingly, two taxa of native bees (Megachile [Megachile] and Megachile [Eutricharaea]) used the artificial nests and were also observed visiting apple crops. There were significantly more native bees using trap‐nests in Orange (n = 65), where orchards are surrounded by agricultural landscapes, than in Bilpin (n = 2), where orchards are surrounded by native forests. Our findings show that artificial nest enhancements are used by native bees, as well as other nontarget invertebrate taxa, some of which can be predators of bees (ants, wasps, and spiders). Nesting habitat augmentation thus has potential to be used as a conservation tool, especially in areas where nesting sites are limited. However, future studies should also consider measures to reduce colonization by non‐target taxa. To support insect pollinators research studies and agri‐environment schemes have sought to supplement floral resources, but relatively little is known about the value of different types of nesting habitat enhancements (e.g. trap‐nests or bee hotels). We deployed eight replicates of each of three types (bamboo reed, hardwood block and sand/cement brick) of trap‐nests at five orchards in two apple and cherry growing regions (Bilpin and Orange, New South Wales) in Australia. Our research highlights the potential of trap‐nests to support native bees in agricultural landscapes through habitat augmentation, with particular benefits in altered landscapes that have relatively little native habitat.
... 'Bee washing', a term coined by MacIvor and Packer (2015), refers to items or actions claiming to support declining bee populations, and thus claiming to be pro-environment, without due diligence or scientific support. MacIvor and Packer (2015) use the example of the widespread promotion and sale of bee hotels as an initiative to increase wild bee populations which is not backed by science (MacIvor and Packer 2015). ...
... 'Bee washing', a term coined by MacIvor and Packer (2015), refers to items or actions claiming to support declining bee populations, and thus claiming to be pro-environment, without due diligence or scientific support. MacIvor and Packer (2015) use the example of the widespread promotion and sale of bee hotels as an initiative to increase wild bee populations which is not backed by science (MacIvor and Packer 2015). Since their publication, the market has been flooded with additional bee washing initiatives, including the well-known multi-million dollar crowd-sourcing campaigns for the 'Flow hive' (Marcum and Blair 2017) and sustainability initiatives involving rooftop honeybee hives, often concentrated in cities (Casanelles-Abella and Moretti 2022) and associated with businesses (Egerer, M. and Kowarik, I., 2020). ...
Article
Full-text available
Concern around declining bee populations globally has become an environmental issue of mainstream importance. Policymakers, scientists, environmental non-government organizations, media outlets and the public have displayed great interest in conservation actions to support pollinators. As with many environmental causes, green washing, or in this case ‘bee washing’, has become rampant. Bee washing can lead to multiple negative consequences, including misinformation, misallocation of resources, increasing threats and steering public understanding and environmental policy away from evidence-based decision-making. Here I will discuss the multiple potential consequences of bee washing on efforts to conserve declining wild bees and promote wild bee health.
... The two types of nests were equally attractive to cavity nesting bees and are both suggested to farmers (Table 1). Wooden log nests are easy to build, cheap (6 USD/year), but farmers must change them every year to avoid the propagation of parasites in the cavities, thus repeating the same effort each year (MacIvor & Packer, 2015). Wooden tray nests are much more costly to build (26.66 USD/year) and also more complex as they require a carpenter with adequate tools to carve the grooves of different diameters. ...
... However, this type of nest has a much longer lifespan (at least 5 years), is easy to clean, and can thus be reused year after year, with little effort. It is also an efficient way to remove parasitized cocoons (MacIvor & Packer, 2015). ...
Article
Full-text available
Food production is highly dependent on pollination services provided by insects; 75% of the leading global food crops need animals for successful production. Pollinators, including managed and wild bees, are declining in many parts of the world. The loss of natural habitats providing nesting sites is considered as one of the main factors driving the decline of crop-visiting wild bees. The researchers had hypothesized that providing bee hotels in cherry orchards may be a useful strategy to support visitors of cherry flowers (Prunus avium). To test this hypothesis, observation was made on the attractiveness of bee hotels to wild bees in cherry orchards in Sefrou Province (Morocco). Bee hotels were installed at the border of two cherry orchards. Surrounding landscapes were described and pollinator communities were observed and sampled within bee hotels, cherry flowers, and also within the surrounding landscape. Bumblebees (Bombus spp.), Mason bees (Osmia spp.), sand bees (Andrena spp.), and sweat bees (Lasioglossum spp.) are the most abundant genera representing almost two-thirds of all wild bee visitors of cherry trees. Mason bees (Osmia spp.) are the most abundant bees nesting in bee hotels with almost three-quarters of all insects recorded. Bee hotels could therefore be used to sustain or even increase cavity-nesting bees visiting cherry orchards in Morocco.
