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Journal of Apicultural Research
ISSN: 0021-8839 (Print) 2078-6913 (Online) Journal homepage: http://www.tandfonline.com/loi/tjar20
Effects of short-term exposure to mobile phone
radiofrequency (900MHz) on the oxidative
response and genotoxicity in honey bee larvae
Marinko Vilić, Ivana Tlak Gajger , Perica Tucak, Anamaria Štambuk , Maja
Šrut , Göran Klobučar , Krešimir Malarić , Ivona Žura Žaja, Ana Pavelić ,
Marin Manger & Mirta Tkalec
To cite this article: Marinko Vilić, Ivana Tlak Gajger , Perica Tucak, Anamaria Štambuk , Maja
Šrut , Göran Klobučar , Krešimir Malarić , Ivona Žura Žaja, Ana Pavelić , Marin Manger & Mirta
Tkalec (2017) Effects of short-term exposure to mobile phone radiofrequency (900MHz) on the
oxidative response and genotoxicity in honey bee larvae, Journal of Apicultural Research, 56:4,
430-438, DOI: 10.1080/00218839.2017.1329798
To link to this article: http://dx.doi.org/10.1080/00218839.2017.1329798
Published online: 04 Jul 2017.
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ORIGINAL RESEARCH ARTICLE
Effects of short-term exposure to mobile phone radiofrequency (900 MHz) on the
oxidative response and genotoxicity in honey bee larvae
Marinko Vilic
´
a
, Ivana Tlak Gajger
b
, Perica Tucak
c
, Anamaria S
ˇtambuk
d
, Maja S
ˇrut
d
,Go
¨ran Klobuc
ˇar
d
,
Kres
ˇimir Malaric
´
e
, Ivona Z
ˇura Z
ˇaja
a
, Ana Pavelic
´
d
, Marin Manger
d
and Mirta Tkalec
d
*
a
Faculty of Veterinary Medicine, Department of Physiology and Radiobiology, University of Zagreb, Zagreb, Croatia;
b
Faculty of
Veterinary Medicine, Department for Biology and Pathology of Fish and Bees, University of Zagreb, Zagreb, Croatia;
c
Ministry of Agriculture Veterinary and Food Safety Directorate, Zagreb, Croatia;
d
Faculty of Science, Department of Biology,
University of Zagreb, Zagreb, Croatia;
e
Faculty of Electrical Engineering and Computing, University of Zagreb, Zagreb, Croatia
(Received 4 January 2017; accepted 9 May 2017)
Exposure of different animal species to radiofrequency electromagnetic fields (RF-EMF) could cause various biological effects
such as oxidative stress, genotoxic effects and dysfunction of the immune system. However, there are a lack of results on
oxidative stress response and genotoxicity in the honey bee (Apis mellifera) after exposure to RF-EMF. This study was per-
formed to investigate the effects of exposure to RF-EMF on the activity of catalase, superoxide dismutase, glutathione S-trans-
ferase, lipid peroxidation level and DNA damage in honey bee larvae. Honey bee larvae were exposed to RF-EMF at 900 MHz
and field levels of 10, 23, 41 and 120 V m
−1
for 2 h. At a field level of 23 V m
−1
the effect of 80% AM 1 kHz sinusoidal and
217 Hz modulation was investigated as well. Catalase activity and the lipid peroxidation level decreased significantly in the
honey bee larvae exposed to the unmodulated field at 10 V m
−1
compared to the control. Superoxide dismutase and glu-
tathione S-transferase activity in the honey bee larvae exposed to unmodulated fields were not statistically different compared
to the control. DNA damage increased significantly in honey bee larvae exposed to modulated (80% AM 1 kHz sinus) field at
23 V m
−1
compared to the control and all other exposure groups. These results suggest that RF-EMF effects in honey bee lar-
vae appeared only after exposure to a certain EMF conditions. The increase of the field level did not cause a linear dose-re-
sponse in any of the measured parameters. Modulated RF-EMF produced more negative effects than the corresponding
unmodulated field. Although honey bees in nature would not be exposed to such high field levels as used in our experiments,
our results show the need for further intensive research in all stages of honey bee development.
