Content uploaded by Fabio Sgolastra
Author content
All content in this area was uploaded by Fabio Sgolastra on Jan 14, 2021
Content may be subject to copyright.
insects
Article
Use of a Managed Solitary Bee to Pollinate Almonds:
Population Sustainability and Increased Fruit Set
Jordi Bosch 1, *, Sergio Osorio-Canadas 2, Fabio Sgolastra 3, * and Narcís Vicens 4
Citation: Bosch, J.;
Osorio-Canadas, S.; Sgolastra, F.;
Vicens, N. Use of a Managed Solitary
Bee to Pollinate Almonds: Population
Sustainability and Increased Fruit Set.
Insects 2021,12, 56. https://doi.org/
10.3390/insects12010056
Received: 26 November 2020
Accepted: 8 January 2021
Published: 11 January 2021
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional clai-
ms in published maps and institutio-
nal affiliations.
Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1Centre for Research on Ecology and Forestry Application, 08193 Bellaterra, Spain
2Departamento de Ecología de la Biodiversidad, Instituto de Ecología,
Universidad Nacional Autónoma de México, AP 70-275, 04510 Mexico City, Mexico;
s.osorio.canadas@gmail.com
3Dipartimento di Scienze e Tecnologie Agro-Alimentari, Alma Mater Studiorum Universitàdi Bologna,
Viale Fanin 42, 40127 Bologna, Italy
4Diputacióde Girona, Servei de Medi Ambient, 17004 Girona, Spain; nvicens@ddgi.cat
*Correspondence: jordi.bosch@uab.cat (J.B.); fabio.sgolastra2@unibo.it (F.S.)
Simple Summary:
Methods to rear Osmia bees to pollinate fruit trees have been developed in various
parts of the world. These bees are excellent pollinators but evidence that their populations can
be sustained in orchards and their use results in increased fruit production is scarce. We released
an Osmia cornuta population at one end of an almond orchard. Then, we surveyed the pollinators
visiting the almond flowers and measured fruit set in trees located at increasing distances from the
nesting stations. We found that fruit production was higher in the trees that received more Osmia
visits. Importantly, this result was obtained against a strong background of honeybees, which were
10 times more abundant than Osmia. The Osmia population obtained at the end of the flowering
period was 1.28 larger than the population initially released. Our study demonstrates that Osmia
populations can be sustained in orchard environments and that even a small population of a highly
effective pollinator may have a significant impact on fruit set. Our results are encouraging for the
use of Osmia populations and for the implementation of measures to promote wild pollinators in
agricultural environments.
Abstract:
Osmia spp. are excellent orchard pollinators but evidence that their populations can be
sustained in orchard environments and their use results in increased fruit production is scarce. We
released an Osmia cornuta population in an almond orchard and measured its population dynamics,
as well as visitation rates and fruit set at increasing distances from the nesting stations. Honeybees
were 10 times more abundant than O. cornuta. However, the best models relating fruit set and bee
visitation included only O. cornuta visitation, which explained 41% and 40% of the initial and final
fruit set. Distance from the nesting stations explained 27.7% and 22.1% of the variability in initial
and final fruit set. Of the 198 females released, 99 (54.4%) established and produced an average of
9.15 cells. Female population growth was 1.28. By comparing our results with those of previous
O. cornuta studies we identify two important populational bottlenecks (female establishment and
male-biased progeny sex ratios). Our study demonstrates that even a small population of a highly
effective pollinator may have a significant impact on fruit set. Our results are encouraging for the use
of Osmia managed populations and for the implementation of measures to promote wild pollinators
in agricultural environments.
Keywords:
pollination service; Osmia cornuta;Apis mellifera; population dynamics; managed pollinators;
crop pollination
1. Introduction
Approximately three-quarters of the world’s crops benefit from animal pollination [
1
],
and a significant part of this pollination service is provided by wild pollinators [
2
–
5
]. How-
Insects 2021,12, 56. https://doi.org/10.3390/insects12010056 https://www.mdpi.com/journal/insects
Insects 2021,12, 56 2 of 11
ever, the current context of agricultural intensification, characterized by low crop diversity,
increased crop field size, loss of semi-natural habitats, and increased pesticide use is clearly
detrimental to pollinator abundance and diversity [
6
–
9
]. As a result, wild pollinators are
notoriously scarce in many agricultural landscapes [
10
–
12
]. For this reason, the general
perception is that wild pollinator populations are insufficient to provide adequate levels
of pollination in intensively farmed areas, and populations of managed pollinators are
usually introduced to enhance pollination services.
Fruit trees are highly dependent on pollinator visitation because they bloom for a
short period of time in spring when weather conditions are often suboptimal for insect
activity and because most cultivars are self-incompatible. For this reason, honey bee
hives are usually introduced in orchards at a rate of 2–6 hives per ha, with each hive
containing several thousands of foragers [
13
–
15
]. However, honey bees are only fully
active at temperatures above 12–14
◦
C [
16
]. In addition, because they have long foraging
ranges [
17
–
19
], and are highly generalist foragers, they often visit other flower species [
14
].
Finally, honey bees are not very effective fruit tree pollinators, mainly due to their low visit
legitimacy (many of the visits result in no contact between the bee and the stigmas; [
20
–
25
]).
These shortcomings, along with the risks associated with relying on a single species, have
prompted the search for alternative pollinators, and methods to manage various Osmia
species as orchard pollinators have been developed in different parts of the world [
26
,
27
].
These Osmia species are only active for a couple of months in spring and fly under marginal
weather conditions [
16
]. In addition, they have short foraging ranges [
28
–
30
] and show a
strong preference for fruit tree pollen [
21
,
23
,
31
–
35
]. Finally, Osmia visit legitimacy on fruit
three flowers is close to 100%, and fruit set in flowers receiving a single visit is similar to
fruit set in hand-pollinated flowers [
20
,
21
,
23
–
25
,
36
]. For these reasons, population densities
recommended for orchard pollination with Osmia spp. are as low as 1250–2000 bees per ha
(with a 1.6 male/female ratio) [
26
]. However, even if the pollinating effectiveness of Osmia
spp. has been amply documented, evidence that the use of Osmia populations results in
increased fruit production is still scarce [11,23,37,38].