... Members of the public wishing to be actively involved with pollinator conservation efforts or attract additional insect biodiversity into their gardens often use similar artificial nests ('bee hotels'). However, efforts to support wild bees often go unrewarded because the bee hotels are of an incorrect design or manufactured from inappropriate materials (MacIvor & Packer 2015;von Königslöw et al 2019;Alton & Ratnieks 2020). Additionally, artificial nests are often criticised because they offer an artificially high density of nesting cavities, they encourage high levels of pathogens and parasites, and may provide data on abundance and diversity that does not necessarily correlate with that obtained by other pollinator sampling methods (Roulston & Karen Goodell 2011;Prendergast et al. 2020). ...
Article
Insect pollinators are suffering global declines, necessitating the evaluation and development of methods for long-term monitoring and applied field research. Accordingly, this study evaluated the use of trap nests ("bee hotels") as tools for investigating the ecology of cavity nesting Hymenoptera within Irish agricultural landscapes. Three trap nests consisting of 110 mm diameter plastic pipe containing 100 cardboard nest tubes of varying diameter were placed at eight apple orchards and eight oilseed rape sites and left in the field for five months. Sealed nest tubes occurred at 15 of the 16 sites, and in 77% of the 48 nests. However, only 7% of the 4800 individual nest tubes were sealed, and only 4% produced cavity-nesting Hymenoptera. Three cavity nesting bee species (Hylaeus communis, Osmia bicornis, Megachile versicolor) and two solitary wasp species (Ancistrocerus trifasciatus, A. parietinus) emerged from nest tubes. There were significant differences among species in terms of emergence date and the diameter of nest tubes from which they emerged, the latter allowing the calculation of niche width and niche overlap, and informing choice of tube size in future studies/conservation efforts. Trap nests, therefore, offer a valuable tool for fundamental ecological research and a model system for investigating interactions between stem-nesting species within their wider ecological networks. The ability of trap nests to actually increase farmland pollinator abundance and diversity as part of agri-environment schemes requires additional investigation. However, used in sufficient numbers, these trap nests provide valuable biogeographical data for cavity nesting Hymenoptera and offer a viable means for long term monitoring of these species in Irish farmland.
... (Russo 2016;Morales et al. 2017). Much less common are studies that quantitatively document possible negative impact of solitary alien bees (MacIvor and Packer 2015;Fitch et al. 2019;LeCroy et al. 2020), including the recent survey on M. sculpturalis (Geslin et al. 2020). Although these results were mostly based on correlational evidence, the lack of indisputable interaction amongst native and non-native bees should not be interpreted as the lack of impact (Stout and Morales 2009). ...
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Full-text available
There is a growing interest to document and better understand patterns and processes involved in non-native bee introductions and subsequent colonisation of new areas worldwide. We studied the spread of the East Asian bee Megachile sculpturalis in Serbia and southeastern Europe; the bee was earlier established in the USA (since 1994) and western Europe (since 2008). Its establishment in Serbia remained dubious throughout most of 2017-2019, following its first detection. We hereby report on its establishment and spreading, which were corroborated in 2019 under specific circumstances. Owing to an exceptionally poor blooming of Styphnolobium japonicum in 2019, we recorded a high activity density of M. sculpturalis concentrated on a scarce key food resource. We present a novel quantitative approach for an improved early detection of M. sculpturalis, based on the interplay between the bee local occurrence pattern and dynamics of key food-plant(s) availability. This approach seems particularly effective during the early-phase colonisation, at initially low population density of introduced bees. We address the importance of integration of the genuine plant usage patterns with context-specific bee assessment options in establishing effective monitoring. The improved understanding of M. sculpturalis local dynamics triggered the questions about possible origin(s) and modes of its dispersal east of the Alps. To explore the possible scenarios of M. sculpturalis introduction(s), we extended the study to a wider spatio-temporal context-the region of SE Europe (2015-2019). The two complementary study approaches (at local and regional scale) provided more comprehensive evidence of bee dispersal history and the detection patterns in varied recording contexts. Based on this two-scale approach, we suggest that a diffusive mode of M. sculpturalis introduction into Serbia now seems to be a more plausible scenario (than a long-distance jump). We argue that the integration of outcomes from the contrasting approaches (a systematic surveillance, based on plant resources and a broad-scale opportunistic recording) could be of great methodological relevance for the development of future monitoring protocols.
