SCIENCE

Influence of electric fields on flying insectsD.B. Watson, Ph.D., L.M. Faithfull, B.E., and N.J. Williams, B.E.

Indexing term: Electromagnetics

Abstract: The paper describes experiments on blowflies and fruitflies in electrostatic fields. At a high enoughfield strength, yet not so high as to cause flashover, the insect becomes paralysed; however, if the field is quicklyremoved the insect eventually regains control and can fly away apparently unharmed. This occurs whether theinsect is originally in contact with one of the electrodes or in flight between the electrodes. It is suggested thatthe nervous system is affected by conduction currents resulting from corona discharges due to local fieldenhancement at irregularities on the insects' bodies.

1 Introduction

Several years ago one of the authors was standing on topof a square church tower which had four lightning conduc-tors, one at each corner. Four large swarms of insects hadgathered and were flying close to the tops of the lightningrods, the size of each swarm being roughly equal. Suddenlythe four swarms flew off, then after a few minutes theysimultaneously reappeared. The most satisfactory explana-tion for this observation seemed to be that the insects wereattracted by the electrostatic field at the top of each lightn-ing rod, and when this collapsed due to, say, the passing ofa cloud, then the swarms dispersed. Thus, the idea of alaboratory investigation of this phenomenon was con-ceived.

A more practical second reason for looking into thebehaviour of insects in electric fields is that this might leadto some advance in methods of pest control. At presentsome commercial control systems use an ultraviolet lightto attract flying insects. As these fly between the high-voltage grilles they are killed by a discharge of current.The flying-insect control system uses high-voltage elec-trodes as a means of fiery extermination, but not as ameans of attraction. The observation on the tower,however, suggests that certain types of flying insect couldbe attracted toward regions in which the electrostatic fieldis* significantly higher than the average earth field; indeedthe tendency for some insects to swarm around a highpoint such as a tree top could be another manifestation ofthe same phenomenon.

Recent environmental reports have been concentratedon electric fields less than about 9 kV/m, typical of thoseunder high-voltage transmission lines. At these fields, somebiological effects in large and small animals are being dis-covered [1, 2]. In particular, there are reports of adverseeffects on honey bees [1, 3], including such unusual behav-iour as killing each other and blocking up the entrance totheir hive. Encouraged by these reports, spurred on by thememory of the church-tower observations, and motivatedto discover what happens to an insect in an electric field,the following experiments were carried out.

2 Experimental

2.1 Divergent fieldsInitially a rod-sphere electrode system was used, sur-rounded by a polymer mesh tube as shown in Fig. 1. Thegap between the electrodes was set to 300 mm. A blowfly

Paper 2717A, first received 23rd August 1982 and in revised form 8th July 1983The authors are with the Department of Electrical Engineering, University of Can-terbury, Christchurch 1, New Zealand

Fig. 1 Blowfly enclosed between rod-sphere electrodes

was inserted between the electrodes and the AC voltagewas raised. The blowfly was expected to fly aroundbetween the electrodes, but, in fact, it normally crawledabout on the mesh. Also, it was hoped to attract it towardthe rod electrode, but it seemed insensitive to the field untilthe voltage was raised to 100 kV, when it attempted toapproach the spherical electrode, but collapsed whenwithin about 10 mm of the sphere. The same behaviouroccurred when direct voltage was applied, irrespective ofpolarity.

Because of the failure to achieve flight behaviour, andthe interesting collapse when close to the sphere electrode,it was decided to confine the insect to a much smalleruniform-field electrode system, where its static behaviourcould be studied more closely.

2.2 Uniform fieldsBlowflies were tested individually in a 25 mm gap betweentwo parallel-plate aluminium-disc electrodes, each 150 mmdiameter, taking care that the insect was well toward thecentre of the electrode system to avoid electrostatic fieldconcentration at the edges. The applied voltage was raisedat a uniform rate so that each test was completed in about1 min. At low field strength the fly is apparently unaffected(Fig. 2a). As the field is raised the fly may exhibit a certaindegree of agitation (Fig. 26), but is not otherwise harmed.Above this field there is a higher field at which the fly rollsover onto its back paralysed (Fig. 2c), a well defined event.After the voltage is removed the fly can recover within afew minutes; however, if the voltage is maintained too longor is further increased the fly dies.

As a comparison, fruitflies were also tested. Eventhough they are of different size, the linear dimensions of a

high voltage high voltage high voltage

a b c

Fig. 2 Blowflies between parallel plate electrodes

a Low fields b Stimulated by higher fields c Paralysis

390 IEE PROCEEDINGS, Vol. 130, Pt. A, No. 7, NOVEMBER 1983

blowfly being about eight times those of a fruitfly, theybehaved in much the same way at slightly higher averagefield strengths. Table 1 gives the comparative results ofparalysis tests on both types of fly at 50 Hz.

parallel-plate electrodes. A blowfly inserted via a PVCtube into the right-hand chamber sees the ultraviolet lightwhich lures it to fly between the electrodes.