The use of a managed pollinator is only sustainable if population levels can be main-
tained from year to year. Ideally, populations should be able to grow on site during the
flowering period of the target crop. If this is not possible, population losses can be compen-
sated by rearing populations under artificial conditions, as done with bumblebee colonies
used for crop pollination [
39
]. However, attempts to mass-rear Osmia populations under ar-
tificial conditions have not been successful [
40
,
41
]. Therefore, to ensure the sustainability of
Osmia spp. as managed pollinators it is essential to understand the dynamics of populations
introduced in orchards. There are various factors potentially limiting the growth of Osmia
populations released in orchards. These factors include: (a) Winter mortality (some of the
cocoons introduced in the orchard contain individuals that are either dead or too weak to
emerge); (b) low female establishment (some of the females that have successfully emerged
out of their cocoon may be predated, too weak to start nesting activities, or disperse and
nest away from the release site); (c) low fecundity (nesting females provision a low number
of cells and therefore lay few eggs); (d) progeny mortality (a part of the progeny does not
reach the adult stage due to either developmental failure or parasitism); (e) male-biased
progeny sex ratio (nesting females produce a high proportion of males, which are less
costly to produce than females [42]).
In this study, we released a population of the European species Osmia cornuta in an
almond orchard. Almond pollination is particularly challenging because almonds bloom
very early in the year (February-March) and have an unusually high bearing capacity (as
many as 40% of the flowers may bear fruit) [
43
]. We measured female establishment and
population growth, as well as flower visitation rates and fruit set at increasing distances
from the nesting site. Our first objective was to assess whether the O. cornuta population
had an impact on fruit production. Given the short foraging range of O. cornuta, we
expected fruit set to be negatively correlated to distance from the nesting site. Our second
objective was to establish whether the O. cornuta population could be increased on site
Insects 2021,12, 56 3 of 11
and, by comparing our results with those of previous studies, to identify the main factors
limiting population growth.
2. Materials and Methods
The study was conducted in an almond orchard in Vila-Seca (Tarragona, NE Spain). The
orchard measured 0.5 ha and had 5 rows of the main cultivar Ferragnes intermixed with
4 rows of the pollinizer cultivar Cristomorto (both cultivars are self-incompatible) (Figure A1).
Each row had 16 trees. The orchard was located within a matrix of farmland, including cereal
fields, fallow land, and a mixture of olive, carob, and hazelnut orchards. There were no honey
bee hives in the orchard or its surroundings (at least within a 500 m radius).
In early February 1992, prior to almond bloom, we set up 3 nesting stations for
O. cornuta at one edge of the orchard (Figure A1). Nesting stations were made with wooden
boxes with the front side open held 1.5 m above the ground on four metal fence posts.
Each station contained 15 perforated solid wood blocks with 25 inserted paper straws
(length: 15 cm; inside diameter: 8 mm). On 22 February, before the orchard started to
bloom, we placed 198 females and 360 males (M/F sex ratio: 1.8) within their cocoons in
open cardboard boxes inside the nesting stations. These cocoons had been wintered at 4
◦
C
since October 1991.
To determine the number of females that established at the nesting stations, we
inspected each nesting cavity with an otoscope. This was done at night, when females were
roosting inside their nests, seven times during the nesting period (approximately twice
a week).
During peak bloom, we conducted pollinator counts on the central row of Ferragnes.
An observer slowly walked around each tree and noted all pollinators seen visiting the
flowers. Total observation time was 10 h and 40 min (40 min ×16 trees).
To assess fruit set (% of flowers that set fruit) at increasing distances (0–80 m) from the
O. cornuta nesting shelters, we marked all trees in 3 Ferragnes rows (48 trees), including
the row used in the pollinator counts (Figure A1). Before bloom we tagged 4 branches on
each tree and counted the number of flower buds on each branch (
mean ±SE = 264.4 ±1.6
flower buds per branch). In April, we counted the number of initiated fruits on each branch
(initial fruit set). Following the natural fruit drop, fruits were counted again in June when
they had reached their final size (final fruit set).
To obtain a measure of maximum potential fruit set, we marked 5 additional Ferragnes
trees (Figure A1) on which we tagged 4 branches and counted flower buds as described
above. These trees were checked daily and newly opened flowers were hand-pollinated
with Cristomorto pollen. Initial and final fruit set was assessed as described above.
After petal fall, when O. cornuta nesting activity had ceased, we removed nesting
materials and kept them in the laboratory. In September, when development was com-
pleted, we pulled out paper straws and analyzed the contents of each nest. We quantified
the number of male and female offspring produced (female cocoons are typically larger
than male cocoons and are found in the inner cells of the nest [
42
]). We also recorded
offspring mortality.
Statistical Analysis
Initial and final fruit set were analyzed separately. To analyze whether fruit set
declined with distance from the O. cornuta nesting stations, we used Linear Mixed Models
(LMMs) with a tree as a random factor. We considered a linear relationship (model with
distance as the only explicative variable) and a quadratic relationship (model with distance
and distance
2
). We then used a model inference approach [
44
] to select the best-fit model
based on AICc values using maximum likelihood criteria. Models with
∆
AICc < 2 were
considered equal to the best model [
44
]. We then run a LMM model with REML to obtain
unbiased parameter estimates. We calculated a likelihood-ratio-based R
2
of the best models
as a measure of explanatory power.
Insects 2021,12, 56 4 of 11
To analyze whether fruit set was related to O. cornuta and/or A. mellifera visitation,
we conducted a model selection procedure with LM models, testing all possible explana-
tory variable combinations through a multi-model inference approach with the ‘dredge’
function (‘MuMin’ package, [
45
]). We again selected the best models based on AICc values.
Following model selection, we used a model averaging procedure (with averaged variable
coefficients) based on AICc. This was done with the ‘model.avg’ function (‘MunMin’ pack-
age), which yields model-averaged estimates of variable coefficients and p-values for each
explanatory variable. Conditional average and full average approaches yielded almost
identical results. We show only conditional average results. Finally, we calculated the
adjusted-R
2
of the best models (containing significant explanatory variables) as a measure
of explanatory power.
Percent initial and final fruit set were arcsin-transformed. The distribution of residuals
was visually inspected for homoscedasticity and the normality assumption was tested with
the Shapiro test. All analyses were conducted with the ‘nlme’ [
46
] and MuMin packages in
R [47]. All means are followed by standard error (SE).
3. Results
Maximum temperatures during the days following the release of the O. cornuta popu-
lation (22 February) ranged between 14 and 18
◦
C. The first females engaged in nesting
activities were observed on 28 February. Of the 198 females released, 182 emerged out of
their cocoons (91.9 % winter survival). The maximum number of females established in the
nesting stations was counted on March 3 (99 females; 54.4% of the emerged females).
We recorded 1114 pollinators visiting the almond flowers. Although there were
no hives in sight, the most frequent pollinator was, by far, Apis mellifera (74.1% of the
visits recorded), followed by O. cornuta (7.3%). Other visitors included various flies
(16.2%), hoverflies (4.11%, Eristalis tenax,Eupeodes sp.), and wild bees (1.1%, Andrena
nigroaenea,Andrena sp., Eucera sp., Bombus terrestris,Xylocopa violacea). Most (70.4%) O.
cornuta visitation occurred within 30 m from the nesting stations (Figure 1). Apis mellifera
visitation followed an irregular pattern across the orchard and tended to be higher towards
the two ends of the orchard (Figure 1).