... Planting "pollinator gardens" is often the top recommendation but we note that its efficacy in improving bee health is rarely empirically demonstrated (but see Baldock et al., 2019;Leve et al., 2019). Installing "bee hotels" (i.e., fixtures with hollow tubes for bees to nest in) was a common recommendation for conserving pollinators in cities but it has since been shown to not benefit native bees (MacIvor & Packer, 2015). We need research to understand which features of urban environments enhance pollinator health to help inform bee conservation initiatives in cities. ...
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Full-text available
With growing urbanization, it is becoming increasingly important to design cities in a manner that sustains and enhances biodiversity and ecosystem services. Native bees are critical pollinators that have experienced substantive declines over the past several decades. These declines have captured the attention of the public, particularly urbanites, prompting a large interest in protecting pollinators and their habitats in cities across North America and Europe. Unfortunately, we currently lack research about specific features of urban environments that can enhance the fitness of pollinators. We carried out an intensive study of Bombus impatiens, the Common Eastern Bumblebee, in the city of Toronto (Canada's largest city), to better understand landscape parameters that provide high-quality habitat for this species and likely other generalist bees. We divided the city into 270 grid cells and sampled a large number of worker bees, which were then genotyped at twelve hypervariable microsatellite loci. The genetic data allowed us to quantify the effective number of colonies and foraging distance for bumblebees in our study area. We then asked how the city's landscape and human population demography and income are associated with the availability of high-quality habitat for B. impatiens. Several aspects of Toronto's landscape influenced colony density and foraging range. Urbanization had a clear effect on both colony density and foraging distance of workers. On the other hand, functional (i.e., not cosmetic) green space was often associated with higher quality habitats for bumblebees. Our study suggests several planning strategies to enhance habitat quality for bumblebees and other pollinators in cities.
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Several management practices have been suggested to mitigate the global pollinator decline in agro‐ecosystems, including wildflower strips and Farming with Alternative Pollinators (FAP). FAP proposes to dedicate 25% of the field area to seed Marketable Habitat Enhancement Plants (MHEP) around the main crop, occupying 75% of the field. However, wild pollinators may not rely fully on the resources that fields provide due to differences in flying period and host‐plant preferences, and need additional resources from wild flowering plant communities. Here we aim to compare wild pollinator communities between FAP fields, monoculture of pollinator dependent crops and the nearby wild flowering plants. We developed two experimental trials with two main crops (faba bean and eggplant) in 16 fields in North‐West Morocco and we compared wild pollinator richness and wild pollinator specialization between FAP fields, control fields and the nearby wild flowering plants. We recorded a significantly higher pollinator richness in FAP fields compared to wild flowering plants and monoculture. Pollinator specialization index (i.e. degree of interaction specialization at the species level) did not differ significantly between the three treatments in faba bean trial (i.e. FAP, control and wild plants), whilst in eggplant trial, wild plants harboured significantly more specialist species than FAP fields. Yet, no significant differences in pollinator specialization index were reported between the other treatments in eggplant trial (i.e. FAP vs. control and control vs. wild plants). Moreover, 28% of the pollinator species collected, were only observed on wild plants, particularly thistles. These results highlight the potential of FAP approach as a tool for pollinator conservation in farmlands. However, the FAP approach alone is not sufficient to cater the diverse pollinators present in the agro‐ecosystem, and hence, the maintenance of the surrounding wild flowering plants is necessary to support pollinators in farmlands.
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Pollination services are a frequently overlooked component of urban ecosystems. As cities look to become more sustainable and incorporate more urban green spaces, these pollinator services are coming to the forefront, and educating the public about the habitat and foraging needs of urban pollinators is becoming more important. Increasingly popular features in urban gardens are “bug hotels”, which are artificial structures that humans can install to create habitat or shelter for urban insect pollinators. In a college-level Urban Ecology class, we use a structured classroom activity to teach students about pollinator needs, but also place the activity in a larger context of a discussion about the value of urban landscapes, as well as the importance of evaluating sources of information. Here we describe the steps of a research activity that students undertake to design a “bug hotel”, as well as suggestions for how to extend the activity beyond the classroom.
