@ ISB 2000Int J Biometeorol (2000) 44:I72-I8I

otit(it\\t \l{l(ll

Marko Nieminen' Matti Leskinen' Juha Helenius

Ibppler radar detection of except¡onal mass-migfationoú aphids into Finland

Received: 11 March 2ü)0 / Revised: 24 April 2üÐ / Accepted: 2ó April 2000

Abstract Our objective was to detect mass migraúonsof insects of economic significance by insect traps and a

Doppler weather radar. Migrants were sampled by suc-tion traps, tow nets and light traps in the Helsinki region.We used radar to observe the migrating insects, and tra-jectories to backtrack mass migrations of aphids (Ho-moptera, Aphididae) in spring 1988. The aphid migra-tions were clearly observed in trap catches and by radar.The first migration, mainly involving Euceraphis betu-lae, ocotrred on 18 May and was Eacked back to north-ern Poland. The second migration, mainly of Rhopalo'sìphwn padi (a serious pest of small-grain cereals), oc-curred 3 days later and was tracked back to a large area

covering Latvia and western Russia south of St Peters-burg. The third migration included both E betulae and

ay. It originated from Es-a provide exact quantita-Trapping efficiency de-

pends strongly on wind speed and insect size- Radarecho intensity is very snongly related to the sizes of in-sects in the large volume of air measured, and the sizesare not known accurately. Weather data, especially tem-peraturc, can be used in t ofaphids, and air-parcel the

sõurce areas of migrants. tingaphid migrations, combined with radar observations, are

useful for warning purposes and to intensify insect trap-

ping. This would contribute to more efFrcient agriculturalpest management.

Keywords Aphid migrations ' Doppler weatherradar '

Euceraphis betula¿ ' Rhopalosiphum padi ']ù/indtrajectory

M. Nieminen (E)DepartmentDivision of I7,FIN-00014 de-mail: marko.nieminen@helsinki.fiFax: +358-0-19I73OL

M. LeskinenDepartment of Meteorology, P.O. Box 4,FIN-00014 University of Helsinki, Finlande-mail: matti.leskinen@helsinki.fi

J. HeleniusAgroecology, Deptrtment of Plant Production,P.O. Box 27, FIN-00014 University of Helsinki, Fi¡lande-mail: juhahelenius@helsinki.fi

lnboduction

Many aphid species perform long-distance migrationsrelatively regularly (e.g. Taylor 1986; Loxdale et al.

1993). The main role of these movements of local aphid

the barley yellow dwarf virus (BYDV), a pathogen espe-

cially affecting oats and barley (e.g. Raatikainen and

Înnilä 1961; Bremer 1965; Rautapãä 1977: Kurppabird cherry, Prz-different gfasses1988 the Finnish

population of R padi peaked following a strong over-wintering population (Kurppa 1989a, b)- In addition,there weie ãt least one, but probably several mass migra-tions of R. padi into Finland in May 1988 and many ofthe individuals immigrating were infected by BYDV

there was anotherøe (Koch), whichdula (Roth). \Vith

the exception of oviparous females in the autumn, allgenerations are fully winged (Heie 1982). Though birch,and especially silver birch, is important to Finnish forest-

ry, E- betulac has not been extensively studied in Fin-land. This is probably due to the fact that birches seem totolerate aphid infestations quite well, and partly because

of the problems of preventing the damage in forests atreasonable cost. However, during 1988 it was thoughtthat aphid infestations (probably E. betulae) resulted inearly senescence, the development of extra branches andcrown deformations in plantationq of young birches(Erkki Annila and Risto Hagqvist, personal communica-tion). Mass migrations of Euceraph¡s have also been rc-ported in Fintand on 2-5 June 1959 (Kanervo 1962) and28 tr'{ay 1968 (Kari Veps?iläinen, personal communica-tion). However, E betulae was separated from Epunctipennis (Zett.) only in 1976 @lackmzn I977;Heie1982) and some of the earlier mass appearances of Euce-raphìs may have included E. punctipennis or both spe-cies. In late May 1995, a mass migration of birch aphidscaused a situation resembling "a snowstorm" in south-eastern Finland (Mikkola 1996)-

In the spring of 1988 a project was initiated to studymigrations of insects by various methods, and to investi-gate the possibility that they cause so-called clear-airechoes that are fr,equently observed by the Dopplerweather radar of the University of Helsinki. One of theai¡ns was to study whether these methods could be in-cluded in an aphid outbreak forecast developed at thattime by Kurppa (1989a). Forecasting R. padi outbreaksis largely based on counting the numbers of winter eggs

on bird cherries. As stated by Ku¡ppa (1989a), the infes-tations in 1988 demonstrated that, besides the local pop-ulation, the detection of migrants that can colonize cropfields in a very sensitive state is crucially important toprevent yield losses as efFrciently as possible.

