SHORT COMMUNICATION

Reproducibility and desensitization of the powerfrequency magnetic field effect on movement of thecommon cutworm Spodoptera lituraJeong-Min RYU1, Nam-Jung KIM2, Ran WON3, Sang-Dae LEE4 and Kwon-Seok CHAE1

1 Department of Biology, Teachers’ College, Kyungpook National University, Daegu, Korea2 Department of Agricultural Biology, National Academy of Agricultural Science, Suwon, Korea3 Department of Biomedical Laboratory Science, Dongseo University, Busan, Korea4 Department of Biological Science, Seonam University, Namwon, Korea

Correspondence

Kwon-Seok Chae, Department of Biology,Teachers’ College, Kyungpook NationalUniversity, 702-701 Daegu, Republic ofKorea. Email: kschae@knu.ac.kr

Received 26 August 2009;accepted 14 September 2009.

doi: 10.1111/j.1748-5967.2009.00248.x

Abstract

All creatures on Earth, including human beings, can be influenced by the powerfrequency electromagnetic field (EMF), even though the consequence and degreeof the effect may vary due to regional context, species, etc. Most of the outstandingscientific achievements about the EMF effect on life have come from behavioralstudies. In such studies, in contrast to the geomagnetic field or static magnetic field(MF), the oscillating MF has attracted far less attention so far. Following a previousreport, to attain deep basic knowledge about the effect of an extremely low fre-quency (ELF) MF on animal behavior, we characterized the 60-Hz MF-responsivemovement activity of common cutworm larvae using sophisticated experimentalschemes. The MF-exposed third instar larvae showed significantly reduced loco-motive activity compared to the matching sham-exposed larvae. Moreover,repeated MF exposure to the same larvae up to three times also showed similarbehavioral responsiveness even though the extent of movement decrease wasattenuated by the repetition time. These results suggest that sinusoidal powerfrequency MF could disrupt the normal locomotory activity of insect larvae, andthe insects may show adaptive desensitization to the same MF.

Key words: magnetic field, extremely low frequency, insect, behavior, Spodoptera litura,reproducibility.

Introduction

During the last few decades, various electromagnetic fields(EMFs) have dramatically expanded in industrialized coun-tries across the world, and EMF has become one of the mostinfluential environmental factors (Kheifets 2001; Irigarayet al. 2007). The World Health Organization has orches-trated the “International EMF Project” since the 1990s toprotect public health in response to public concern aboutEMF by assessing the scientific evidence of the possiblehealth effects of EMF (http://www.who.int/peh-emf/en/).Among the artificial EMFs, power frequencies of 50 or60 Hz have existed for the longest time span on Earth. All

living creatures including humans inevitably may have beeninfluenced by the power frequency EMF everywhere, eventhough the consequences and degree of the effect may varydue to region and species.

As a scientific approach to address concerns about theeffect of EMF on life, the effects of various EMFs, includ-ing power frequency on cultured cells, microorganisms,insects, mammals and humans have been investigated forthe last two decades (Feychting et al. 2005; Wiltschko &Wiltschko 2006). The most prominent scientific achieve-ments of such endeavors have come from behavioralstudies in which birds, amphibians and insects have beenused frequently (Feychting et al. 2005; Wiltschko &

Entomological Research 39 (2009) 406–409

© 2009 The AuthorsJournal compilation © 2009 The Entomological Society of Korea and Blackwell Publishing Asia Pty Ltd

Wiltschko 2006). The work has largely concentrated onhow such animals sense the geomagnetic field (GMF) andmigrate for long distances by exploiting the so-called geo-magnetic map. Insects such as butterflies, moths, bees, antsand cockroaches have served as successful experimentalmodels from the beginning in behavioral studies. In thisline of work, the tested animals were exposed to a staticmagnetic field (SMF), GMF or artificial magnetic fields(MF).