The plates are 150 mm long in the direction of flight,

Table 1 : Mean interelectrode AC (RMS) field required to produce paralysis

Fruitf lies

Mean Number Standard Meandeviation

kV/m

410 9

kV/m kV/m

40 324

Table 2: First noticed effectsfield

Effect

AgitationParalysis

Blowflies

Mean Number

kV/m

254 10335 10

Blowflies

Number Standarddeviation

kV/m

18 69

Blowflies on Mylar

Mean Number Standarddeviation

kV/m

590 20

(agitation) field compared with

Standard Meandeviation

kV/m kV/m

23 34653 558

kV/m

88

paralysis

Blowflies on Mylar

Number

1313

Standarddeviation

kV/m

9992

The onset of the agitated state is not a well definedevent and, therefore, it was difficult, in many cases, todetermine the onset field. Table 2 gives the recorded agita-tion onset field for some of the blowflies, together withtheir corresponding paralysis field for comparison.

2.3 Insulated electrodeThe surface of one parallel-plate electrode was nowcovered by a 0.1 mm thick insulating layer of Mylar. Thefly was placed between the electrodes as before. As indi-cated in Figs. 2 and 3 the fly usually settles on the lower

Fig. 3 Blowfly insulated from lower electrode

electrode, and when this was the one covered by the Mylarsheet a much higher AC voltage had to be applied toparalyse the fly. However, when the electrode system wasinverted so that the fly was now sitting on the uncoveredaluminium electrode, the required voltage for paralysis wasthe same as in Section 2.2. Results are included in Table 1.Using the V test the difference between the fields requiredto produce paralysis, with and without the Mylar layer, isstatistically highly significant at less than the 0.1% prob-ability level.

Thus, when the fly is standing on an insulating film it ispartially protected from the effect of the electric field, sug-gesting that paralysis is due to conduction current flowingthrough the fly's body, entering at the feet and possiblyleaving as a local discharge in the air at the fly's uppersurface.

2.4 Fly trappingThe apparatus of Fig. 4 was set up to test the idea of flytrapping by paralysis as they attempt to fly between

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high voltage

oUVIamp

Fig. 4 Blowfly, paralysed inflight, drops through hole in lower electrodeinto collecting box

the lower plate having a rectangular hole at its centre, suchthat the minimum flight path while in the field before rea-ching the hole is 55 mm. The vertical distance between theplates could be varied between 30 mm and 50 mm. If thefield is high enough, an AC field of 400 kV/m provedsufficient, the fly is paralysed in flight and drops into thecollecting box. A video tape recording showed the flycoasting through the field region having lost its power offlight, dropping as it travels. If the field is too low or if thefly is moving at too high a speed, on entering the electrodesystem, it traverses the electrodes and escapes at the exit(Table 3). In these cases the fly drops out, dazed, to recoverafter a few seconds.

This test shows that paralysis occurs even when the flyis not in contact with the electrodes; in which case, anyconduction current flowing through its body must orig-inate and terminate at local discharge sites due to fieldenhancement at opposite sides of the body surface.

3 Discussion

Because of the difficulty in getting insects to fly normallybetween the electrodes, these experiments have thrown

Table 3: Observations on blowflies in the fly-trapping experi-ment

Field kV/m (RMS)

Dropped into collection box

Flew through but stunned

320

0

4

360

1

3

400

7

3

391

very little light on the riddle of the insect swarms aroundlightning rods. To attain a free flight situation a very muchlarger test facility would be required. However, the experi-ments have demonstrated that, when walking, a blowflycan be attracted toward a ball electrode. In such a diver-gent field situation, there will be electromechanical forcesattracting the fly toward the electrode; experiments with adead fly demonstrated these forces and also alignmentforces which tend to line up the wings in the field direction.It was observed, in both divergent and uniform fields, thatthe wings of live blowflies were affected by electromechani-cal forces at fields of the order of 200 kV/m. As theseexperimental fields are much higher than the earth'snormal field of about 0.2 kV/m, the reason for the liveblowfly being attracted toward the ball electrode cannotyet be regarded as the same as that for the attraction of aswarm of flying insects from a great distance to the top ofa lightning conductor. On the other hand, the earth's fieldcan reach up to 50 kV/m in thundery weather [4] and maybe several times this value near a lightning conductor.