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Loss of natural habitat through land‐use change threatens bees. Urbanisation is a major, increasing form, of habitat loss, and a novel, pervasive form of disturbance known to impact bee diversity and abundance in a variety of often inconsistent ways. We conducted a comprehensive, semi‐quantitative review, involving 215 studies, on responses of bees to urban landscapes, and local and landscape variables proposed to influence bee abundance and diversity. Urban areas tend to be favourable habitat for bees compared with agricultural ones, but compared with natural areas, urban areas often host more abundant populations yet fewer species. Factors associated with urban landscapes, including changes in foraging resources and nesting substrate types and availability, contribute to changes in abundance, species richness, and composition of native bee assemblages. However, the conclusions of studies vary greatly because of the difference in the ecological traits of bees, habitats surveyed, and geographic region, as well as noise in the data resulting from inconsistencies in sampling methodology, and definitions of ‘urban’ and ‘natural’. Identifying what biotic and abiotic features of cityscapes promote or threaten the persistence of urban bee diversity is critical. We provide a comprehensive evaluation of how bees (both in aggregate and according to their ecological guild) have responded to the urban environment, identify gaps in knowledge in urban bee ecology, and make recommendations to advance our understanding of bees in urban environments to promote conservation of diverse bee communities. Reviewing 215 studies, factors associated with urban landscapes, including changes in foraging and nesting resources, contribute to changes in abundance, species richness, and composition of native bee assemblages. Urban areas tend to be favourable habitat for bees compared with agricultural ones, but compared with natural areas, urban areas often host more abundant populations yet fewer species. Seven key knowledge gaps are identified, and recommendations on how to advance urban bee conservation.
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Pollinators are fundamental to maintaining both biodiversity and agricultural productivity, but habitat destruction, loss of flower resources, and increased use of pesticides are causing declines in their abundance and diversity. Using historical records, we assessed the rate of extinction of bee and flower-visiting wasp species in Britain from the mid-19th century to the present. The most rapid phase of extinction appears to be related to changes in agricultural policy and practice beginning in the 1920s, before the agricultural intensification prompted by the Second World War, often cited as the most important driver of biodiversity loss in Britain. Slowing of the extinction rate from the 1960s onward may be due to prior loss of the most sensitive species and/or effective conservation programs.
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Despite the global trend in urbanization, little is known about patterns of biodiversity or provisioning of ecosystem services in urban areas. Bee communities and the pollination services they provide are important in cities, both for small-scale urban agriculture and native gardens. To better understand this important ecological issue, we examined bee communities, their response to novel floral resources, and their potential to provide pollination services in 25 neighborhoods across Chicago, IL (USA). In these neighborhoods, we evaluated how local floral resources, socioeconomic factors, and surrounding land cover affected abundance, richness, and community composition of bees active in summer. We also quantified species-specific body pollen loads and visitation frequencies to potted flowering purple coneflower plants (Echinacea purpurea) to estimate potential pollination services in each neighborhood. We documented 37 bee species and 79 flowering plant genera across all neighborhoods, with 8 bee species and 14 flowering plant genera observed on average along each neighborhood block. We found that both bee abundance and richness increased in neighborhoods with higher human population density, as did visitation to purple coneflower flower heads. In more densely populated neighborhoods, bee communities shifted to a suite of species that carry more pollen and are more active pollinators in this system, including the European honey bee (Apis mellifera) and native species such as Agapostemon virescens. More densely populated neighborhoods also had a greater diversity of flowering plants, suggesting that the positive relationship between people and bees was mediated by the effect of people on floral resources. Other environmental variables that were important for bee communities included the amount of grass/herbaceous cover and solar radiation in the surrounding area. Our results indicate that bee communities and pollination services can be maintained in dense urban neighborhoods with single-family and multi-family homes, as long as those neighborhoods contain diverse and abundant floral resources.
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Sex ratio, parental investment and body size of Osmia lignaria, a sexually dimorphic, solitary bee, were studied by using field trap-nesting techniques in northeastern Utah. Each half of the flight season was studied separately because previous observations suggested that the offspring sex ratio changed with time. The results confirm sex ratio and investment theory over the entire nesting season almost equal investment was made in each sex. However, female production was emphasized early and male production late. The seasonal transition in the sex of offspring is related to seasonal decline in floral resource availability and to lower female foraging efficiency With time, production of the smaller male offspring becomes more advantageous because the probability of adult or offspring mortality is lower than if larger offspring were produced. Small male production is emphasized later in the season because they are probably more fit than equally sized (or larger) females. Conversely, large females increase parental fitness more than comparably sized males and they are produced early in the nesting season when resources are most abundant and parents are in best condition.