Insects 2020, 11, x 5 of 13
3. Results
Maximum temperatures during the days following the release of
the O. cornuta population (22 February) ranged between 14 and 18 °C.
The first females engaged in nesting activities were observed on 28 Feb-
ruary. Of the 198 females released, 182 emerged out of their cocoons
(91.9 % winter survival). The maximum number of females established
in the nesting stations was counted on March 3 (99 females; 54.4% of
the emerged females).
We recorded 1114 pollinators visiting the almond flowers. Alt-
hough there were no hives in sight, the most frequent pollinator was,
by far, Apis mellifera (74.1% of the visits recorded), followed by O. cor-
nuta (7.3%). Other visitors included various flies (16.2%), hoverflies
(4.11%, Eristalis tenax, Eupeodes sp.), and wild bees (1.1%, Andrena ni-
groaenea, Andrena sp., Eucera sp., Bombus terrestris, Xylocopa violacea).
Most (70.4%) O. cornuta visitation occurred within 30 m from the nest-
ing stations (Figure 1). Apis mellifera visitation followed an irregular
pattern across the orchard and tended to be higher towards the two
ends of the orchard (Figure 1).
Figure 1. Number Apis mellifera and Osmia cornuta individuals recorded visit-
ing almond flowers at increasing distances from the O. cornuta nesting sta-
tions.
Almond bloom was over by 23 March. At that time, O. cornuta fe-
males that were still alive foraged mostly on Diplotaxis erucoides (Bras-
sicaceae), a common weed in the surroundings of the orchard. By 29
March O. cornuta nesting activity had ceased. The number of nests pro-
duced was 203. These nests contained 253 female and 653 male cells.
Offspring mortality was 7.1%. Most of this mortality was due to un-
known causes (5.3%), and the rest to parasitism by the cleptoparasitic
mite Chaetodactylus osmiae (1.8%). The live female population recovered
was 241, and the live male population 601 (M/F sex ratio: 2.5).
Both initial and final fruit set significantly declined with distance
from the O. cornuta nesting stations (Figure 2). The quadratic model
provided the best fit, with distance explaining 27.7% of the variability
in initial fruit set and 22.1% of the variability in final fruit set (Table 1).
Importantly, the random factor tree also had a strong effect on fruit set
(initial: 38.7%; final: 37.1%). The linear models yielded similar results
(Table A1 and Figure A2 in Appendix A). Final fruit set was strongly
related to initial fruit set (Pearson’s r = 0.88; p < 0.0001; Figure 3).
Figure 1.
Number Apis mellifera and Osmia cornuta individuals recorded visiting almond flowers at
increasing distances from the O. cornuta nesting stations.
Almond bloom was over by 23 March. At that time, O. cornuta females that were
still alive foraged mostly on Diplotaxis erucoides (Brassicaceae), a common weed in the
surroundings of the orchard. By 29 March O. cornuta nesting activity had ceased. The
number of nests produced was 203. These nests contained 253 female and 653 male cells.
Offspring mortality was 7.1%. Most of this mortality was due to unknown causes (5.3%),
and the rest to parasitism by the cleptoparasitic mite Chaetodactylus osmiae (1.8%). The live
female population recovered was 241, and the live male population 601 (M/F sex ratio: 2.5).
Insects 2021,12, 56 5 of 11
Both initial and final fruit set significantly declined with distance from the O. cornuta
nesting stations (Figure 2). The quadratic model provided the best fit, with distance
explaining 27.7% of the variability in initial fruit set and 22.1% of the variability in final
fruit set (Table 1). Importantly, the random factor tree also had a strong effect on fruit
set (initial: 38.7%; final: 37.1%). The linear models yielded similar results (Table A1 and
Figure A2 in Appendix A). Final fruit set was strongly related to initial fruit set (Pearson’s
r = 0.88; p< 0.0001; Figure 3).
Insects 2020, 11, x 6 of 13
Figure 2. Initial (A) and final (B) fruit set (% of flowers setting fruit in April and June, respectively) at
increasing distances from the Osmia cornuta nesting shelters. Each dot represents a branch. Broken
lines indicate mean fruit set in hand-pollinated trees (five trees; four branches per tree). The gray
bands represent 95% confidence intervals.
Figure 3. Relationship between initial (April) and final (June) fruit set (% of
flowers setting fruit). Each dot represents a branch. The gray band represents
the 95% confidence interval.
Table 1. Output of Linear Mixed Model (LMM) relating initial fruit (A) and final fruit set (B) to dis-
tance from the Osmia cornuta nesting shelters. Parameter- (t) and p-values are provided for the best-
fitted models based on AICc selection. R2m and R2c are the marginal and conditional R2 of the best-
fitted models.
A. Initial Fruit Set B. Final Fruit Set
t-Value p-Value t-Value p-Value
(Intercept) 25.27 <0.0001 22.63 <0.0001
Distance −3.33 <0.0001 −2.85 0.0001
Distance2 2.12 0.0395 1.75 0.0857
R
2m: 0.28; R2c: 0.67 R2m: 0.22; R2c: 0.59
The best models analyzing the effect of O. cornuta and A. mellifera
visitation on fruit set included only O. cornuta visitation, which ex-
plained 41% of the initial fruit set and 40% of the final fruit set (Table 2;
Figure 4). Even then, the levels of fruit set obtained across the orchard
Figure 2.
Initial (
A
) and final (
B
) fruit set (% of flowers setting fruit in April and June, respectively)
at increasing distances from the Osmia cornuta nesting shelters. Each dot represents a branch. Broken
lines indicate mean fruit set in hand-pollinated trees (five trees; four branches per tree). The gray
bands represent 95% confidence intervals.
Table 1.
Output of Linear Mixed Model (LMM) relating initial fruit (A) and final fruit set (B) to
distance from the Osmia cornuta nesting shelters. Parameter- (t) and p-values are provided for the
best-fitted models based on AICc selection. R
2
m and R
2
c are the marginal and conditional R
2
of the
best-fitted models.
A. Initial Fruit Set B. Final Fruit Set
t-Value p-Value t-Value p-Value
(Intercept) 25.27 <0.0001 22.63 <0.0001
Distance −3.33 <0.0001 −2.85 0.0001
Distance22.12 0.0395 1.75 0.0857
R2m: 0.28; R2c: 0.67 R2m: 0.22; R2c: 0.59
Insects 2020, 11, x 6 of 13
Figure 2. Initial (A) and final (B) fruit set (% of flowers setting fruit in April and June, respectively) at
increasing distances from the Osmia cornuta nesting shelters. Each dot represents a branch. Broken
lines indicate mean fruit set in hand-pollinated trees (five trees; four branches per tree). The gray
bands represent 95% confidence intervals.