The emphasis of this report is (1) to demonstrate theprogress of the mass migrations of E. betulae and R.

padi into Finland in spring 1988, (2) to find out the ori-gins of the migrations by trajectory calculations, (3) toelucidate whether these kinds of mass migration can bedetected by insect trap and/or radar data, and (4) to studythe possibility of using these information sources to pre-vent economically damaging losses.

Matcrials and methods

Meteorological data

Basic meteorological data from Fioland are available from theFiDnish Meteorological Institute (FMI) (1988, 1989) and datafrom the former Soviet Union were from the HydrometeorologicalState Comnittee of the USSR (1988-1989). For analyses of the

weather and climatic conditions we used ¿nalysis charts (Deutscher'Wetterdienst 1987, 1988a, b) and climatological overviews (WMO1987, 1988).

173

mostlysible tomodelsas reli-way of

identifying the source of small insects and pollutants bome by aircurrents.

section of l0a-10-5 m2, depending on signal-averaging senings.

ume. If all the insects had the same radar cross-section, radar ¡e-

sum of E. betulae individu-als in a Ú m3. APhid echoes

would, oes, and the numberdensity using echo intensitY(Drake 1981).

Aphids are small compared to weather radar wavelenglhs.Therefore, it can be estimated that the rada¡ cross-section of an

aphid is inversely proportional to the fourth power of the wave-Éngth and directly proportional to the sixth power of its body di-améter (Banao 1973). Individuals of R. padi are much smallerthan those of E. belulae and they also vary considerably in size'The rada¡ cross-section of R. padi is probably about two orders of

sect migration. Because of its horizontally polarized sigual, the.az-imuthafvariation of echo intensity may reveal the common orien-tation of the insect bodies. This is because the radar cross-sectionis at the maximum when the longitudinal axis of the body is alongthe polarization axis of radiation (e'g' Riley 1975, 1985). A Dopp-

L74

f ig. I The locations of trapsand the Doppler weather radar.The larger-scale map shows thelocations of weathet stâtionssupplying the data fm thisstudy (O). The smaller-scalemap shows the locations of theequipment [O weather radar,tow net, suctioD trap and ligbttrap at the Porthania building(Porth.), and tow net onTilfturila water tower;* weather stations;* light-traps; n Viikki fieldsl

oN

65

60

55

ler weather radar measures the Doppler shift frequencies of echoesand the mean motion of scatterers is derived by using these datafrom varying azimuths. Usually this motion is called wind, butmore precisely, in the case of insects, radar actually measures theirmean vectorial ground speed, which is a composite measure ofwind speed and the insects' flight activity.

Insect data

The insect traps used in this study were situated in the Helsinki a¡-ea near the southem coast of Finland (Fig. l). Iûsect samples werccollected by one Rothamsted-type suction trap (on the roof of thePorthania Buildiug, Helsinki), two tow-net traps (on the roof ofPorthania Building and on the top of Ttkkurila water tower,Vantaa) and five light naps. The suction trap (Jobnson and Taylor1955; Macauley et al. 1988) has a 2.l-m-high tube with a fan atthe bottom. The inlet diameter of the trap is 40 cm and it sucksabout 30ü) m3 airlh. In the collector tube there is a dense metalmesh (mesh size 0.4 mm), which leads insects into a collector jar.The net of the tow-net traps (Raatikainen l9ó0) was 1.0 m in di-ameter and 1.9 m long and the mesh size of the net was 1.5 mm.In light traps (Jalas 1960) we used 1ó0-W (Thorn) mixed lightbulba. Lights were switched ot at 22OO hours and off at 03ü)hours (Finnish summer time=+3 h UTC; in late May in Helsinkithere is no astronomical da¡k¡ess, but civil darkness lasts for 4 h)

with a timer. All traps werc usually emPtied several times a daywhen an obviously iavoruable situation for migration occüred,and otherwise at least once a week.