Efectos a corto plazo de la exposicio
´n a la radiofrecuencia de los tele
´fonos mo
´viles (900 MHz) sobre la
respuesta oxidativa y la genotoxicidad en las larvas de abejas
La exposicio
´n de diferentes especies animales a campos electromagne
´ticos de radiofrecuencia (CEM-RF) podrı
´a causar diver-
sos efectos biolo
´gicos tales como estre
´s oxidativo, efectos genoto
´xicos y disfuncio
´n del sistema inmunolo
´gico. Sin embargo,
hay una falta de resultados sobre la respuesta al estre
´s oxidativo y la genotoxicidad en la abeja de la miel (Apis mellifera)
despue
´s de la exposicio
´n a CEM-RF. Este estudio fue realizado para investigar los efectos de la exposicio
´n a CEM-RF sobre la
actividad catalasa, supero
´xido dismutasa, glutatio
´n S-transferasa, nivel de peroxidacio
´ndelı
´pidos y dan
˜o al ADN en larvas de
abejas. Las larvas de abejas fueron expuestas a CEM-RF de 900 MHz e intensidades de campo de 10, 23, 41 y 120 V m
−1
dur-
ante 2 h. A 23 V m
−1
de intensidad de campo, se investigo
´tambie
´nelefectodel80%demodulacio
´ndeamplitudde1kHzsinu-
soidal y 217 Hz de modulacio
´n. La actividad catalasa y el nivel de peroxidacio
´nlipı
´dica disminuyeron significativamente en las
larvas de abejas expuestas al campo no modulado de 10 V m
−1
en comparacio
´n con el control. La actividad supero
´xido dismu-
tasa y glutatio
´n-S-transferasa en las larvas de abejas expuestas a campos no modulados no fue estadı
´sticamente diferente en
comparacio
´n con el control. El dan
˜o en el ADN aumento
´significativamente en larvas de abejas expuestas a campos modula-
dos (80% AM 1 kHz sinusoidal) de 23 V m
−1
en comparacio
´n con el control y todos los dema
´sgruposdeexposicio
´n. Estos
resultados sugieren que los efectos de CEM-RF en larvas de abejas de miel aparecieron so
´lo despue
´s de la exposicio
´naciertas
condiciones de los campos electromagne
´ticos. El aumento de la intensidad del campo no causo
´una dosis-respuesta lineal en
ninguno de los para
´metros estudiados. El CEM-RF modulado produjo ma
´s efectos negativos que el campo no modulado cor-
respondiente. Aunque las abejas melı
´feras en la naturaleza no estarı
´an expuestas a intensidades de campo tan altas como las
utilizadas en nuestros experimentos, nuestros resultados indican la necesidad de una mayor investigacio
´n en todas las etapas
del desarrollo de las abejas melı
´feras.
Keywords: antioxidative enzymes; DNA damage; genotoxicity; honey bee larvae; lipid peroxidation; oxidative damage;
radiofrequency electromagnetic fields
Introduction
The use of devices that emit radiofrequency electro-
magnetic fields (RF-EMF), such as mobile phones, has
arisen rapidly in recent years. In 2015, there were
more than 7 billion mobile cellular subscriptions esti-
mated, with a tendency of further growth worldwide
(The International Telecommunication Union [ITU],
2016). With increasing usage of mobile phones, public
*Corresponding author. Email: mirta.tkalec@biol.pmf.hr
©2017 International Bee Research Association
Journal of Apicultural Research, 2017
Vol. 56, No. 4, 430–438, https://doi.org/10.1080/00218839.2017.1329798
concern about possible adverse health effects from
exposure to mobile phone signals have occurred.
Although the absorbed energy from mobile phones
cannot break chemical bonds, the results of many stud-
ies have shown that exposure to radiofrequency radia-
tion at the operating frequency of mobile phones can
induce various biological effects. These include oxida-
tive stress in human cells (Luukkonen, Hakulinen, Ma
¨ki-
Paakkanen, Juutilainen, & Naarala, 2009; Moustafa,
Moustafa, Belacy, Abou-El-Ela, & Ali, 2001), plants (Tka-
lec, Malaric
´, & Pevalek-Kozlina, 2007), earthworms
(Tkalec, S
ˇtambuk, S
ˇrut, Malaric
´, & Klobuc
ˇar, 2013),
genotoxic effects in human lymphocytes (Esmekaya
et al., 2011), earthworms (Tkalec et al., 2013), plants
(Tkalec, Malaric
´, Pavlica, Pevalek-Kozlina, & Vidakovic
´-
Cifrek, 2009) and other ecological effects in different
organisms (for review see Cucurachi et al., 2013). Fur-
thermore, alterations of antioxidant defense system
parameters such as glutathione (GSH) concentration or
glutathione peroxidase (GSH-Px), superoxide dismutase
(SOD) and catalase (CAT) activity, have also been doc-
umented in human cells (Moustafa et al., 2001) and rats
(Aydin & Akar, 2011), earthworms (Tkalec et al.,
2013), plants (Tkalec et al., 2007).