Figure 3. Relationship between initial (April) and final (June) fruit set (% of
flowers setting fruit). Each dot represents a branch. The gray band represents
the 95% confidence interval.
Table 1. Output of Linear Mixed Model (LMM) relating initial fruit (A) and final fruit set (B) to dis-
tance from the Osmia cornuta nesting shelters. Parameter- (t) and p-values are provided for the best-
fitted models based on AICc selection. R2m and R2c are the marginal and conditional R2 of the best-
fitted models.
A. Initial Fruit Set B. Final Fruit Set
t-Value p-Value t-Value p-Value
(Intercept) 25.27 <0.0001 22.63 <0.0001
Distance −3.33 <0.0001 −2.85 0.0001
Distance2 2.12 0.0395 1.75 0.0857
R
2m: 0.28; R2c: 0.67 R2m: 0.22; R2c: 0.59
The best models analyzing the effect of O. cornuta and A. mellifera
visitation on fruit set included only O. cornuta visitation, which ex-
plained 41% of the initial fruit set and 40% of the final fruit set (Table 2;
Figure 4). Even then, the levels of fruit set obtained across the orchard
Figure 3.
Relationship between initial (April) and final (June) fruit set (% of flowers setting fruit).
Each dot represents a branch. The gray band represents the 95% confidence interval.
Insects 2021,12, 56 6 of 11
The best models analyzing the effect of O. cornuta and A. mellifera visitation on fruit
set included only O. cornuta visitation, which explained 41% of the initial fruit set and 40%
of the final fruit set (Table 2; Figure 4). Even then, the levels of fruit set obtained across the
orchard were lower than those obtained in the five hand-pollinated trees (initial fruit set:
57.8
±
2.0; final fruit set: 36.9
±
1.8%) (Figure 2), indicating that pollination services could
still be increased.
Table 2.
Output of LM model averaging relating initial (A) and final (B) fruit set to Osmia cornuta and
Apis mellifera visitation. Estimated coefficients and their p-values are provided. Adjusted R
2
of the
best-fitted model (containing only O. cornuta visitation as predictor variable) are provided.
(A) Initial Fruit Set.
Estimate SE Adjusted SE zValue Pr(>|z|)
(Intercept) 0.5545 0.028 0.031 17.62 <2e−16
O. cornuta 0.0099 0.003 0.003 3.08 0.002
A. mellifera −0.0004 0.001 0.001 0.39 0.689
Adjusted R2: 0.44
(B) Final Fruit Set
Estimate SE Adjusted SE zValue Pr(>|z|)
(Intercept) 0.489 0.043 0.046 10.63 <2e−16
O. cornuta 0.009 0.003 0.003 2.70 0.007
A. mellifera −0.001 0.001 0.001 1.17 0.241
Adjusted R2: 0.36
Insects 2020, 11, x 7 of 13
were lower than those obtained in the five hand-pollinated trees (initial
fruit set: 57.8 ± 2.0; final fruit set: 36.9 ± 1.8%) (Figure 2), indicating that
pollination services could still be increased.
Figure 4. Relationship between Osmia cornuta visitation and initial (A) and final (B) fruit set (% of
flowers setting fruit in April and June, respectively). Each dot represents a tree. Gray bands represent
95% confidence intervals.
Table 2. Output of LM model averaging relating initial (A) and final (B) fruit set to Osmia cornuta and Apis
mellifera visitation. Estimated coefficients and their p-values are provided. Adjusted R2 of the best-fitted model
(containing only O. cornuta visitation as predictor variable) are provided.
(A) Initial Fruit Set.
Estimate SE Adjusted SE z Value Pr(>|z|)
(Intercept) 0.5545 0.028 0.031 17.62 <2e−16
O. cornuta 0.0099 0.003 0.003 3.08 0.002
A. mellifera -0.0004 0.001 0.001 0.39 0.689
Adjusted R2: 0.44
(B) Final Fruit Set
Estimate SE Adjusted SE z Value Pr(>|z|)
(Intercept) 0.489 0.043 0.046 10.63 <2e−16
O. cornuta 0.009 0.003 0.003 2.70 0.007
A. mellifera -0.001 0.001 0.001 1.17 0.241
Adjusted R2: 0.36
4. Discussion
Our first objective was to assess whether the O. cornuta population
had an impact on almond production. Most of the O. cornuta observed
were recorded within 30 m from the nesting stations. Although Osmia
females are able to locate their nest from distances as far as 500–1800 m
[28,29], populations established in orchards concentrate most of their
foraging within 50 m of the nesting sites [23,30]. The negative relation-
ship between fruit set and distance from the nesting stations closely
paralleled the distribution of O. cornuta across the orchard. The contri-
bution of O. cornuta to pollination service was further confirmed by the
analysis identifying O. cornuta visitation (but not A. mellifera visitation)
as a significant predictor of initial and final fruit set. This result is re-
markable given that A. mellifera visitation was 10 times higher than O.
cornuta visitation. Apis mellifera is consistently reported as the dominant
pollinator species in commercial orchards (e.g., [11,12,48]). However,
its contribution to pollination service is strongly limited by its low per-
Figure 4.
Relationship between Osmia cornuta visitation and initial (
A
) and final (
B
) fruit set (% of
flowers setting fruit in April and June, respectively). Each dot represents a tree. Gray bands represent
95% confidence intervals.
4. Discussion
Our first objective was to assess whether the O. cornuta population had an impact
on almond production. Most of the O. cornuta observed were recorded within 30 m
from the nesting stations. Although Osmia females are able to locate their nest from
distances as far as 500–1800 m [
28
,
29
], populations established in orchards concentrate
most of their foraging within 50 m of the nesting sites [
23
,
30
]. The negative relationship
between fruit set and distance from the nesting stations closely paralleled the distribution
of O. cornuta across the orchard. The contribution of O. cornuta to pollination service was
further confirmed by the analysis identifying O. cornuta visitation (but not A. mellifera
visitation) as a significant predictor of initial and final fruit set. This result is remarkable
given that A. mellifera visitation was 10 times higher than O. cornuta visitation. Apis
mellifera is consistently reported as the dominant pollinator species in commercial orchards
Insects 2021,12, 56 7 of 11
(e.g., [
11
,
12
,
48
]). However, its contribution to pollination service is strongly limited by its
low per-visit pollination effectiveness on fruit tree flowers [
20
–
25
]. Our results are limited
to a single orchard and a single year. However, they are in line with a previous study that
found a significant impact of O. cornuta visitation on final seed-set in a pear orchard in
which A. mellifera was 7 times more abundant [
23
]. These results are also in agreement with
studies showing yield increases in orchards pollinated with other Osmia species [
11
,
37
,
38
].
