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http://dx.doi.org/10.12899/asr-2116
Annals of Silvicultural Research
45(1), 2020: 99-104
https://journals-crea.4science.it/index.php/asr
99
ABSTRACT Hive thermoregulation is fundamental for the normal development of bee colonies and, consequently, hive productivity
and honey bee health. External conditions mainly affect the walls of the hive. Therefore, hive construction materials and thermal
conductivity features can inuence its thermoregulation efciency. The present trial made a comparison of experimental hives (mo-
died Dadant-Blatt of 10 frames) made with cork as thermal insulator and conventional hives made entirely with rwood to evaluate
their effects on thermoregulation of Apis mellifera ligustica colonies in Northwerstern Sardinia (Italy). The cork-modied beehives
consisted of common conventional beehives modied by replacing the wooden walls with cork walls (pressed cork), whereas the
control beehives (wooden hives) consisted of standard Dadant-Blatt beehives entirely made of rwood. Environmental (especially
nest internal temperature) parameters were assessed periodically. The daily temperature pattern of cork-modied beehives was
more regular than that of control beehives. In addition, bees had a more efcient winter thermoregulation in cork-modied beehives
compared with control hives.
KEYWORDS: cork beehives, thermal insulation, thermoregulation, forest product.
The use of cork in the thermoregulation of the hive: an innovation
attempt to enhance non-wood products and beekeeping in
Mediterranean forests
Ignazio Floris1, Michelina Pusceddu1, Elia Raccimolo1, Antonio Casula2, Giuliano Patteri2, Alberto Satta1*
Received: 15/01/2020 Accepted: 31/08/2020 Published: 30/10/2020
Introduction
The microclimatic conditions of the hive play an
important role in colony homeostasis, particular-
ly in the maintenance of optimal nest temperature
regardless of external conditions (Dyer and Seeley
1987, Ruttner 1988, Heinrich and Esch 1997). Sta-
ble conditions inside the hive have a positive effect
on brood rearing, and the state of colonies at low
temperatures (overwintering) and over summer
in hot conditions (Winston 1991). Although honey
bees (workers) are able to respond efficiently to
environmental conditions using physiological and
behavioural mechanisms for temperature control,
this response involves an energy cost, such as con-
sumption of honey stocks (Esch and Bastian 1968,
Kronemberg and Heller 1982, Heinrich 1996). When
the honeybee colony perceives the temperature in-
side the nest as too high, its foragers collect water,
thus increasing ventilation and evaporative cooling
by fanning their wings at the hive entrance (Lindau-
er 1955, Kiechle 1961, Lensky 1964, Kühnholz and
Seeley 1997). When the elevated temperature is lo-
calized in a specific point of the nest, the workers re-
spond with heat shielding (Starks and Gilley 1999).
In contrast, when the temperature is perceived as
too low, the worker bees respond by producing met-
abolic heat and forming a cluster (Heinrich 1981 and
1995, Kronenberg and Heller 1982, Harrison 1987).
Although honeybees maintain the temperature of
their nest elevated (Seeley 2014) mainly to acceler-
ate brood development (Milum 1930), thermoregu-
lation may also influence the sanitary status of bee
colonies. In fact, the optimal temperature for the re-
production of the parasitic mite Varroa destructor
in beehives is between 32.5 and 33.4 °C (Fremuth
1985, Le Conte and Arnold 1987 and 1988, Le Con-
te et al. 1990). Above 36.5 °C mite reproduction is
significantly reduced and above 38 °C female mites
die without reproducing (Le Conte et al. 1990). For
this reason, the original natural host of Varroa, Apis
cerana maintains the temperature of the brood high-
er than A. mellifera (Le Conte et al. 1990, Yang et
al. 2010). Thermoregulation may play an important
role in contrasting also other diseases, such as the
fungus Ascosphaera apis, agent of the chalkbrood
disease, which needs a temperature of approximate-
ly 30° C to germinate (Bailey 1966; 1981). In this
case, it is necessary that the colony recognizes the
infested larvae early and then increases the brood-
comb temperature to limit the pathogen effects. This
mechanism was defined as social fever by Starks et
al. (2000). Finally, the elasticity in thermoregulation
capacity is exploited by bees in defence against
predators as well (Ono et al. 1995).