The most consistently demonstrable result of theseexperiments is that a fly can be temporarily paralysed byan electric field. The required field is not greatly affectedby the size of the insect, small fruitflies and large blowfliesbeing dropped by about the same magnitude of fieldstrength. Probably the greatest variation in field requiredto produce paralysis arises from the characteristics of indi-vidual flies within a group, some being more susceptibleand others more resistant.

The observations that.the required paralysis stress ishigher when the insect is standing on an insulating filmand that insects having similar geometrical structure(blowflies, fruitflies) require a similar field strength toproduce paralysis, are consistent with the hypothesis thatparalysis results from conduction currents flowing throughthe insect due to local field enhancement and subsequentcorona discharges at insect irregularities or protrusions.

The characteristics of discharge in air from pointed elec-trodes have been extensively investigated and documented[4, 5]. At very low voltages before local ionisation occurs,the saturation currents are of the order 10~14 A. As thevoltage is raised, ionisation causes a rapid increase incurrent with Trichel pulses at negative points and streamerpulses at positive points; individual current pulses even-tually reaching as high as hundreds of microamperes. Thefrequency of Trichel pulses is initially quite high, of theorder of several kilohertz, rising with increase in appliedvoltage to several megahertz. Steamer pulses have initiallya very low frequency of only a few hertz, rising to thekilohertz region as voltage is increased. Typically, whilethe local field at the point electrode would attain the airionisation potential of 3000 kV/m, for a point-plane elec-trode system the Trichel and streamer pulses occur over amean field range from about 100 to 600 kV/m [5], similarto that required to paralyse insects in the present experi-ment. It is suggested that, on the neurological level, thecurrent pulses excite neuromuscular activation [6], the agi-tated state; and that paralysis occurs when the insect isoverwhelmed by the intensity or frequency of the currentpulses. It may be relevant to note that the reported effectson honey bees [1] appear to correlate to induced currentrather than field strength. There remains the possibility,however, that loss of muscular control, and paralysis,observed at the much higher fields of the present work,result directly from internal polarisation.

The uniform field tests partially explain the behaviourin divergent fields. Assuming the fly is weakly attractedtoward the spherical electrode, it enters regions of increas-ing field strength in which it first tends to lose control, theagitated state, then, as it approaches closer, it reaches theparalysis field and drops.

There are no reports of dead or temporarily paralysedflies dropping from lightning conductors or from high-voltage electrical equipment. In this respect, as theworking stress at the surface of an overhead power line isusually much higher than the stress required to paralysethe insects in the present investigation, it is suggested thatflying insects would drop before being able to reach theconductors, as in the fly-trapping experiment.

The trapping experiment has shown how flying insectscan be paralysed in flight. In this way, insects may becollected alive for, say, scientific purposes. Selective trap-ping could result from control of field strength and theactive length of the electrode in the direction of flight; theflies having lowest inertia (small low-speed flies) being thefirst to drop. Large high-speed flies might travel ballisti-cally through the electrodes while paralysed, only to reviveand fly off once out of the field. The same selectivity couldbe useful in an insect 'electric-fence' system, allowing thepassage of valuable insects like bees while trapping thesmall insects like aphids. At higher field strengths, the trap-ping system could also be used for insect killing, withoutthe need for electrical flashover; thereby avoiding noise,interference problems and the unpleasant smell of burning.

4 Conclusion

Flying insects can be temporarily paralysed, and killedwithout flashover, by moderate electrostatic fields. A sug-gested reason is that the nervous system is affected byconduction current resulting from corona discharges,owing to local field enhancement at irregularities on theinsects' bodies.

5 Acknowledgments

The authors wish to thank Flying Insect Control Limitedfor making their equipment available; A. Duder, ZoologyDepartment, University of Canterbury, who provided mostof the insects, and A. Bright, Audio Visual Aids Depart-ment, University of Canterbury, for making a video tape ofthe fly-trapping experiments.

References

1 KAVET, R.: 'Biological effects of electric fields: EPRI's role', IEEETrans., 1982, PAS-101, pp. 2115-2121

2 ROGERS, L.E, GILBERT, O., LEE, J.M. Jun., and BRACKEN, T.D.:'BPA 1100-Kv transmission system development—environmentalstudies', ibid., 1979, PAS-98, pp. 1958-1969

3 MARINO, A.A., and BECKER, R.O.: 'Hazard at a distance: effects ofexposure to the electric and magnetic fields of high voltage transmis-sion lines', Med. Res. Eng., 1977, 12, pp. 6-9

4 MOORE, A.D. (Ed.): 'Electrostatics and its applications' (Wiley, 1973)5 NASSER, E.: 'Fundamentals of gaseous ionization and plasma elec-

tronics' (Wiley, 1071)6 BRAZIER, M.A.B.: The electrical activity of the nervous system'

(Pitman, 1973, 3rd edn.)

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