In contrast, oscillating MF, which shows repetitive varia-tion of amplitude and/or phase periodically, has attractedfar less scientific attention compared to the SMF in behav-ioral studies until recently. To study the behavioral effectsof an extremely low frequency (ELF) MF, mice and ratshave mainly been used. A variety of sinusoidal- or pulsed-ELF MF either induced or suppressed the movement ofmice and rats depending on the specific MF exposurecontext (Pesic et al. 2004; Liu et al. 2008). Very recently,human subjects have shown particular response to an ELFMF; thereby, nonlinear changes in occipital electroen-cephalograms and postural tremors have been observed(Legros & Beuter 2005, 2006; Carrubba et al. 2007).However, very little information about the behavior ofinsects, a plausible experimental organism, under an ELFMF has been accumulated so far. A set of sinusoidal ELFMF (~10 Hz) exposure notably reduced the feeding andmolting times of the silkworm Bombyx mori withoutadverse effects on larval growth, implying that the MFderegulated the normal behavior possibly through affect-ing the endocrine or nervous systems (Qadri et al. 2006).An exposure of power frequency (50 Hz) sinusoidal MF(~70 mT) on comb-building oriental hornet workers Vespaorientalis deformed the size and symmetry of the combmarkedly (Ishay et al. 2007). Moreover, the MF exposuredecreased the number of cells and eggs per comb. Theselimited experimental results convincingly suggest that anoscillatory MF affects insects in terms of behavior, growthand the endocrine system. Somewhat in accordance withthese reports, we showed very recently that a 60-Hz sinu-soidal MF notably changed the duration and onset time ofthe movement of the common cutworm Spodoptera liturain the laboratory (Chae 2008).

Even though these studies have accumulatedevidence supporting that oscillatory ELF MF could inducea variety of behavioral effects in insects, further investiga-tion is necessary to understand the detailed consequencesof the effects and the underlying mechanism for signaltransduction from magnetoreception to regulatoryeffectors.

Following previous results (Chae 2008), in the presentstudy we investigated the power frequency MF effect on themovement of the common cutworm with more sophisticatedbehavioral assay schemes.

Materials and methods

Insects

Second instar larvae of Spodoptera litura were maintainedin the same way as described in a previous report (Chae2008) until needed for experiments. The temperature in therearing room was 25 � 0.5°C with a photoperiod of 16 hlight : 8 h dark. Twenty-four hours before the behavioraltests, each larva was moved to a Petri dish (35 mm diameter)and then stabilized individually.

Magnetic field exposure

The MF was generated by rectangular Helmholtz coils thathad been used in our previous studies (Chae 2008; Koh et al.2008). Briefly, the homogeneity of MF over the sample stageinside the coil systems was more than 95%, as measured bya gaussmeter (TES 1390; TES Electrical Electronic Corp,Taipei, Taiwan). The intensity of the ambient stray MF wasless than 0.05 mT and the temperature inside the exposuresystem was regulated at 25 � 0.5°C throughout the experi-ments. Sham-exposed larvae were tested in the same mannerwith a switch-off state.

Behavioral analysis

Throughout the experiments, movement of the tested larvaewas captured using a camcorder (Samsung, Seoul, Korea).The larval movement was analyzed on a computer monitorwith the assistance of a person in a blind manner. Anymovement lasting more than 5 s was counted except foreating or excretion movement.

Statistics

Statistical analysis was performed by one-way analysis ofvariance (anova), and all the statistical values were pre-sented as mean � SEM. Throughout, P < 0.05 was regardedas significant. All the experiments in this study wererepeated two or three times separately.