Our study also shows that initial fruit set (commonly used as a proxy of pollination service)
is a good predictor of final fruit set in almonds.
Our second objective was to establish whether the O. cornuta population could be
increased on site, and to identify the main factors limiting population growth. We released
198 females of which 182 emerged out of their cocoons (8.1% winter mortality). This winter
mortality is similar to values obtained in other O. cornuta populations managed for orchard
pollination (Table 3). Of the 182 females that emerged, 99 (54.4%) established in the nesting
stations. Failure to establish can be caused by lack of vigor of emerging females [
49
],
predation [
50
–
52
], and dispersal of pre-nesting females [
53
], but attributing a relative
weight to each of these three factors is not easy. At any rate, the percent establishment
obtained in our study was similar to values obtained with other O. cornuta populations
released in orchards (Table 3). The 99 females that established in the nesting stations
produced 906 cells (an average of 9.15 cells/female). This fecundity is again close to
fecundity in other O. cornuta populations released in orchards (Table 3). Of these 906 cells,
253 contained female progeny (2.6 M/F sex ratio). This is considerably higher than that
the sex ratio of the population released (1.8), but similar to sex ratios obtained from other
O. cornuta populations released in orchards (Table 3). Developmental mortality (5.3%)
and parasitism (1.8%) were also similar to mortality levels obtained in previous studies
(Table 3). Ultimately, the female population recovered was 1.28 times larger than the
female population released, providing evidence that Osmia populations can be sustained
in orchard environments ([26] and references therein, [54]).
Table 3.
Population parameters of Osmia cornuta populations released in orchards. All means
followed by standard error (SE).
This Study Other Studies 7
Winter mortality 18.1% 5.5 ±0.7 (n= 8)
Female establishment 254.4% 50.7 ±5.1 (n= 14)
Fecundity 39.2 9.8 ±0.9 (n= 10)
Progeny sex ratio 42.6 2.5 ±0.18 (n= 12)
Developmental mortality 55.3% 8.5 ±0.9 (n= 15)
Parasitism 61.8% 1.6 ±0.4 (n= 17)
1
Individuals that did not emerge out of their cocoons;
2
Proportion of emerged females that established at the
nesting stations provided;
3
Number of eggs laid;
4
Males/females;
5
Proportion of progeny that died from
unknown causes; 6Proportion of progeny that died from cleptoparasitism; 7[21,23,53,55–58].
The results of Table 3allow us to identify two important bottlenecks in the dynamics
of O. cornuta populations managed for orchard pollination. The first one occurs in the
establishment phase, during which the effective female population is reduced by ca. 50%.
Previous studies have shown that Osmia establishment can be enhanced by releasing
populations within their natal nests, rather than as loose cocoons [
33
,
55
,
59
–
61
]. This result is
probably mediated by olfactory nest cues that enhance the tendency of females to re-nest in
their natal nesting site (philopatry; [
61
]). In relation to this, an olfactory attractant similar to
that developed for Osmia lignaria [
54
] could potentially enhance establishment of O. cornuta
populations released in orchards. The second important bottleneck is the production of a
male-biased sex ratio. In O. cornuta, the sex ratio of populations trap-nested in semi-natural
areas is 1.5
±
0.06 (n= 4 populations) [
62
,
63
], a figure that closely matches the theoretically
optimal sex ratio based on male and female body weights (1.7) [
42
]. However, progeny
sex ratios obtained from populations released in orchards are consistently higher (ca. 2.5;
Table 3). In other words, in populations nesting in orchards, a considerable fraction of the
Insects 2021,12, 56 8 of 11
parental investment is devoted to the production of surplus males. The causes underlying
these differences in sexual allocation are unclear, but a greater proportion of female cells
in managed populations could be obtained by increasing the diameter [
56
,
64
,
65
], or the
length [
55
,
65
] of the nesting cavities offered in orchard operations. A more balanced sex
ratio (closer to 1.7) would increase the reproductive and pollinating potential of populations
recovered from orchards.
5. Conclusions
Our study demonstrates that even a small population of a highly effective pollinator
may have a significant impact on crop pollination service and fruit set. Our results are
encouraging not only for the use of Osmia spp. as managed pollinators but also for the
implementation of measures to protect wild pollinator communities in orchard environ-
ments. In addition to Osmia spp., various species of Andrena,Eucera and Bombus are highly
effective fruit tree pollinators [
5
,
22
,
24
,
25
,
36
]. In the current scenario of pollinator declines,
agri-environmental measures to enhance wild populations of these valuable pollinators
could have important economic returns in terms of enhanced pollination service and
fruit yields.
Supplementary Materials:
The following are available online at https://www.mdpi.com/2075-445
0/12/1/56/s1, dataset. Table S1: Fruit set vs. Distance; Table S2: Osmia vs. fset row1.
Author Contributions:
J.B., conceived the research. J.B., S.O.-C., and N.V. conducted the study.
S.O.-C. analyzed the data. J.B., S.O.-C., N.V., and F.S. wrote the manuscript. J.B. secured funding.
All authors have read and agreed to the published version of the manuscript.
Funding:
This study was supported by the DGICYT (project AGR. 91-0988-CO3) and the Spanish
MEC (FPI grant to NV).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data are available in the Supplementary Materials.
Acknowledgments:
We are grateful to J. Calzadilla and M. A. Escolano for their help with field work
and nest analysis.
Conflicts of Interest: The authors declare no conflict of interest.
Appendix A
Table A1.
Output of Linear Mixed Model (LMM) relating initial fruit (A) and final fruit set (B) to
distance from the Osmia cornuta nesting stations considering a linear fit. R
2
m and R
2
c are the marginal
and conditional R2of the best-fitted models.
(A) Initial Fruit Set
t-Value p-Value
(Intercept) 32.03 <0.0001
Distance −4.73 <0.0001
R2m: 0.24; R2c: 0.66
(B) Final Fruit Set
t-Value p-Value
(Intercept) 29.22 <0.0001
Distance −4.31 0.0001
R2m: 0.19; R2c: 0.59
Insects 2021,12, 56 9 of 11
Insects 2020, 11, x 10 of 13
Table A1. Output of Linear Mixed Model (LMM) relating initial fruit (A) and final fruit set (B) to
distance from the Osmia cornuta nesting stations considering a linear fit. R
2
m and R
2
c are the marginal
and conditional R
2
of the best-fitted models.