The insect samples were sorted by orders (except Homoptera,which were sorted by superfamilies) using binoculars on petridishes on paper with a 1-mm grid. The individuals were dividedinto three ìize classes: smaller than 2 mm, 2-4 mm and greaterthan 4 mm in length. The species of aphid (Aphidoidea) from sixsamples from different traps aod different periods were identifiedby Osmo Heikinheimo. No other arthropods besides insectrs werecaught during the study period.

Insect density calculations

in a given time. The efflrcieucy of the tow net is also depeûdent oD

wind speed.Tow-uet catches were comPared to con€cted suction-traP Dum-

ber densities during 20 shof periods in May-July 1989. Traps

25 t0 35 û ,15 f).E

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sponds to the height difference between the anemometer at 10 mu'nd the water-toùer roof at 50 m above the ground (I-auniainen

traps were all at roof level, and no additional corrcctions were ap-plied.

Resulb

Phenological data

Field observations of the occurrence and developmeú of R- padipopulations were madeKurppa (1989b). Therepopulations. Therefore,betulae was estimâtedsums. A threshold valupractice in developmental studies of forest t¡ees and herbivores

leneraly follow closely the development of their host plant (e.g'

Strong et al. 1984).Tñe th¡eshold value of the temPerature sum (above 5'C) for ^B'

yeaß.

0 2 3

wind spc€d/(nß)

were situated on the roof of the Po¡thania building about 20 mapart, and an anemometer between these traps was used for windmeasu¡ements. [nstances when both Eaps collected aphids withinthe same size class are shown in Fig. 2. The tow-net number den-sity was calculated from the product of the inlet area,_win9 speed

anä the duration of the catching period (air volume). Suction-trap

midges were multiplied by two if the average wind speed was less

than 5 m s-I.Wind measurements at nearby weather staúons were used in

mometer (height l0 m) were used for the Trkkurila tow net situat-ed 4 km ESE of the anemometer. In addition, the wiod speed forthe Tlkkurila water-tower was multiplied by 1.5, which corre-

Insect densities

The corrected numbers of individuals in 1000 m3 air ofdifferent insect taxa and size classes in the suction trapand tow nets, and in the Porthania light trap (in Helsinki)

individuals were in the light trap on 2l ivlay (Kauri

parasitoid larva inside their aMomen, but the proportionòf such individuals was not estimated. Many R. padiwere very small and their ovariesthis phenomenon is known as ooge(e.g. Rankin et al. 1986), which athese were migrants.

O <zmO 2-4smo lohnson

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L76

Table I The corrected numbers of individuals of differcnt insect tåxa at differcûtaphid species in samples identifred (by osmo Heikinheimo) in spring 1988. See Fig.

collecúng periods, and the perceDtages of different1 for the locations of üapping sites

Data Insect taxa 29 Aprll- 16-19 May1ó May

L9-24May 24-25May 25-27 IllaY 21MaY- 1-3 June1 June

Tiklru¡ila, tow net(individualdtOO0 m3)

Porthania tow net(individuals/1000 m3)

Porthania, suction tap(individualV1000 m3)

Aphidoidea 0Diptera 0.48Other taxa 0.02

Aphidoidea 0Diptera 0.07Other taxa 0.01

AphidoideaDipteraOther taxa

5.378.160.30

11.18.1030.35

0.350.550

0.170.200.01

0.200.240.01

0.03

9.42T.2L0.07

5.95r.160.03

0.o21.380.03

11.30.850.r2

4.t20.r20.06

4.250.590.02

0.166.740.02

42.2h57.5b

0.4b

No dataNo dataNo data

No dataNo dataNo data

99.4c

II.30.650.02

6.900.33o.02

10.31.600.26

L.4I1.050.01

L.675.010.r7

0.011.1ó0.02

98.6u1.41

0.060.070

Porthania, light trap Aphidoidea(individuals/min)

Blflf.tä."

Aphid samples (7ø)

datadatadata

NoNoNo

0

0

No dataNo dataNo data

E. betulaeR. padiOther spp.

No dataNo dataNo data

0.080.310.002

9r.98.1

98.70.90.4 0.6c

t 27 May;b 27-30Mayi " 30 May

Phenology

which implies an inmigrants per hectareusually rcmain well(Helenius 1989).