Results and discussion

Generally, the results and interpretation of an animal behav-ioral assay are very dependent on experimental design. Toexamine more precisely the movement of Spodoptera lituraresponding to a sinusoidal power frequency MF, weexploited a modified experimental scheme (Fig. 1) from ourprevious study (Chae 2008). Several control movements ofthe larvae were determined in a series of experiments. Eachlarva was stabilized individually for 24 h before the experi-ments. The normal level of sham movement (sham B, 190 +

Magnetic field effect on S. litura

407Entomological Research 39 (2009) 406–409© 2009 The Authors. Journal compilation © 2009 The Entomological Society of Korea and Blackwell Publishing Asia Pty Ltd

78 s) was measured for 1 h inside the Helmholtz coils, whichwas compared to the MF movement (92 + 27 s) by the MFexposure (Fig. 2). The values of “sham A” (99 + 34 s) andthe “sham” (139 + 28 s) were calculated after subtractionfrom the “operational control” (65 + 25 s). Unexpectedly,the MF reduced larval movement with the present experi-mental scheme, suggesting that the MF could suppressnormal movement of the larvae in a more stabilized state. Totest whether this effect could be repeated on the same larvae,the same MF was exposed three times with a 1-h intervalbetween repetitions in a separate subsequent experiment.Similar to the first MF exposure, the second exposure sup-pressed larval movement significantly while the total dura-tions of the movement of the sham and the MF in the secondexposure were not comparable to those of the first exposure(Fig. 3). This MF-induced suppression of larval movement

was not shown in the third exposure. In fact, the sham andMF level of the third exposure were slightly higher than theMF movement of the second exposure, implying that the MFeffect was notably attenuated. These results suggest that theMF effect was repeated on the same larvae while the larvaewere desensitized to the MF eventually. However, furtherstudy is required to address questions including how long thedesensitization could be sustained. If the desensitizationremained for a long duration, the same MF may not induceany remarkable effect on larval movement for a while.

operational control1 h

stabilization1 day

Sham A1 h

stabilization1 day

Sham B1 h

(a)

(b)

(c)MF (1mT)

1 hsham1 h

stabilization1 day

stabilization1 day

MF 11 h

sham 11 h

interval 11 h

MF 21 h

sham 21 h

interval 21 h

MF 31 h

sham 31 h

(c)

(d)

Figure 1 Diagram of the experimental scheme. (a) Measurement of the operational control, movement activity of the larvae from mimickingof the positioning of each 35-mm Petri dish containing a larva onto the sample stage, for 1 h outside the coil after the larva was stabilized fora day. (b) Measurement of the consecutive sham, Sham A and Sham B for 1 h each without MF exposure inside the coil. (c) Measurement ofthe 1 h of sham followed by 1 h of MF (1 mT) exposure inside the coil. Note that (b) serves as a control experiment for (c), thereby genuine MFmovement, “MF”, was obtained by subtraction of Sham B from the apparent MF movement. (d) Reproducibility test using repeated sham- andMF-exposure with intervals. Numbers following each sham, MF, and interval indicate the serial number of the repetition.

200

150

250

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men

t (se

c)

300

(45)

(45)(18)

(18)

0

50

100

tota

l dm

ove

Sham A Sham Bsham MF

Figure 2 MF exposure reduces the movement activity of the larvae.MF exposure showed remarkably reduced movement compared toSham B while Sham A and the sham showed marginal difference.Values in parentheses are numbers of tested larvae for eachexperiment.

200

150

250

300ur

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sec)

(18)

(18)

0

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100tota

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ove (18)

(18)(9)

(9)

sham 1 MF 1

*

*

sham 2 MF 2 sham 3 MF3

Figure 3 Reproducibility and attenuation of the MF effect on move-ment of the same larvae. Consecutive sham and MF exposure wererepeated three times with intervals. In the first and second expo-sures, the MF significantly reduced movement. In the third exposure,the MF effect was negligible, and the mean values of sham and MFwere slightly higher than that of the MF in the second exposure.Numbers following each sham and MF indicate the serial number ofthe repetition. *P < 0.05 was regarded as significant.

J.-M. Ryu et al.