(A) Initial Fruit Set
t-Value p-Value
(Intercept) 32.03 <0.0001
Distance −4.73 <0.0001
R
2
m: 0.24; R
2
c: 0.66
(B) Final Fruit Set
t-Value p-Value
(Intercept) 29.22 <0.0001
Distance −4.31 0.0001
R
2
m: 0.19; R
2
c: 0.59
Figure A1. Structure of the study orchard and location of the Osmia cornuta
nesting stations.
Figure A2. Initial (A) and final (B) fruit set (% of flowers setting fruit in April
and June, respectively) at increasing distances from the Osmia cornuta nesting
shelters. Each dot represents a branch. Broken lines indicate mean fruit set in
hand-pollinated trees (5 trees; 4 branches per tree). The gray bands represent
95% confidence intervals.
References
1. Klein, A.-M.; Vaissière, B.E.; Cane, J.H.; Steffan-Dewenter, I.; Cunningham, S.A.; Kremen, C.; Tscharntke,
T. Importance of pollinators in changing landscapes for world crops. Proc. R. Soc. B 2007, 274, 303–313.
Figure A1. Structure of the study orchard and location of the Osmia cornuta nesting stations.
Insects 2020, 11, x 10 of 13
Table A1. Output of Linear Mixed Model (LMM) relating initial fruit (A) and final fruit set (B) to
distance from the Osmia cornuta nesting stations considering a linear fit. R
2
m and R
2
c are the marginal
and conditional R
2
of the best-fitted models.
(A) Initial Fruit Set
t-Value p-Value
(Intercept) 32.03 <0.0001
Distance −4.73 <0.0001
R
2
m: 0.24; R
2
c: 0.66
(B) Final Fruit Set
t-Value p-Value
(Intercept) 29.22 <0.0001
Distance −4.31 0.0001
R
2
m: 0.19; R
2
c: 0.59
Figure A1. Structure of the study orchard and location of the Osmia cornuta
nesting stations.
Figure A2. Initial (A) and final (B) fruit set (% of flowers setting fruit in April
and June, respectively) at increasing distances from the Osmia cornuta nesting
shelters. Each dot represents a branch. Broken lines indicate mean fruit set in
hand-pollinated trees (5 trees; 4 branches per tree). The gray bands represent
95% confidence intervals.
References
1. Klein, A.-M.; Vaissière, B.E.; Cane, J.H.; Steffan-Dewenter, I.; Cunningham, S.A.; Kremen, C.; Tscharntke,
T. Importance of pollinators in changing landscapes for world crops. Proc. R. Soc. B 2007, 274, 303–313.
Figure A2.
Initial (
A
) and final (
B
) fruit set (% of flowers setting fruit in April and June, respectively)
at increasing distances from the Osmia cornuta nesting shelters. Each dot represents a branch. Broken
lines indicate mean fruit set in hand-pollinated trees (5 trees; 4 branches per tree). The gray bands
represent 95% confidence intervals.
References
1.
Klein, A.-M.; Vaissière, B.E.; Cane, J.H.; Steffan-Dewenter, I.; Cunningham, S.A.; Kremen, C.; Tscharntke, T. Importance of
pollinators in changing landscapes for world crops. Proc. R. Soc. B 2007,274, 303–313. [CrossRef] [PubMed]
2.
Winfree, R.; Williams, N.M.; Gaines, H.; Ascher, J.S.; Kremen, C. Wild bee pollinators provide the majority of crop visitation
across land-use gradients in New Jersey and Pennsylvania, USA. J. Appl. Ecol. 2008,45, 793–802. [CrossRef]
3.
Garibaldi, L.A.; Steffan-Dewenter, I.; Winfree, R.; Aizen, M.A.; Bommarco, R.; Cunningham, S.A.; Kremen, C.; Carvalheiro, L.G.;
Harder, L.D.; Afik, O.; et al. Wild Pollinators enhance fruit set of crops regardless of honey bee abundance. Science
2013
,339,
1608–1611. [CrossRef] [PubMed]
4.
Hevia, V.; Bosch, J.; Azcárate, F.M.; Fernández, E.; Rodrigo, A.; Barril-Graells, H.; González, J.A. Bee diversity and abundance in a
livestock drove road and its impact on pollination and seed set in adjacent sunflower fields. Agric. Ecosyst. Environ.
2016
,232,
336–344. [CrossRef]
5.
Park, M.G.; Raguso, R.A.; Losey, J.E.; Danforth, B.N. Per-visit pollinator performance and regional importance of wild Bombus and
Andrena (Melandrena) compared to the managed honey bee in New York apple orchards. Apidologie
2016
,47, 145–160. [CrossRef]
6. Klein, A.-M.; Brittain, C.; Hendrix, S.D.; Thorp, R.; Williams, N.; Kremen, C. Wild pollination services to California almond rely
on semi-natural habitat. J. Appl. Ecol. 2012,49, 723–732. [CrossRef]
7.
Woodcock, B.A.; Bullock, J.M.; Shore, R.F.; Heard, M.S.; Pereira, M.G.; Redhead, J.; Ridding, L.; Dean, H.; Sleep, D.; Henrys, P.; et al.
Country-specific effects of neonicotinoid pesticides on honey bees and wild bees. Science
2017
,356, 1393–1395. [CrossRef]
[PubMed]
8.
Hass, A.L.; Kormann, U.G.; Tscharntke, T.; Clough, Y.; Bosem Baillod, A.; Sirami, C.; Fahrig, L.; Martin, J.-L.; Baudry, J.;
Bertrand, C.; et al. Landscape configurational heterogeneity by small-scale agriculture, not crop diversity, maintains pollinators
and plant reproduction in Western Europe. Proc. R. Soc. B 2018,285, 20172242. [CrossRef] [PubMed]
Insects 2021,12, 56 10 of 11
9.
Sirami, C.; Gross, N.; Baillod, A.B.; Bertrand, C.; Carrié, R.; Hass, A.; Henckel, L.; Miguet, P.; Vuillot, C.; Alignier, A.; et al.
Increasing crop heterogeneity enhances multitrophic diversity across agricultural regions. Proc. Natl. Acad. Sci. USA
2019
,116,
16442–16447. [CrossRef] [PubMed]
10.
Kennedy, C.M.; Lonsdorf, E.; Neel, M.C.; Williams, N.M.; Ricketts, T.H.; Winfree, R.; Bommarco, R.; Brittain, C.; Burley, A.L.;
Cariveau, D.; et al. A global quantitative synthesis of local and landscape effects on wild bee pollinators in agroecosystems. Ecol.
Lett. 2013,16, 584–599. [CrossRef] [PubMed]
11.
Pitts-Singer, T.L.; Artz, D.R.; Peterson, S.S.; Boyle, N.K.; Wardell, G.I. Examination of a managed pollinator strategy for almond
production using Apis mellifera (Hymenoptera: Apidae) and Osmia lignaria (Hymenoptera: Megachilidae). Environ. Entomol.