However, it is very important to emphasizes that
the mentioned effects of RF-EMF are still controversial
because there are several studies that have not found
biological effects after exposure to RF-EMF or have
some shortcomings, especially in studying genotoxic
effects (for reviews see Ruediger, 2009; Verschaeve,
2009; Vijayalaxmi & Prihoda, 2012).
Previous studies regarding effects of RF-EMF at fre-
quencies from 900 MHz to 2.4 GHz in honey bee colo-
nies investigated mostly adult honey bees. Mall & Kumar
(2014) found no effect on brooding, honey production
and foraging behavior, but other studies have shown
decreased colony strength and egg laying rate of the
queen (Sharma & Kumar, 2010), induction of “worker
piping” which is associated with swarming (Favre, 2011)
and the alteration of some biochemical parameters (total
carbohydrates, glucose, glycogen, total lipids, cholesterol,
protein, hexokinase, alkaline phosphatase, glucose 6-
phosphatase and free amino acids) in the hemolymph of
the drone (Kumar, Neha, & Preeti, 2013). However,
according to our knowledge, studies on the influence of
the RF-EMF on oxidative stress and/or genotoxicity in
honey bee larvae after exposure to radiofrequency at
900 MHz are not yet available.
The effects of RF-EMF on biological systems may
depend on physical properties of radiation such as fre-
quency, straight, modulation, exposure time (Kwee &
Raskmark, 1998; Luukkonen et al., 2009; Sarimov, Malm-
gren, Markova
`, Persson, & Belyaev, 2004) as well as on
biological parameters such as species or development
stage (Cucurachi et al., 2013). Therefore, in this study
we wanted to investigate the oxidative stress parame-
ters and genotoxicity in honey bee larvae, a vulnerable
stage of development, after short-term exposure to RF-
EMF under controlled laboratory conditions at the
operating frequency of mobile phones (900 MHz) with
different field levels and modulation.
We have used the field level of 10 V m
−1
as a stan-
dard value of operating mobile devices, then 23 V m
−1
as
the value of the electric field when establishing the con-
nection of mobile devices, 41 V m
−1
as the maximum
value when dialing, and 120 V m
−1
as a limit value (taking
factor 100) above the value (1 V m
−1
) that honey bees
could be exposed in the natural environment. Although
values of field strength used in this study were higher
than those under natural conditions, we used them to
trace a linear dose-response relationship of RF radiation
and/or possible occurrence of an increased bioactivity of
RF-EMF under specific conditions. In addition, based on
results of our previous investigations on the earthworms
and plants (Tkalec, Malaric
´, & Pevalek-Kozlina, 2005;
Tkalec et al., 2007; Tkalec et al., 2009; Tkalec et al.,
2013), we compared the effects of unmodulated and
modulated wave of RF radiation at 23 V m
−1
as the most
common field level. Namely, RF signal having a carrier
frequency of 900 MHz and amplitude modulated at
217 Hz is similar to that used by the global system for
mobile communication (GSM) telephone system.
The objective of this study was therefore to answer
the questions: (a) could exposure to RF-EMF at
900 MHz cause oxidative stress and genotoxic effect in
larval stage of honey bees under laboratory conditions;
and (b) is there a possibility of a different bioactivity of
RF-EMF after honey bee larvae exposure to RF-EMF at
specific field levels and modulation.
Material and methods
Honey bees (Apis mellifera)
Honey bee larvae were used in the experiment. Frames
with combs containing young honey bee brood were
taken from one hive, type Langstroth – Root. After
frames were taken from hives they were wrapped with
moist flannel and placed in a carton box. Frames with
honey bee brood were transported from bee yard to the
laboratory every day under the same conditions. Upon
arrival in the laboratory, prior to exposure, the honey
bee larvae approximately five to six days old, were ran-
domly collected from delivered combs, using entomologi-
cal tweezers, stored on a moist filter paper in Petri dishes
with perforated lids and immediately placed in a Gigahertz
transversal electromagnetic (GTEM) cell for irradiation
for 2 h. All sampled larvae had approximately equal
weight (about 30 g). The study was reviewed and
approved by the Ethics Committee of the Faculty of
Veterinary Medicine University of Zagreb. (Class: 640-01/
13-17/76; Record Number: 251/61-01/139-13-9).