The days when the threshold temperature for the bud-

burst of birches (58'Cd^") was achieved, according to

weather stations, are shõwn in Fig. 3. The amount ofbirch pollen in Helsinki near the coastline had a distinctmaximum on 14 May (Pentti Sorsa, p€rsonal communi-cation), which agrees very well with the calculated effec-

on the map in value of45-50oC¿"u1or th in south-

ern Finlan'<l was ' If a re-duction of 15oC¿"u in the threshold for the budburst inthe southern area¡'is true, then the dates in Fig. 3 wouldbe only 2-3 days earlier.

Migrations and source arcas

On l8 May a cold front went over Finland from west to

east. Air temperatures were over 20oC to the east of tbatfront. Most of me clear-air radar echoes were observed

as a uniform layer. This layer was, at noon, 1.5 km thick

to the east ofthe radar, but 1.2km in the southern sectorand 1.0 km in the western sector. Above the uniformecho layer only single scatterers, birds or large insects,

were observed. The rain area of the cold front had al-ready reached westerning of rain showers habetween St Petersburgperaturc in Tallinn was 22"C and, with a vertical lapse

iate of 10oC km-I, the temperature at the top of the echo

from the west.The 48-h three-dimensional trajectories originated

from the southeastern corner of the Baltic hoper (nofth-

ern Poland) roughly following the coastline via the large

islands of Estonia (Fig. 3)' All levels were between 300 mand 400 m in the beginning, and moved relativelysmoothly in the end to cover heights of 25Þ1000 m.Most of the time the trajectories were over the cold sea-

Local populations of E- betula¿ in the more northernland areai were not mature according to the effectivetemperature sum, especially in the cooler coastal areas-

Therefore, noftrem Poland or areas southeast from there

were the most probable source of the aphids. The flighttime from northern Poland to Helsinki (at 12 UTC) was

24-28 h according to trajectories, which means that the

towa¡ds yellow (Dixon 1985). The wind direction ac-

fig.3 Izfr pan¿l backwa¡d [a-jectories of May 18, 1988 (12

UTC -48 h) sørting at five dif-ferent pressure levels indicatedat tbe end of trajectories, andisoclines of dates when the ef-fective temperan¡re sum(T.58"statbers (in metres above sea lev-el). Right panel wind speed atdifferent trajectories (pressurelevels from left to righl 97, 95,93,91 anLd 89lcPa)

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cording to Doppler radar was from the SSIW and themaximum wind speed of about 17 m s-l was at the top ofthe echo layer. In the trajectory calculations the windspeed was about 13 m s-l at maximum. Winds in the

sòundings were around 10 m s-1, but were 15 m s-l inVisby at 00 UTC and in St Petersburg at 12 UTC. There-fore, the wind speeds were slightly higher than what the

trajectory son for this isprobably wind, which is

known to (Browning and

Pardoe 1973). This has also been found in a study oftheHelsinki weather radar data in frontal systems in general(H?ikkinen 1987). The higher wind speed naturallymeans a shorter time of flight from the source areas than

indicated by the trajectories. Using the distance travelledfrom northern Poland, and assuming a constant Propor-tional error in wind speed [(13-17)lI7=247o] as ob-served by radar, the flight time would have been about

6 h shorter. The aphids could therefore have been overthe Baltic on the morning of 17 May.

Greater numbers of E. betulae probably migrated on2l l.lay together with the first R. padi; this we considerthe second migration (Fig. 4; Table 1). The radar was notoperated during this weekend period. These migrantsseemed to originate frorn a large area covering Latviaand western Russia south of St Petersburg. At least some

of the E betulae may have a¡rived in those areas during

Jokioinen when local populations of R. padì obviously

22 Mray from Belorussia, and from 23 May from Belo-russia to southeastern Poland. The weather was suitable

for migration to southem Finland only on May 2l and

the following night. This is because of a low pressure ar-

ea that had its centre nea¡ the Gulf of Finland (Fig. 4).The low pressure effectively blocked the migration tosouthwestern and western Finland and, at the same time,provided good conditions for migration into eastern Fin-land and thence to the east.