In the last decades, hive models made of poly-
styrene or plastic have been proposed, especially to
meet the needs of cold climates. However, the use
of these materials is not always satisfactory, due to
their fragility or excessive impermeability, whereas
conventional wooden hives do not have these dis-
advantages. On the other hand, a hive that is too
isolated from the external environment could show
problems related to internal overheating that, in this
case, could not always be efficiently compensated by
the regulatory function of bees (Büdel 1968). Anoth-
er method that has been evaluated experimentally
1 Dipartimento di Agraria, Sezione di Patologia vegetale ed Entomologia, Università di Sassari
2 Agenzia Fo.Re.STAS - Agenzia forestale regionale per lo sviluppo del territorio e dell’ambiente della Sardegna
*Corresponding author: albsatta@uniss.it
Research paper Collection: “4th Italian National Congress of Silviculture”
Torino, 5-9 November 2018
Annals of Silvicultural Research
IgnazIo FlorIs, MIchelIna Pusceddu, elIa raccIMolo, antonIo casula, gIulIano PatterI, alberto satta
The use of cork in the thermoregulation of the hive: an innovation attempt to enhance non-wood products and beekeeping in Mediterranean forests
100
to mitigate the effects of cold climate, but with lim-
ited results, is the black colour of the hives (Madren
1995). Considering that the thermoregulatory capac-
ity of the bees has its limitations and the influence
of the external environment on the hive is exerted
mainly through the walls of the hive, and only mini-
mally through the entrance of the hive, the choice of
the material used to make the beehive is important
(Satta and Floris 2004). In various environments and
civilizations, hive construction was influenced by
the availability of suitable materials (earth materials,
such as sun-dried mud and fired clay, or plant materi-
als, such as hollowed log, cork bark, woven cylinder,
Ferula stems and wooden boards) (Crane 1999). In
Sardinia (Italy), North Africa and other Mediterrane-
an regions, the cork bark cylinder (in horizontal or
vertical position) was commonly used (Crane 1999).
It is well known that cork is superior in insulating
properties to wood, because it has a thermal conduc-
tivity of about 0.052 W/mK compared to a value of
0.10-0.12 W/mK of firwood or pine at 20 °C. In addi-
tion, cork is lighter and more resistant to mold. On
the basis of these characteristics, the traditional use
of cork in hive construction, and the economic inter-
est in this non-wood forest product, we tested the in-
sulating properties of cork as construction material
of modern hives and its impact on thermoregulation
of Italian bee colonies in comparison to traditional
beehives made of firwood.
Materials and Methods
Hive models
The study was performed in an experimental
apiary in Northwestern Sardinia from December
2016 to April 2017, at the experimental farm of the
Department of Agriculture of the University of Sas-
sari (latitude 40˚46’23”, longitude 8˚29’34”).
Based on the Dadant-Blatt model, modern cork-
wooden hives (52.0 x 34.8 cm), with 3-cm-thick walls
made mostly of cork (83%) and only a thin inner la-
yer of wood (0.5 cm). These cork-modified beehives
were handcrafted in the woodworking facility of the
regional Forestry Agency (Forestas - Sardinia) (Fig.
1). Standard Dadant-Blatt wooden hives were used
as control (Fig. 1). Differences in thermal insulation
capability of the experimental hive and the control
hive were preliminarily estimated by computing the
thermal power transmitted outside (P) for each
type of hive with the following formula:
P = λStot [(Ti–Te)/D]
where:
λ = Thermal conductivity determined considering
a wall thickness equal to 3.0 cm in the cork/wooden
hive and 2.5 cm in the firwood hive
Stot = external surface of the two hive models (1.11 m2)
Ti = internal temperature
Te = external temperature
D = thickness of the hive walls
The thermal power transmitted outside was
calculated for the months of December, January, Fe-
bruary, March and April considering the temperature
of the brood chamber (35 °C) as internal temperatu-
re and the monthly average of 9.86 °C for December,
7.29 °C for January, 9.73 °C for February, 10.37 °C
for March and 11.49 °C for April as external tempe-
rature.
Experimental hive group
Two experimental groups of four hives each were
used in the experiment. Colonies of a local strain of
Apis mellifera ligustica, containing about the same
amount of adult bees, brood (eggs, larvae and sealed
brood) and stocks of honey and pollen, preliminarily
monitored using one-sixth of a Dadant–Blatt frame
(188 cm2) as a unit of measure (Marchetti 1985), were
placed into each cork-modified or control beehive.