Exposure
Exposure of honey bee larvae to a homogeneous elec-
tromagnetic field was carried out in a GTEM cell as
Short-term exposure to mobile phone radiofrequency on honey bee larvae 431
previously described (Tkalec et al., 2005). The larvae
were exposed to RF-EMF at 900 MHz and field levels of
10, 23, 41 and 120 V m
−1
for 2 h. Corresponding power
flux densities were 0.3, 1.4, 4.2 and 38.2 W m
−2
, respec-
tively. At field level of 23 V m
−1
the effect of 80% AM
1 kHz sinusoidal and 217 Hz modulation was investi-
gated as well. The measurements lasted for a week.
One exposure session was displayed over the day i.e.,
one exposure condition lasted 2 h and was repeated
two times. For each exposure condition eight Petri
dishes (PD) with four larvae each were used. Of the
eight exposed PD, 6 PD were used for the analysis of
oxidative stress parameters and 2 PD for genotoxicity
analysis. For each exposure condition PD were placed
at the center of the GTEM cell, in the same plane, but
perpendicular to the electric field (Tkalec et al., 2005;
Tkalec et al., 2009). The area where Petri dishes were
placed had the most uniform field distribution
( ± 0.1 dB) as measured with the electric probe (Hola-
day HI-4455) and verified with the finite element
method (Malaric
´, Bartolic
´, & Malaric
´,2005). Unexposed
larvae kept in the GTEM cell under the same conditions
as the experimental groups but without exposure to RF
radiation were used as a control. For control, three
repetitions with in total 14 PD were done and as no
statistical difference among repetitions was found for
any investigated parameters we pooled the results of
control groups together. The exposure was performed
at the microwave laboratory of the Faculty of Electrical
Engineering and Computing in Zagreb at the room tem-
perature of 23 ± 1 ˚C. The temperature measurement
in the GTEM cell was done on the surface of the larva
prior to and immediately after the exposure with K2 K/J
Thermometer, Fluke. The difference in temperature
which was measured immediately before and after the
exposure was less than 0.1 ˚C.
Oxidative stress parameters
Tissue extracts were prepared from a composite sample
of four larvae from one Petri dish. Tissue was homoge-
nized in cold 50 mM potassium phosphate buffer (pH
7.0) containing 0.5 mM EDTA with TissueLyser II (QIA-
GEN) for 60 s at 15 Hz. The homogenate was cen-
trifuged twice at 15,000 g for 15 min at 4 ˚C. The
protein concentration in the supernatant was deter-
mined according to Bradford (1976), using bovine serum
albumin as a standard. Obtained supernatants were used
for biochemical assays.
The level of lipid peroxidation was measured as the
formation of thiobarbituric acid reactive substance
(TBARS), a by-product of lipid peroxidation that reacts
with thiobarbituric acid (Legeay, Achard-Joris, Baudri-
mont, Massabuau, & Bourdineaud, 2005). Supernatants
(300 μl) were mixed with 200 μl of cold 20% (w/v)
trichloracetic acid (TCA) to precipitate proteins. The
precipitate was pelleted by centrifugation (10,000 g for
15 min at 4 ˚C) and the obtained supernatant was
reacted with 400 μl of 1% (w/v) thiobarbituric acid pre-
pared in 20% TCA. After heating at 95 ˚C for 30 min,
the mixture was cooled in an ice bath. The absorbance
of the supernatant was measured at 532 nm and correc-
tion for unspecific turbidity was done by subtracting the
absorbance at 600 nm. The content of TBARS was cal-
culated using an extinction coefficient of
155 mM
−1
cm
−1
and expressed per mg of proteins.
Catalase (CAT) activity (EC 1.11.1.6) was assayed by
measuring the decrease in absorbance at 240 nm
(ε=36mM
−1
cm
−1
) according to Aebi (1984). The reac-
tion mixture consisted of 50 mM potassium phosphate
buffer (pH 7.0), 10 mM H
2
O
2
and 25 μl of sample. CAT
activity was expressed in units per mg of proteins. One
unit was defined as the amount of enzyme that hydro-
lyzes 1 μmol of H
2
O
2
per minute, at 25 ˚C and pH 7.0.