The third and the last of the migrations took place on

30 May. At that time Padihadprobably matured in ç the mi-gration most likely southern

ðoast of Gulf of Finl individu-als had already left the more southern areas. Many clear-air echoes over the Gulf of Finland were observed in the

morning at 0820 hours (local summer time) about 4 h af-ter sunrise. The echo layer was nearly 2 km thick and the

radar-derived wind speed was quite unifonn about 10 m

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L78

X'ig. 4 Meteorological situationon 21 May 1988 (12 UTC).Armws surface wind vectors,length relative to speed accord-ing to the scale in upper leftcorncr. Z the positions of low-pressure cenúes; a secondarycentre war¡ analysed NE of theprimary one on the Gulf of Fin-land.

- The northernmost

limir of immig¡ation (20-25M"y); - - - the a¡ea of thestrongest immigration by Rho-palosiphum padi according toKurppa (1989b)

66

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63

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t920ztr¿23242526n2829303132OE

s-l from the SSW. The echo layer had clearly strongerecho€s in the di¡ections ENE and WSW suggesting tbatthe insects' bodies had a significantly uniform orienta-tion in the SSE/NNW direction. The common orientationof insects means that they, as a gloup, have a non-zeroair speed. The mean trajectory of insects is therefore notthe same Í¡s a meteorological trajectory. If, in this case,

the mean air-speed of insects is I m s-l to the NNW, forexample, then in a constant situation this would meanthat the end-point of a 6-h insect trajectory would devi-ate about 22km(16 km to the left) from the end-point ofthe meteorological trajectory. Thus, other errors in tra-jectory calculations are probably more significant.

On May 30 the 48-b trajectories wer€ from the Kiyevarea in northern Lllrraine passing over Lithuania, Lawia and

the westem coast of Estonian mainland- All trajectories

showed a westem side of the

higb-press g ttre last 24h-T}recalculated the Estonian coast

over the Gulf of Finland to Helsinki was about 4 h, whichis the time from sumise to the time of the radar obsewa-

tions. But, because of the uniformity of the echo layerup to

hania from 0900 to 1600 hours (local summer time) col-lected 161 aphids and 45 midges, the other abundant group

of insects; of the aphids,99Vo werc R padì (Table 1). Thelocal population at Porthania apparently affected the rezulevery little because the wind was blowing ftom the sea-

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Diecussion

A long-range migration of aphids can be detected byf,reld observations and trapping if the local populationhas not reached the stage of winged adults, and is keptmonitored. This was the case in the spring 1988 whenwinged E- betulae w€re extremely abundant before thelocal population contained any adults- However, after theimmigrants had arrived, these individuals, flying a¡oundthe local scale in a search of host plants, may have sub-stantially confounded the data. The effective temPerah¡resum appears to be an efficient way of deducing potentialareas for the source of the migrations, especially if fielddata are unavailable. When combined with reliable tra-jectory calculations, this method can produce inforrna-tion on the actual source areas, allowing international ex-tension and co-operaúon in aphid samplirig and treat-ment. These methods could be used in connection withthe short-time-range (48 h) weather forecast models toforecast possible aphid migrations, and with frequentlyconducted field observations or trapping. On the otherhand, weather radar networks could be used for real-timeobservations of the situation.

The number densities of insects in the air may not bereliably calculated from trap catches. In suction traps theefficiency changes with wind and insect size (Taylor1962) and in tow nets these factors are more significant(Fig. 2). Moreover, wind conditions may vary consider-ably even during short catching periods, making the cal-culations of efficiency difFrcult, especially when the siz-es of the insects are considered. There are yet other con-founding factors in trap catches, including, for example,some midge species which may use traps as swarmingsites thereby increasing their catch, and the fact that the

white tow nets may attract some species from a distance.In southern Finland, the migraúons nearly always ar-

rive over the Gulf of Finland, which is cold in spring andearly summer. For instance, in the case of the migrationon 18 May 1988 the air temperature over the southerncoast was 22C (n Tallinn), 12"C 10 km offthe coast ofHelsinki (in Isosaari), l7"C around the coastline in Hel-sinki (in Kaisaniemi) and 21oC 15 km north of Helsinki(at Helsinki-Vantaa airport). It is probable that the lowestair layers do not contain migrating insects at all, eitherbecause they have ceased flying in the cool temperaturesover the sea or because they have risen up to the warmerair layers, or for both these reasons. Therefore, a traP atthe coastline may miss even most of the migrations as

they actually arrive and, furthermore, the site of the Port-hania traps in the middle of the city may not have attract-ed migrating aphids to land. Even though the traps indi-cated the observed migrations quite clearl¡ the maincatches appeared in all traps somewhat later than the ac-tual arrival of migrations.

