View of Seasonal occurrence of Lygus bugs on field crops in Finland

(1)

Seasonal occurrence of Lygus bugs ^on field crops in Finland

Anna-Liisa Varis

DepartmentofApplied Zoology,POBox27,FIN-00014 Universityof^Helsinki,^Finland

Alongterm surveyofLygus populations (Heteroptera,^Miridae)^wascarried outoneightarable crops insouthern Finland. Sweepnetsamplesweretakenweekly duringthegrowing seasonin 16succes- siveyears. The hibernated adults firstappeared in Mayonoverwintered crops: winter rye, red clover- timothy leyand winterturniprape.Turniprape andleyharbored the mostLygus adults until the end of Juneeven îfsome of them started tomoveto spring cereals(wheat,oats andbarley),_{potato and} sugarbeetafter the emergence of theseplants.The numbers of adultsoncereals remained small until the crops reached theheading stage. Thepeakoccurredinthe first half ofAugust.The adult numbers onpotato and sugarbeet were âlso highest in August. The total number of adults was highest on wheat. Nymphswere^found on all the studied crops.They were^first captured inthe second half of June and reached apeakbetween mid-July andmid-August. Numbers were highest onwheat. The mostcommon ^speciesônall crops was^L. rugulipennis Poppius, constituting 92%of the adults. The otherLygus specieswere^L.gemellatus (Herrich-Schaeffer), L. pratensis(L.) andL.punctatus(Zetter- stedt). Afew specimens ofL. wagneriRemane werealso found.

Keywords: cultivatedplants, growing ^seasons, Heteroptera, Lygus rugulipennis, Miridae

ntroduction

Lygus bugs occur on numerous crop and weed plants throughout the world (Graham_etal. 1984).

In many European countries themost common Lygus species is the European tarnished plant bug, L. rugulipennis Poppius (e.g. Bilewicz 1958, Varis ^1959, Boness 1963). It has been found on 437 host plants in 57 families (Holo- painen and Varis 1991). In Finland the population of Lygus bugs is usuallylow, and their abundance varies considerably from year to year (Varis 1995).They may cause damage early in

thesummerwhen the plants arebeginning their development. The species is univoltine in Fin- land and hibernates as adults. This study de- scribes the seasonal occurrence of Lygus bugs oneight arable crops in southern Finland.

Material and methods

Seasonal occurrence of Lygus species was de- termined from sweep net samples taken in 16 successive years (from 1955to 1970) from eight

©Agricultural and Food ScienceinFinland Manuscriptreceived September 1997

(2)

Varis,A.-L. Lygus bugson

field

^crops^{in Finland}

crops: barley, Hordeum vulgäre L,; oats, Avena saliva L.; spring wheat, Triticum aestivum L.;

winter rye, Secale cerealeL.; potato, Solanum tuberosumL.; sugarbeet, Beta vulgaris L. v. al- tissima Doell.;ley: red clover-timothy,

Trifolium

pratenseL. ^- Phleumpratense L. mixture;and winter turnip rape, Brassica rapa ssp.

oleifera

DC. All the fields were located_on the experi- mental farm of the Agricultural Research Cen- tre,nearHelsinki (60° 17’N,25°04' E). The fields

were managed according tonormal agricultural practices.

Of the crops, the perennial ley was usually kept for 3-4 years. It was cut yearly atthe full headingstage oftimothy^atthe end of Juneorin the beginning of July, Rye and winter turnip rape were sown in the previous year in August, the other crops were spring sown in May. Cereals, potato, and turnip rape were harvested in Au- gust-September andsugarbeetwasharvested in October. Samples werecollected weekly witha sweepnet, diameter34 cm, and they consisted of30 double sweeps. Lygus nymphs could ^not be identifiedtospecies. Crops and details ofsam- pling arethe same asin Varis(1995), where the yearly variations and the possibilities of fore- casting the abundance of Lygus populations are discussed. More information about the climato- logicaldata,theuseofinsecticides,etc.is given in connection with that publication.