Glutathione S-transferase (GST) activity (EC 1.8.1.7)
was determined at 340 nm using 1-chloro-2,4-dini-
trobenzene (CDNB) according to a modified method of
Bocchetti & Regoli (2006). The reaction mixture con-
tained 100 mM potassium phosphate buffer (pH 6.5),
2 mM CDNB, 2.5 mM glutathione (GSH), and 50 μlof
sample. GST activity was expressed in units per mg of
proteins, where one unit is defined as the amount of
enzyme producing 1 μmol of GS-DNB conjugate per
min under the conditions of the assay.
The activity of superoxide dismutase (SOD) (EC
1.15.1.1) was assayed by the xanthine oxidase/cy-
tochrome cmethod modified according to McCord and
Fridovich (1969). The reaction mixtures contained
0.01 mM cytochrome c and 0.5 mM xanthine in 50 mM
potassium phosphate buffer (pH 7.8) with 0.1 mM
EDTA. Reactions were started by adding xanthine oxi-
dase in an amount sufficient to cause change in the
absorbance of 0.025 per min. One unit of SOD inhibits
the rate of reduction of cytochrome c by 50% in a cou-
pled system, using xanthine and xanthine oxidase at pH
7.8 at 25 ˚C.
Comet assay
The alkaline Comet assay (single cell gel electrophoresis
assay) was performed according to the basic procedure
of Singh, McCoy, Tice, and Schneider (1988) with slight
modifications. To obtain the cell suspension, each larva
was placed in 1.5 ml Eppendorf tubes and dilacerated
with a Potter–Elvehjem tissue homogenizer (Braun Bio-
tech, Sartorius, Goettingen, Germany) in 500 μlof
phosphate buffer saline (PBS; 1.45 M NaCl, 60 mM
Na
2
HPO
4
,40mMKH
2
PO
4
; pH 7.0). The homogenates
obtained in this manner were filtered through a 70 μm
sieve, centrifuged at 200 g for 10 min at 4 ˚C, resus-
pended in 400 μl of PBS, centrifuged again at 180 g for
10 min at 4 ˚C and finally resuspended in 60 μl of PBS.
50 μl of a cell suspension was mixed with 50 μl of 0.5%
low melting point (LMP) agarose and transferred to
432 M. Vilic
´et al.
microscope slides precoated with 1% normal melting
point (NMP) agarose. After solidification for 2.5 min in
a freezer, a third layer consisting of 80 μl of 0.5% LMP
agarose was added and left to solidify as described
above. The cells were lysed in freshly made lysis solu-
tion (2.5 M NaCl, 100 mM ethylenediaminetetraacetic
acid (EDTA), 10 mM Tris–HCl, 10% dimethyl sulfoxide,
1% Triton X-100, pH 10) for 1 h at 4 ˚C. After rinsing
with redistilled water, the slides were placed in a hori-
zontal gel box, covered with the cold alkaline buffer
(0.3 M NaOH, 1 mM EDTA pH > 13) and left for
15 min. Electrophoresis was performed in the same buf-
fer at 35 V (1.16 V cm
−1
) and 300 mA for 15 min at
4 ˚C. After electrophoresis, the slides were neutralized
in cold neutralization buffer (0.4 M Tris–HCl, pH 7.5)
for 2 ×5 min, then fixed in methanol: Acetic acid (3:1)
for 5 min and stored in the dark at room temperature.
Prior to examination, the slides were rehydrated,
stained with 10 μg/ml ethidium bromide and examined
using a Zeiss Axioplan epifluorescence microscope. For
each slide 100 nuclei were analyzed. The extent of
DNA migration was determined as the percentage of
DNA in the tail (% tDNA) using the Comet five image
analysis system (Kinetic Imaging Ltd.; UK).
Statistical analysis
All results were expressed as means followed by corre-
sponding standard errors (SE). The oxidative stress
parameter analysis was made from a composite sample
of four larvae from one Petri dish and in total there
were 6 PD for each exposure group (n= 6) and 12 PD
for control group (n= 12). Exception was exposure
group at 120 V m
−1
for TBARS measurement in which
we only analyzed 4 PD (n= 4). Comet test was made
on each larva and in total there were 2 PD with four
larvae each, for each exposure and control group
(n= 8). Statistical analysis was performed using Statistica
12 (StatSoft, Inc.; USA) software package. After testing
for normal distribution (Kolmogorov–Smirnov test of
normality), the results were tested by the analysis of
variance (ANOVA) to determine the differences
between the groups and multiple comparisons between
means were determined by Tukey HSD test. Statistical
difference was considered significant at p< 0.05. The
ANOVA result is reported as an F-statistic and its asso-
ciated degrees of freedom and p-value.