The Doppler weather radar data are very useful in de-termining the movement of insects during migrations. Inthe cases studied here, the migrants were most probablyaphids in the uniform radar echo layers. However, it isnot always possible to relate radar echoes to insects ob-

L79

served at ground level. This was clearly demonstrated bykwin and Th¡esh (1988) as their airborne traps collecteddifferent species at different levels. Nevertheless, in theirstudy the number densities of insects correlated verywell with radar echo intensities at coÍesllonding levels.In Helsinki there has been a situation when a clear air

The results of trajectory models are in principle reli-able if the wind flow fields analysed are accurate

enough. In our study, the FMI limited-area forccast mod-el (SLAM) that analysed the windfield from observa-

tions was, at the time, being replaced by a high-resolu-tion limited-area model (HIRLAM) developed through

M had a 150-kmsize can be de-higher resoliltion

of the model, in addition to other improved features, willmake the fields of meteorological variables analysed

more accurate. Doppler weather radars that are sensitiveenough to clear-air echoes can be used in testing the tra-jectory calculations in sinrations that we have presented

here. Ttre main interest is to position accurately the low-level wind maxima that the ¡nodel can produce but maywrongly place. This error may cause great€r enors in tra-jectory calculations than if these wind maxima were notpredicted by the model at all.

For a forther evaluation of trajectory calculations, wetried to obtain data on aphids from the probable areas oforigin of the migrants in the former USSR. The only in-formation was from Ion Chi¡iac in Moldova saying thatin birches there were plenty of aphids in May 1988, as isusual there, and that the R. padi populations were dense

and infecæd by BYDV. Though some of the 48-h trajec-tories originated relatively near to Moldova, the migrantswere most likely from some more northern areas. Inwestern Europe, R. padi was having a plentiñrl year as

well (Woiwod et al. 1989).'What caused the aphid s¡1þre¡ks in 1988 is an inter-

esting question. The winter of 198G1987 was extremelycold in central and northern Europe and the summer of1987 was colder than nonnal in the Baltic area andnorthwards from about 50o latitude (Deutscher Wetter-dienst 1987; WMO 1987). It seems probable that these

weather conditions resulted in low populations of bothaphids and their predators and parasitoids in the springand summer of 1987. A wanner perid in late Augustand September in Ftnland p'robably provided suiøbleconditions for aphid populations to grow rapidly in an

environment of minimum control by predators or paras-

180

1988 (Kurppa 1989a)- Strong radar echoes in Helsinkicoincided with the high catches of R- padi in September1987 (M.L., personal observations). These echoes wereobserved in northwesterly winds that carried insects ontothe Gulf of Finland and probably frrther south. There-fore, it is possible that some of the 1988 spring migrantswere offsprings of the Finnish 1987 autumn generation.

January, February and March in 1988 had average tem-peratur€s a couple of degrees warmer than normal ineastern Europe (Deutscher Wetterdienst 1988b). Thisprobably decreased the mortality of overwintering aphideggs, and increased the number of migrating adults in theareas southeast/south of Finland.

The A. padi migations were the major contribution,along with a strong domestic population, to damagingdensities of summer generations in cereals. Samples ofthe newly arrive{ winged aphids were collècted for se-

rological tests and many of the field populations wereshown to be vectors of barley yellow dwarf virus(Kurppa 1989a). Aphids were conrolledby the most ex-tensive insecticide sprayings (mostly dimethoate prod-ucts) ever perforrned in Finland: over 807o of the fieldswere sprayed, a large proportion of them twice (Kurppa1989b). [n the future radar detection of such mass migra-tions, or invasions, could be used to warn farmers andmrrsery managers of their imminent arrival, whichwould, by giving accurate timing for pesticide sprayings,serve to minimise environmentally hannful insecticideapplications-

Acknowledgements lüe thaok Osmo Heikinheimo for help in de'termining aphid samples, and Kalle Ee¡ola for providing us with

tise was invaluable. TÞo anonymous referees are thaoked for valu-able commeuts on the manuscript. Societas pro Fauna et FloraFennica supported the study tbrough a glant to M.N. The experi-ments comply with the curent laws of Finland.

Referuncss

Battan LI (1973) Radar observation of the atmosphere. Universityof Chicago Press, Chicago

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