Data collection
Temperature inside and outside each experimen-
tal (cork-wooden) or control (wooden-only) beehive
was monitored using ibutton mini-sensors (model
DS 1923-F5#). Inside the hive, the sensors were pla-
ced in central position between two combs contai-
ning brood (central combs) or between two combs
containing pollen or honey stores (side combs).
Outside the hive, the sensors were placed at a short
distance from the hive entrance and kept suspen-
ded with a small wooden support. Temperature data
were recorded hourly throughout two periods of two
weeks each, the first between the 15th and 28th of Ja-
nuary 2017 and the second between the 18th and 31th
of March 2017.
Data from each experiment were analysed se-
parately by fitting linear models using Generalized
Least Squares (GLS) in R software (R Development
Core Team 2018) with nlme package (Pinheiro et al.
2017). A compound symmetry correlation structure
was considered (Pinheiro and Bates 2000).
Figure 1 - Experimental beehive model (cork-wooden beehive,
on the left), with walls made by cork, and standard Dadant-Blatt
beehive (wooden beehive, on the right), with walls made by
rwood, used as control hive.
Annals of Silvicultural Research
IgnazIo FlorIs, MIchelIna Pusceddu, elIa raccIMolo, antonIo casula, gIulIano PatterI, alberto satta
The use of cork in the thermoregulation of the hive: an innovation attempt to enhance non-wood products and beekeeping in Mediterranean forests
101
Another experiment was performed in April to
assess the time necessary to restore the optimal tem-
perature (~ 35 °C) in the brood chamber when the
hive is opened to check the colonies. In this case,
the brood-comb containing the sensor was extracted
and kept out of the hive for five minutes. After this
time, the brood-comb was put back into place and
the beehive closed. The sensors had been program-
med to record the temperature every minute.
Results
Theoretical isolation capacity of the experi-
mental hives
Figure 2 clearly shows the greater thermal di-
spersion of wooden hives compared to cork-wooden
hives, corresponding, on average, a difference of ap-
proximately 74,569 cal./h. It is also evident that the
differences in the thermal dispersion between the
two hive models increase when the outside tempera-
ture decreases, as it occurred in January.
Temperature trend inside the hives
In January, the daily mean temperature in the bro-
od chamber showed significantly higher values (F =
48.7, P<0.0001) and a more regular trend in the cork-
wooden hives, with an oscillation of only 0.34 °C (T.
min. = 34.53 °C and T. max = 34.87 °C), compared to
control hives, which had lower values (T. min = 30.40
°C and T. max = 32.59 °C) and a more marked oscilla-
tion (2.18 °C) (Fig. 3). In the same month, the tempe-
rature of the side combs was also significantly higher
in the cork-wooden hives (F = 117.3, P<0.0001), with
a variation of 5.88°C (T. min. = 17.63 °C and T. max =
23.51 °C), compared to the control hives, which sho-
wed an average variation of 8.36 °C (T. min = 9.52
°C and T. max = 17.88 °C) (Fig. 4). A stronger rela-
tionship, with a significant positive linear regression
between the side comb and the outside temperature,
was found for wooden-only hives compared to cork-
wooden ones (P = 0.0001 and R2 = 73.68 vs P = 0.0487
and R2 = 28.63).
As observed in January (Figs. 3 and 4), in March a
greater oscillation of the brood chamber temperature
occurred in the wooden-only hives (T. min. = 35.21 °C
and T. max = 34.50 °C) compared to the cork-wooden
ones (T. min. = 35.42 °C and T. max = 35.61 °C) (Fig.
5) but in this case the difference between the trend
of temperature in the two type of hive was not sig-
nificant (F = 1.2, P = 28.31). Moreover, the difference
between T. max and T. min. was much less marked
for both wooden-only hives and cork-wooden hives
(0.29 vs 0.19) compared to that observed in January.
Because brood was found also in the side combs in
March, the temperature in this area of the hive was
not recorded during that month.
Fig. 2
0
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14
0
20000
40000
60000
80000
100000
120000
140000
December January February March April
Temperature °C
Thermal power cal./h
Cork-wooden hive Wooden hive Mean monthly temperature
Fig.3
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12
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38
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°C
Brood chamber temperature
°C
Cork-wooden hive Wooden hive Outside temperature
Fig. 4
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External comb temperature °C
Cork-wooden hive Wooden hive Outside temperature
Fig. 5
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16
Outside temperature
°C
Brood chamber temperature (
°C)
Cork-wooden hive Wooden hive Outside temperature
36.0
35.5
35.0
34.5
34.0
Figure 2 - Thermal power transmitted outside (main y axis)
calculated for cork-wooden hives and wooden beehive (control
hive). The mean monthly outside temperature is also represented
in the gure (secondary y axis).