Results
Oxidative stress parameters
The GST activity in the honey bee larvae exposed to
unmodulated RF-EMF at frequency of 900 MHz and field
levels of 10, 23, 41, 120 V m
−1
was not statistically different
when compared to the control (Figure 1). The lowest GST
activity was measured in the larvae exposed to modulated
(80% AM 1 kHz sinus) field at 23 V m
−1
, and it was signifi-
cantly lower (p<0.05, F
(6,41)
= 2.435) than in the larvae
exposed to unmodulated field at 23 V m
−1
(Figure 1).
The CAT activity was significantly decreased
(p< 0.05, F
(6,41)
= 3.416) in the honey bee larvae
exposed to unmodulated RF-EMF at field level of
10 V m
−1
when compared to control group, as well as
with group exposed to unmodulated RF-EMF at field
level of 23, 41, 120 V m
−1
and modulated (217 Hz) field
at 23 V m
−1
(Figure 2).
The SOD activity in the honey bee larvae exposed to
RF-EMF at frequency of 900 MHz and field levels of 10,
23, 41, 120 V m
−1
was not statistically different compared
Figure 1. GST activity in the honey bee larvae exposed to unmodulated RF-EMFs at 900 MHz and field levels of 10, 23, 41,
120 V m
−1
, as well as modulated field (80% AM 1 kHz and 217 Hz) of 23 V m
−1
for 2 h. Results are presented as mean ± SE (n=6
for exposure groups and n= 12 for control). Columns with different letters are significantly different according to the Tukey HSD
test at p< 0.05.
Short-term exposure to mobile phone radiofrequency on honey bee larvae 433
to control (Figure 3). However, the lowest SOD activity
was measured in the larvae exposed to unmodulated RF-
EMF at field level of 10 V m
−1
and it was significantly
lower (p< 0.05, F
(6,41)
= 2.789) than in the larvae
exposed to a modulated (217 Hz) field at 23 V m
−1
.
The TBARS concentration was significantly
decreased (p< 0.05, F
(6,39)
= 3.155) in honey bee larvae
exposed to unmodulated RF-EMF at a field level of
10 V m
−1
when compared to the control group
(Figure 4). Although the content of TBARS in all other
exposed groups was lower than in the control
group, there was no significant difference between the
groups.
Comet assay
DNA damage was significantly increased (p< 0.05,
F
(6,49)
= 17.304) in honey bee larvae exposed to modu-
lated (80% AM 1 kHz sinus) field at 23 V m
−1
in com-
parison to the control and all other exposure groups
(Figure 5). Other treatments did not trigger significant
DNA damage in comparison to the control.
Figure 2. CAT activity in the honey bee larvae exposed to unmodulated RF-EMFs at 900 MHz and field levels of 10, 23, 41,
120 V m
−1
, as well as modulated field (80% AM 1 kHz and 217 Hz) of 23 V m
−1
for 2 h. Results are presented as mean ± SE (n=6
for exposure groups and n= 12 for control). Columns with different letters are significantly different according to the Tukey HSD
test at p< 0.05.
Figure 3. SOD activity in the honey bee larvae exposed to unmodulated RF-EMFs at 900 MHz and field levels of 10, 23, 41,
120 V m
−1
, as well as modulated field (80% AM 1 kHz and 217 Hz) of 23 V m
−1
for 2 h. Results are presented as mean ± SE (n=6
for exposure groups and n= 12 for control). Columns with different letters are significantly different according to the Tukey HSD
test at p< 0.05.
434 M. Vilic
´et al.
Discussion
Our results demonstrate that 2 h-exposure to RF-EMF
at 900 MHz and field levels of 10, 23, 41, 120 V m
−1
as
well as modulated field of 23 V m
−1
, with the modula-
tion of 217 Hz or 80% AM 1 kHz sinus, induced alter-
ations of the antioxidant enzymes activity and lipid
peroxidation level as well as DNA damage in the honey
bee larvae. It was interesting that effects of RF-EMF
were observed only under certain conditions; they did
not follow a linear dose-response relationship, and they
strictly depended on the measured parameters, field
levels and modulation.