2018
,
47, 364–377. [CrossRef] [PubMed]
12.
Roquer-Beni, L.; Alins, G.; Arnan, X.; Boreux, V.; García, D.; Hambäck, P.A.; Happe, A.-K.; Klein, A.-M.; Miñarro, M.;
Mody, K.; et al. Management-dependent effects of pollinator functional diversity on apple pollination services: A response-effect
trait approach. (Unpublished; Manuscript in Preparation).
13. McGregor, S.E. Insect Pollination of Cultivated Crop Plants; Agricultural Research Service: Washington, DC, USA, 1976; p. 411.
14. Free, J.B. Insect Pollination of Crops, 2nd ed.; Academic Press: London, UK, 1993.
15. Delaplane, K.S.; Mayer, D.R. Crop Pollination by Bees; CABI: Wallingford, UK, 2000.
16.
Vicens, N.; Bosch, J. Weather-dependent pollinator activity in an apple orchard, with special reference to Osmia cornuta and Apis
mellifera (Hymenoptera: Megachilidae and Apidae). Environ. Entomol. 2000,29, 413–420. [CrossRef]
17.
Visscher, P.K.; Seeley, T.D. Foraging strategy of honeybee colonies in temperate deciduous forest. Ecology
1982
,63, 1790–1801.
[CrossRef]
18.
Beekman, M.; Ratnieks, F.L.W. Long-range foraging by the honey-bee, Apis mellifera L. Funct. Ecol.
2000
,14, 490–496. [CrossRef]
19.
Hagler, J.R.; Mueller, S.; Teuber, L.R.; Machtley, S.A.; Van Deynze, A. Foraging range of honey bees, Apis mellifera, in alfalfa seed
production fields. J. Insect Sci. 2011,11, 1–12. [CrossRef] [PubMed]
20.
Bosch, J.; Blas, M. Foraging behaviour and pollinating efficiency of Osmia cornuta and Apis mellifera on almond (Hymenoptera,
Megachilidae and Apidae). Appl. Entomol. Zool. 1994,29, 1–9. [CrossRef]
21.
Vicens, N.; Bosch, J. Pollinating efficacy of Osmia cornuta and Apis mellifera (Hymenoptera: Megachilidae, Apidae) on ‘Red
Delicious’ apple. Environ. Entomol. 2000,29, 235–240. [CrossRef]
22.
Thomson, J.D.; Goodell, K. Pollen removal and deposition by honeybee and bumblebee visitors to apple and almond flowers.
J. Appl. Ecol. 2001,38, 1032–1044. [CrossRef]
23.
Monzón, V.H.; Bosch, J.; Retana, J. Foraging behavior and pollinating effectiveness of Osmia cornuta (Hymenoptera: Megachilidae)
and Apis mellifera (Hymenoptera: Apidae) on “Comice” pear. Apidologie 2004,35, 575–585. [CrossRef]
24.
Eeraerts, M.; Vanderhaegen, R.; Smagghe, G.; Meeus, I. Pollination efficiency and foraging behaviour of honey bees and non-Apis
bees to sweet cherry. Agric. For. Entomol. 2020,22, 75–82. [CrossRef]
25.
Roquer-Beni, L.; Arnan, X.; Rodrigo, A.; Bosch, J. What Makes a Good Pollinator? Relationship between Pollinator Traits and
Pollination Effectiveness in Apple Flowers. (Unpublished; Manuscript in Preparation).
26.
Bosch, J.; Kemp, W.P. Developing and establishing bee species as crop pollinators: The example of Osmia spp. (Hymenoptera:
Megachilidae) and fruit trees. Bull. Entomol. Res. 2002,92, 3–16. [PubMed]
27.
Sédivy, C.; Dorn, S. Towards a sustainable management of bees of the subgenus Osmia (Megachilidae; Osmia) as fruit tree
pollinators. Apidologie 2014,45, 88–105. [CrossRef]
28.
Vicens, N.; Bosch, J. Nest site orientation and relocation of populations of the orchard pollinator Osmia cornuta (Hymenoptera:
Megachilidae). Environ. Entomol. 29, 69–75. [CrossRef]
29.
Guédot, C.; Bosch, J.; Kemp, W.P. Effect of three-dimension and color contrast on nest localization performance of two solitary
bees (Hymenoptera: Megachilidae). J. Kansas Entomol. Soc. 2007,80, 90–104. [CrossRef]
30.
Biddinger, D.; Joshi, N.; Rajotte, E.; Halbrendt, N. An immunomarking method to determine the foraging patterns of Osmia
cornifrons and resulting fruit set in a cherry orchard. Apidologie 2013,44, 738–749. [CrossRef]
31.
Torchio, P.F. Use of Osmia lignaria Say (Hymenoptera: Apoidea: Megachilidae) as a pollinator in an apple and prune orchard. J.
Kansas Entomol. Soc. 1976,49, 475–482.
32.
Torchio, P.F. Field experiments with Osmia lignaria propinqua Cresson as a pollinator in almond orchards: II, 1976 studies
(Hymenoptera: Megachilidae). J. Kansas Entomol. Soc. 1981,54, 824–836.
33.
Maeta, Y. Comparative studies on the biology of the bees of the genus Osmia in Japan, with special reference to their management
for pollination of crops (Hymenoptera, Megachilidae). Bull. Tohoku Natl. Agric. Exp. Stn. 1978,57, 1–221. (In Japanese)
34.
Márquez, J.; Bosch, J.; Vicens, N. Pollens collected by wild and managed populations of the potential orchard pollinator Osmia
cornuta (Latr.) (Hym., Megachilidae). J. Appl. Entomol. 1994,117, 353–359. [CrossRef]
35.
Maccagnani, B.; Ladurner, E.; Santi, F.; Burgio, G. Osmia cornuta (Hymenoptera, Megachilidae) as a pollinator of pear (Pyrus
communis): Fruit- and seed-set. Apidologie 2003,34, 207–216. [CrossRef]
36.
Garratt, M.P.D.; Breeze, T.D.; Boreux, V.; Fountain, M.T.; McKerchar, M.; Webber, S.M.; Coston, D.J.; Jenner, N.; Dean, R.;
Westbury, D.B.; et al. Apple pollination: Demand depends on variety and supply depends on pollinator identity. PLoS ONE
2016
,
11, e0153889. [CrossRef] [PubMed]
37.
Bosch, J.; Kemp, W.P.; Trostle, G.E. Bee population returns and cherry yields in an orchard pollinated with Osmia lignaria
(Hymenoptera: Megachilidae). J. Econ. Entomol. 2006,99, 408–413. [CrossRef]
Insects 2021,12, 56 11 of 11
38.