Figure 3 - Trend of the brood chamber temperature (mean ± SE)
throughout a period of two weeks in January in cork-wooden and
wooden beehives (main y axis). The outside temperature is also
represented in the gure (secondary y axis).
Figure 4 - Trend of the side comb temperature (mean ± SE)
throughout a period of two weeks in January (winter) in cork-
wooden and wooden beehives (main y axis). The outside
temperature is also represented in the gure (secondary y axis).
Figure 5 - Trend of the brood chamber temperature (mean ±
SE) throughout a period of two weeks in March (spring) in
cork-wooden and wooden beehives (main y axis). The outside
temperature is also represented in the gure (secondary y axis).
Annals of Silvicultural Research
IgnazIo FlorIs, MIchelIna Pusceddu, elIa raccIMolo, antonIo casula, gIulIano PatterI, alberto satta
The use of cork in the thermoregulation of the hive: an innovation attempt to enhance non-wood products and beekeeping in Mediterranean forests
102
In a winter day (20th of January), the temperature
recorded in the brood chamber over a 24-hour pe-
riod showed more constant values in the cork-woo-
den hives than in the control ones (Fig. 6). In general,
temperature was kept significantly higher (F = 24.99,
P<0.0001) in cork-wooden hives (T. min = 34.01 °C
and T. max = 34.88 °C), with an oscillation of 0.87 °C,
compared to wooden-only hives (T. min = 30.44 °C
and T. max = 33.31 °C), which had a higher oscilla-
tion (2.87 °C). A similar significant trend was obser-
ved in the same day for the temperature recorded in
the side comb (Fig. 7) (F = 177.2, P < 0.0001).
In March, the temperature trend monitored in the
brood chamber over a 24-hour period did not eviden-
ce significant differences between hive groups, with
minimal fluctuations (0.16-0.17 °C) of temperature in
both model of hives (F = 2.5, P=0.119).
The time necessary to restore the temperature
of the brood chamber after opening the hive in April
differed between the two hive types (Fig. 8): after
closing the hive (time 0), the cork-wooden hives
took 14 minutes to reach the temperature of 35.56
°C, whereas the wooden hives took 40 minutes to
reach 35.10 °C.
Discussion
The efficiency of cork in the isolation of the hive,
expected from a theoretical point of view due to its
lower thermal conductivity compared to wood, was
confirmed by the experimental observations. In fact,
in comparison to the conventional firwood beehives,
during January the experimental cork-wooden hives
showed a more regular temperature pattern in the
nest (brood chamber and side combs) and the ability
to restore the optimal thermal conditions more quic-
kly after the opening and closing of the hive, which
simulated its management in apiary. Moreover, the
temperature variations recorded inside the hives
were more dependent on the external temperature
variation in the conventional wooden hives than in
the experimental cork-wooden hives, where the tem-
perature trend was more constant and in line with
the optimal temperatures for brood growth (Winston
1991). The greater thermal insulation capacity of the
cork-wooden hives observed in January was less
evident in March, when differences in temperature
trends between the cork-wooden hives and the con-
ventional firwood hives were not significant. Consi-
dering this scenario, it can be hypothesized that bees
had to consume greater quantities of honey in the
firwood beehives, in order to maintain comparable
thermal conditions with those of the experimental
cork-wooden hives. Indeed, another interesting ef-
fect of hives modified with the use of cork, reported
in a previous study (Satta and Floris 2004), concerns
a lower consumption of honey during winter by the
colonies reared in cork-modified hives compared to
those in the firwood hives, probably due to lower he-
ating needs for winter thermoregulation in the cork-
wooden hives. In that study, an average amount of
approximately 3.5 kg per hive of stored honey was
saved in the winter season. Considering the fairly
mild environmental conditions in which that amount
of honey was saved in the study of Satta and Flo-
ris (2004) and the more efficient thermoregulation
found in the cork-modified beehives in the present
study, there are interesting prospective applications
of cork-wooden beehives in more severe climatic
conditions. Historical findings on traditional apicul-
ture indicate a widespread use of cork for the rustic
hive construction in the Mediterranean area (Floris
and Prota 1989, Crane 1999), with some attempts of
adoption of this material in initial forms of semi-ra-
tional or rational beekeeping (Floris and Satta 2009).