Although results of many studies on mammals show
a significant increase of antioxidant enzyme activity as
well as DNA damage and intensity of lipid peroxidation,
after exposure to short-term RF-EMF (see review Yaky-
menko et al., 2016), the present results could hardly be
compared with them. Firstly, it is generally known that
antioxidant protection against oxidative stress is related
to phylogenetic groups, and it differs between mammals
Figure 4. TBARS concentration in the honey bee larvae exposed to unmodulated RF-EMFs at 900 MHz and field levels of 10, 23,
41, 120 V m
−1
, as well as modulated field (80% AM 1 kHz and 217 Hz) of 23 V m
−1
for 2 h. Results are presented as mean ± SE
(n= 6 for exposure groups, except for group 120 V m
−1
where n= 4, and n= 12 for control). Columns with different letters are
significantly different according to the Tukey HSD test at p< 0.05.
Figure 5. DNA damage (% tDNA) in the honey bee larvae exposed to unmodulated RF-EMFs at 900 MHz and field levels of 10,
23, 41, 120 V m
−1
, as well as modulated field (80% AM 1 kHz and 217 Hz) of 23 V m
−1
for 2 h. Results are presented as mean ± SE
(n= 8). Columns with different letters are significantly different according to the Tukey HSD test at p< 0.05.
Short-term exposure to mobile phone radiofrequency on honey bee larvae 435
and invertebrates (Nikolenko, Saltykova, & Gaifullina,
2011). The second reason relates to the contradictory
results that have been published on the antioxidant
enzyme activity and level of lipid peroxidation after
exposure to RF-EMF. Some authors reported increased
activity of antioxidant enzymes and lipid peroxidation
level after exposure to RF-EMF (Gu¨ler et al., 2012; Khi-
razova et al., 2012; Ozgur et al., 2013; Yurekli et al.,
2006), some reported a decrease in these parameters
(Akbari, Jelodar, & Nazifi, 2014; Jelodar, Nazifi, &
Akbari, 2013) or no RF-EMF effects (Avci, Akar, Bilgici,
& Tunc¸el, 2012; Dasdag et al., 2003; Shehu, Mohammed,
Magaji, & Muhammad, 2016; Stronati et al., 2006; Zeni
et al., 2005), while in some studies antioxidant enzymes
showed variation in their activity (Ayata, Mollaoglu, &
Yilmaz, 2004; Balci, Devrim, & Durak, 2007;Tu¨redi
et al., 2015).
The results of the present study indicate that a short-
term exposure of honey bee larvae to RF-EMF at
900 MHz caused a different response of investigated
parameters.The results could be divided into three
groups: (i) decreased level of some parameters after
exposure to the unmodulated RF-EMF (i.e., the CAT
activity and TBARS content were lower in larvae
exposed to the unmodulated RF-EMF at 10 V m
−1
than in
the control; (ii) decreased level of some parameters at
the modulated RF-EMF compared to the unmodulated
field (i.e., the GST activity in larvae exposed to the mod-
ulated (80% AM 1 kHz sinus) RF-EMF at 23 V m
−1
was
lower than in those exposed to the unmodulated field at
23 V m
−1
); and (iii) increased level of some parameters at
the modulated RF-EMF compared to the unmodulated
field (i.e., DNA damage was increased in larvae exposed
to the modulated (80% AM 1 kHz sinus) RF-EMF com-
pared to the unmodulated field and the control whereas
SOD and CAT activities were increased at the
modulated (217 Hz) RF-EMF at 23 V m
−1
compared to
the unmodulated field at 10 V m
−1
. At the moment the
reason for such behavior is not clear. Our results are not
in accordance with the literature data on the same oxida-
tive stress parameters after short-term exposure to
unmodulated RF-EMF at 900 MHz in earthworms Eisenia
fetida (Tkalec et al., 2013). These authors showed that
CAT activity and lipid peroxidation level increased signifi-
cantly in earthworms after exposure for 2 h to RF-EMF
of 23 and 120 V m
−1
. We suppose that the decreased
activity of CAT (as well as level of TBARS) observed in
the five to six days old honey bee larvae after 2 h expo-
sure to unmodulated RF-EMF of 10 V m
−1
could be due
to the function of this enzyme in cell defense from oxida-
tive stress. It has been known that RF-EMF could, even at
low intensity, induce overproduction of reactive oxygen
species (Burlaka et al., 2013), which in honey bees can be
scavenged by SOD, CAT and GST, the most important
antioxidative enzymes. Farjan et al. (2012) showed that
four to six day old A. mellifera larvae have higher activity
of CAT and GST than SOD under physiological condi-
tions and that enzyme activities change during larvae
development; the activities of SOD, CAT and GST
increase slightly from day 1–6, but then decrease after
day six to the end of honey bee development. Among
these enzymes CAT activity shows the highest decrease
during the development. Therefore, according to the
CAT development profile during larval stage and the fact
that CAT is the most important enzyme decomposing
hydrogen peroxide in honey bee brood (Farjan et al.,
2012) we assume that honey bee larvae were more sen-
sitive to the exposure to the EMF level of 10 V m
−1
than
to the other field levels.