Sheffield, C.S. Pollination, seed set and fruit quality in apple: Studies with Osmia lignaria (Hymenoptera: Megachilidae) in the
Annapolis Valley, Nova Scotia, Canada. J. Pollinat. Ecol. 2014,12. [CrossRef]
39.
Velthuis, H.H.W.; van Doorn, A. A century of advances in bumblebee domestication and the economic and environmental aspects
of its commercialization for pollination. Apidologie 2006,37, 421–451. [CrossRef]
40. van der Steen, J.J.M. Indoor rearing of the solitary bee Osmia rufa L. Proc. Exp. Appl. Entomol. NEV Amst. 1997,8, 81–84.
41.
Ladurner, E.; Maccagnani, B.; Tesoriero, D.; Nepi, M.; Felicioli, A. Laboratory rearing of Osmia cornuta Latreille (Hymenoptera
Megachilidae) on artificial diet. Boll. Dell’ist. Entomol. Univ. Bologna 1999,53, 133–146.
42. Bosch, J.; Vicens, N. Body size as an estimator of production costs in a solitary bee. Ecol. Entomol. 2002,27, 129–137. [CrossRef]
43.
Kester, D.E.; Griggs, W.H. Fruit setting in the almond: The effect of cross-pollinating various percentages of flowers. Proc. Am.
Soc. Hortic. Sci. 1959,74, 206–213.
44.
Burnham, K.P.; Anderson, D.R. Model Selection and Inference: A Practical Information-Theoretic Approach, 1st ed.; Springer: New York,
NY, USA, 2002. [CrossRef]
45.
Barton, K. ‘MuMIn’: Multi-Model Inference. R Package Version 1.43.17. Available online: https://CRAN.R-project.org/package=
MuMIn (accessed on 11 January 2021).
46.
Pinheiro, J.; Bates, D.; DebRoy, S.; Sarkar, D. ‘nlme’: Linear and Nonlinear Mixed Effects Models. R Package Version 3.1-151.
Available online: https://CRAN.R-project.org/package=nlme (accessed on 11 January 2021).
47.
R Core Team. R: A Language and Environment for Statistical Computing, Version 4.0.1; R Foundation for Statistical Computing:
Vienna, Austria, 2020.
48.
Melin, A.; Rouget, M.; Midgley, J.J.; Donadson, J.S. Pollination ecosystem services in South African agricultural systems. S. Afr.
J. Sci. 2014,110, 1–9. [CrossRef]
49.
Sgolastra, F.; Arnan, X.; Pitts-Singer, T.L.; Maini, S.; Kemp, W.P.; Bosch, J. Pre-wintering conditions and post-winter performance
in a solitary bee: Does diapause impose an energetic cost on reproductive success? Ecol. Entomol. 2016,41, 201–210. [CrossRef]
50. Felicioli, A. Le osmie. In Api e Impollinazione; Pinzauti, M., Ed.; Giunta Regionale Toscana: Firenze, Italy, 2000; pp. 159–188.
51.
Kruni´c, M.; Stanisaviljevi´c, L.Ž. The Biology of European Orchard bee Osmia Cornuta (Latr.) (Hymenoptera: Megachilidae); Faculty of
Biology University of Belgrade: Belgrade, Serbia, 2006.
52.
Bosch, J.; Kemp, W.P. How to Manage the Blue Orchard Bee: As an Orchard Pollinator; Sustainable Agriculture Network: Beltsville,
MD, USA, 2001.
53.
Bosch, J.; Vicens, N. Relationship between body size, provisioning rate, longevity and reproductive success in females of the
solitary bee Osmia cornuta.Behav. Ecol. Sociobiol. 2006,60, 26–33. [CrossRef]
54.
Boyle, N.K.; Pitts-Singer, T.L. The effect of nest box distribution on sustainable propagation of Osmia lignaria (Hymenoptera:
Megachilidae) in commercial tart cherry orchards. J. Insect Sci. 2016,17, 41. [CrossRef]
55.
Bosch, J. Osmia cornuta Latr. (Hymenoptera, Megachilidae) as a potential pollinator in almond orchards: Releasing methods and
nest-hole length. J. Appl. Entomol. 1994,117, 151–157. [CrossRef]
56.
Bosch, J. Improvement of field management of Osmia cornuta Latr. for almond pollination (Hymenoptera, Megachilidae).
Apidologie 1994,25, 71–83. [CrossRef]
57.
Bosch, J. Parasitism in wild and managed populations of the almond pollinator Osmia cornuta Latr. (Hymenoptera, Megachilidae).
J. Apicult. Res. 1992,31, 77–82. [CrossRef]
58.
Bosch, J. Comparison of nesting materials for the orchard pollinator Osmia cornuta Latr. (Hymenoptera, Megachilidae). Entomol.
Gen. 1995,19, 285–289. [CrossRef]
59.
Torchio, P.F. Field experiments with the pollinator species, Osmia lignaria propinqua Cresson (Hymenoptera: Megachilidae) in
apple orchards: III, 1977 studies. J. Kansas Entomol. Soc. 1984,57, 517–521.
60.
Torchio, P.F. Field experiments with the pollinator species, Osmia lignaria propinqua Cresson in apple orchards: V, 1979–1980,
methods of introducing bees, nesting success, seed counts, fruit yields (Hymenoptera: Megachilidae). J. Kansas Entomol. Soc.
1985,58, 448–464.
61.
Pitts-Singer, T.L. Olfactory response of megachilid bees, Osmia lignaria,Megachile rotundata, and M. pugnata, to individual cues
from old nest cavities. Environ. Entomol. 2007,36, 402–408. [CrossRef]
62.
Vicens, N.; Bosch, J.; Blas, M. Análisis de los nidos de algunas Osmia (Hymenoptera, Megachilidae) nidificantes en cavidades
preestablecidas. Orsis 1993,8, 41–52.
63.
Bosch, J.; Vicens, N. Sex allocation in the solitary bee Osmia cornuta: Do females behave in agreement with Fisher’s theory? Behav.
Ecol. Sociobiol. 2005,59, 124–132. [CrossRef]
64.
Tepedino, V.J.; Torchio, P.F. The influence of nest-hole selection on sex ratio and progeny size in Osmia lignaria propinqua
(Hymenoptera: Megachilidae). Ann. Entomol. Soc. Am. 1989,82, 355–360. [CrossRef]
65.
Seidelmann, K.; Bienasch, A.; Pröhl, F. The impact of nest tube dimensions on reproduction parameters in a cavity nesting solitary
bee, Osmia bicornis (Hymenoptera: Megachili-dae). Apidologie 2016,47, 114–122. [CrossRef]