Fig. 6.
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Outside temperature
°C
Brood chamber temperature
°C)
Hour
Cork-wooden hive Wooden hive Outside temperature
Fig. 7
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Fig. 8
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34
35
36
37
0 2 4 6 8 10 12 14 24 26 28 30 32 34 36 38 40
Brood chamber temperature
°C
Minutes
Cork-wooden hive Wooden hive
Figure 6 - Trend of the brood chamber temperature (mean
± SE) over 24 hours (20th of January) in cork-wooden and
wooden beehives (main y axis). The outside temperature is also
represented in the gure (secondary y axis).
Figure 7 - Trend of the external comb temperature (mean
± SE) over 24 hours (20th of January) in cork-wooden and
wooden beehives (main y axis). The outside temperature is also
represented in the gure (secondary y axis).
Figure 8 - Rhythm of restoration of the optimal temperature in
the brood chamber in cork-wooden and wooden beehives. The
brood-comb containing the sensor for measuring the temperature
was left outside the hive for 5 minutes and then repositioned in its
place (April, spring).
Annals of Silvicultural Research
IgnazIo FlorIs, MIchelIna Pusceddu, elIa raccIMolo, antonIo casula, gIulIano PatterI, alberto satta
The use of cork in the thermoregulation of the hive: an innovation attempt to enhance non-wood products and beekeeping in Mediterranean forests
103
Based on our preliminary results, the combina-
tion of cork and wood in the construction of modern
hives represents a kind of product innovation since
exploiting in synergy the properties of the two ma-
terials, consents the enhancement of both in a new
application (Wolfslehner et. al. 2019) and could re-
sult in an interesting synergy between apiculture and
woodland management or forestry. In the Mediter-
ranean areas, the agro-silvo-pastoral system covered
by cork oak forests is not only the most widespre-
ad but also a very important hotspot of biodiversity
(Myers et al. 2000). Currently, this type of ecosystem
is threatened, and adequate management and active
use by human are required to maintain its existence.
The main product of the agro-silvo-pastoral system
is cork, but a decline in cork oaks has been obser-
ved in sub-western Europe, due to the substitution
of this natural material with other types of materials
(Bugalho et al. 2011). For this reason, a greater use
of cork in the green building sector could have a po-
sitive economic and environmental impact.
For example, the production of panels for the
construction of cork hives does not require a raw
material of particular quality or a different type of
management of cork forests. In fact, but the cork
used in bee hives, called “granulated”, is obtained
from secondary products of the cork industry. The
exploitation of this “waste resource” is a conside-
rable advantage of this model of beehive, which
ecofriendly. In addition, the use of this hive model
promotes the sustainable use of Mediterranean agro-
forestry systems, thus contributing to their conserva-
tion, with a positive effect on sensitive species such
as the eagle (Mannu et al. 2018). Another added va-
lue of this type of agro-forest ecosystem is the main-
tenance of beekeeping thanks to the availability of
resources throughout the year that positively affect
the development and health of the colonies (Floris et
al. 2016). Moreover, in this context, honey is another
resource in balance with the agro-pastoral activities
and the forest system to be preserved (Croitoru and
Merlo 2005). In conclusion, this new beehive model
provides a new possibility for enhancing a non-wood
product as cork and improving the performance of
hives not only in the forest context. This is in line
with the European Union’s bio-economy strategies,
which support and promote new opportunities for
the forestry sector that may arise from the combina-
tion of sustainable bio-based materials such as cork
and wood (European Commission 2012). Finally, al-
though further studies on a larger number of hives
and during a longer time are required, our study de-
monstrates the effectiveness of cork in the thermal
insulation of hives and, consequently, in their ther-
moregulation.
Acknowledgements
This publication was funded by the “Agenzia Fo-
restas - Sardinia” (regional Forestry Agency) by the
program “Progetto di sviluppo e studio per la riquali-
ficazione dell’apicoltura nei compendi forestali della
Sardegna”. We are also very grateful to Dr. Ana Hele-
na Dias Francesconi for the English revision and to
Dr. Roberto Mannu for the support in the data statis-
tical analysis.
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