This finding corresponds well with previously
reported experimental evidence on non-linear dose-re-
sponse relationship between EMF exposure and biologi-
cal effect (Panagopoulos, Johansson, & Carlo, 2013). For
instance, biological effects such as DNA damage could
show stronger effects at lower field levels of RF-EMF
than at higher field levels (Panagopoulos et al., 2010).
Specifically, several studies reported on the regions of
increased bioactivity called “windows” in which the bio-
logical effects reach a maximum compared to the effects
at smaller or larger values of a physical parameter like
the intensity or frequency of the radiation (Panagopou-
los et al., 2013 and references therein).
Our results showed that only the exposure of honey
bee larvae to modulated field (80% AM 1 kHz sinus) at
23 V m
−1
led to the increase in DNA damage. Parallel
with that, there was a decrease in GST activity indicating
negative effect of modulated EMF on oxidative status in
honey bee larvae. Similar increase in DNA damage after
exposure to modulated field at 23 V m
−1
was previously
reported in E. fetida earthworms (Tkalec et al., 2013). It
has already been reported that frequency modulation can
have significant effect and fields of the same specific
absorption rate (SAR) but of different carrier or modula-
tion frequencies produce different biological effects (dis-
cussed in Panagopoulos et al., 2013). However, it is
important to emphasize that RF-EMF modulated at
217 Hz, which is used in GSM communication, did not
cause any statistically significant changes in investigated
parameters when compared to the control group.
Concerning the DNA damage caused by RF-EMF,
controversial findings have been reported after in vivo
and in vitro exposures to various radiofrequency signals.
While some studies reported DNA damage or cell dam-
age induced by mobile telephony or similar RF radia-
tions at non-thermal intensity levels others did not find
such connection (reviewed in Panagopoulos & Margari-
tis, 2008; Miyakoshi, 2013). Although inconsistent, the
data imply that certain conditions of exposure to RF-
EMF can exert genotoxic properties. One plausible
mechanism for RF-EMF-induced DNA damage is free
radical damage (Burlaka et al., 2013; Phillips, Singh, &
Lai, 2009). Particularly, an extremely low frequency field
(ELF) as well as RF-EMF modulated by ELF can act as a
moderate damaging agent causing mild oxidative stress
responsible for DNA damage (Mihai, Rotinberg, Brinza,
& Vochita, 2014).
436 M. Vilic
´et al.
In conclusion, the results of our study showed that
effects of RF-EMF at 900 MHz in honey bee larvae
appeared only after exposure to the certain EMF condi-
tions. RF-EMF modulated at 1 kHz showed an increase
of DNA damage, while unmodulated RF-EMF produced
alteration in catalase activity and lipid peroxidation at
the lowest field level of 10 V m
−1
. Evidently, the
increase of the field level did not cause a linear dose-re-
sponse relationship in any of the measured parameters.
Although honey bees in nature would not be
exposed to such high field levels as used in our experi-
ments, our results show the need for further intensive
research in all stages of honey bee development, as well
as the intensive research on the possible existence of a
“window” effect under natural conditions during the
annual cycling of bees.
Acknowledgements
The authors wish to thank the participating beekeeper for
sampling collections.
Disclosure statement
No potential conflict of interest was reported by the
authors.
Funding
This work was supported by University of Zagreb, Republic of
Croatia [grant number BM1.77].
ORCID
Ivana Tlak Gajger http://orcid.org/0000-0002-4480-3599
Anamaria S
ˇtambuk http://orcid.org/0000-0002-3177-7694
Maja S
ˇrut http://orcid.org/0000-0002-3120-7843
Go¨ran Klobuc
ˇar http://orcid.org/0000-0002-0838-4593
Kres
ˇimir Malaric
´http://orcid.org/0000-0002-1255-6415
Ana Pavelic
´http://orcid.org/0000-0003-2612-7467
Marin Manger http://orcid.org/0000-0002-1644-6336
Mirta Tkalec http://orcid.org/0000-0003-4733-828X
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