408 Entomological Research 39 (2009) 406–409© 2009 The Authors. Journal compilation © 2009 The Entomological Society of Korea and Blackwell Publishing Asia Pty Ltd

However, if an imprinting effect occurs after the desensiti-zation, the MF effect may be manifested as noted in a pre-vious report where a 50 or 60 Hz frequency-dependentpeculiar imprinting on Ca2+ efflux in the developing embryobrain was observed (Blackman 2006). This question may inpart overlap with the “window effect” in electromagneticbiology research, which has frequently attracted much atten-tion (Litovitz et al. 1992; Belyaev & Alipov 2001). Relatedto this, we are on the way to revealing the relationshipbetween intensity and the exposure period of the MF to theMF-induced suppression of larval movement.

Acknowledgements

We thank Soon-Hwan Kwon and Joon-Sang Lee for assis-tance in the blind behavioral assay. This work was supportedby the Kyungpook National University Research Fund,2007.

References

Belyaev IY, Alipov ED (2001) Frequency-dependent effects ofELF magnetic field on chromatin conformation in Escherichiacoli cells and human lymphocytes. Biochimica et BiophysicaActa 1526: 269–276.

Blackman CF (2006) Can EMF exposure during developmentleave an imprint later in life? Electromagnetic Biology andMedicine 25: 217–225.

Carrubba S, Frilot C, II, Chesson AL Jr, Marino AA (2007)Evidence of a nonlinear human magnetic sense. Neuroscience144: 356–367.

Chae KS (2008) An extremely low frequency magnetic fieldincreases unconditioned larval movement of the commoncutworm, Spodoptera litura: a novel model for a magnetore-ceptive neurobehavioral study. Entomological Research 38:299–302.

Feychting M, Ahlbom A, Kheifets L (2005) EMF and health.Annual Review of Public Health 26: 165–189.

Irigaray P, Newby JA, Clapp R, Hardell L, Howard V, Montag-nier L et al. (2007) Lifestyle-related factors and environmentalagents causing cancer: an overview. Biomedicine and Pharma-cotherapy 61: 640–658.

Ishay JS, Plotkin M, Volynchik S, Shaked M, Schuss Z, BergmanDJ (2007) Exposure to an additional alternating magnetic fieldaffects comb building by worker hornets. Physiological Chem-istry and Physics and Medical NMR 39: 83–88.

Kheifets LI (2001) Electric and magnetic field exposure and braincancer: a review. Bioelectromagnetics 22 (Suppl 5): S120–S131.

Koh EK, Ryu BK, Jeong DY, Bang IS, Nam MH, Chae KS (2008)A 60-Hz sinusoidal magnetic field induces apoptosis of pros-tate cancer cells through reactive oxygen species. InternationalJournal of Radiation Biology 84: 945–955.

Legros A, Beuter A (2005) Effect of a low intensity magnetic fieldon human motor behavior. Bioelectromagnetics 26: 657–669.

Legros A, Beuter A (2006) Individual subject sensitivity toextremely low frequency magnetic field. Neurotoxicology 27:534–546.

Litovitz TA, Montrose CJ, Wang W (1992) Dose-response impli-cations of the transient nature of electromagnetic-field-inducedbioeffects: theoretical hypotheses and predictions. Bioelectro-magnetics 13 (Suppl 1): 237–246.

Liu T, Wang S, He L, Ye K (2008) Anxiogenic effect of chronicexposure to extremely low frequency magnetic field in adultrats. Neuroscience Letters 434: 12–17.

Pesic V, Janac B, Jelenkovic A, Vorobyov V, Prolic Z (2004)Non-linearity in combined effects of ELF magnetic field andamphetamine on motor activity in rats. Behavioural BrainResearch 150: 223–227.

Qadri SM, Dhahira Beevi N, Mani A, Leelapriya T, Dhilip KS,Sanker Narayan PV (2006) Sinusoidal magnetic fields andchawki (silkworm) rearing in sericulture. ElectromagneticBiology and Medicine 25: 145–153.

Wiltschko R, Wiltschko W (2006) Magnetoreception. Bioessays28: 157–168.

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409Entomological Research 39 (2009) 406–409© 2009 The Authors. Journal compilation © 2009 The Entomological Society of Korea and Blackwell Publishing Asia Pty Ltd