Results and discussion

The hibernated adults appeared on cultivated fields in May (Fig. ^{1), at}the sowing time of the spring-sown plants. The first bugs were found in the first half of May on overwintered crops:

rye,turnip rape and ley. They startedto move^to spring cereals in the second half of May or at the beginning of June after the emergence of these crops. From sugarbeet, the first adultswere captured^at the beginning of June and from po- tatoin the second half of June. Therewere some difficulties in getting sweep samples on newly-

emerged crops in the spring without doing any harm^toplants. For thatreasonsamplesweretak- en by sweeping the netabove themas low as possible. ^- From June onwards all the studied crops were colonized. The overwintered adults reached their peak in May-June, the numbers being highestonturnip rape and ley. On the other crops the numberswere rather small and _no distinct peak in theoccurrence of overwintered adults was observed. These Lygus species are highly mobile, and may easily move from one plant species toanother.

The first nymphs were caught in the second half ofJune,the peak occurring from mid-July

tomid-August (Fig.2).As earlier stated by Varis (1995)themean temperatureinJune,when^most eggs are being laid and hatching of nymphs starts,highly affects the total numbers of nymphs during thesummer.The numbers ofnymphswere by far the higheston wheat and second highest on potato. The numberswerelowest onrye.All plants were oviposition hosts,based onthe fact that nymphs were collected from all of them.

Ackording to Bilewicz-Pawihska (1965) the movementof the main species L. rugulipennis from one cropto another was always observed atthe adultstage.Most of the crops in this study have also earlier been identified with oviposi- tionornymphal growth forL. rugulipennis (Varis

1972, Holopainen and Varis ^1991).Nymphswere found untilmid-September.

The peak ofnew adults occurredatthe end of July and atthe beginning of August (Fig. ^1).

The numbers were considerably higher than those of the overwinteredadults,and highest on wheat,where the bugs wereconcentratedon de- veloping grains. Also, other spring cereals and potato harbored considerable^amountsof bugs.

Onthese crops thebugs stayed untilharvest, even ifsome of them started to move to theirover- wintering habitats earlier. Although Lygus bugs arecapable of livingon agreatnumber of plants, these late seasoncrops arevaluable hosts offer- ing plenty of nourishment and shelter for bugs before their hibernation.

The total numbers of nymphs^wereconsider- ably lower than those ofadults (Fig. 3). This may

(3)

partly be dueto the sampling method, because the nymphs often stay in the lower stratum of vegetation and thus avoid sweeping. Schotzko and O'Keeffe (1986) compared sweepnetaccu- racy with that of D-Vac and absolute sampling for determining Lygus hesperus Knight densi- ties in lentils and found that the estimateswere

similar,although sweepnetsampling consistent-

ly gavealower estimate of nymph numbers. Racz and Bernath(1993)found the sweeping method tobe more successful than aninspection of in- dividual plants in tenyears of surveys of maize stands.

It is possible that the insecticidetreatments onsugarbeet and turnip rape in the spring (Varis 1995)may have had someeffect on the catches Fig. I.Numbers of Lygus adults

collected with a sweep netin 1955-1970from eight crops in southern Finland. Numbersaver- agedoverdifferent years.

Fig. 2.Numbers of Lygusnymphs collected with a sweep net in 1955-1970from eight crops in southern Finland. Numbersaver- aged^overdifferent years.

(4)

Varis,A.-L.Lygusbugs

onfield

^crops^{in Finland}

on these crops. Because of the high mobility of the bugs the effect^was,however,mostlikely of short duration. Insecticides were notused atall onother crops, the only exception being the year 1959, when spring cerealsweresprayed against bird cherry-oat aphid, Rhopalosiphum padi (L.).

Because L. rugulipennis constituted 92% of the adults the diagrams mainly reflect the abundance of this species. The other^Lygus species wereL. gemellatus (Herrich-Schaeffer) (4.8%), L. pratensis (L.) (2.2%), and ^L. punctatus (Zetterstedt)(0.8%). A few specimens of L.wag-

neri Remanewere also found.

L. gemellatus occurredonall the crops. The first adultswerecaught from rye and turnip rape

atthe end of May and last adults from ley and cereals at the end of September. The numbers were highest on ley, the peak occurring at the end of June and at the beginning of July. The high numbersonley derived fromoneyear, 1962, when this species exceptionally comprised 42%

of total numbers. In that year the standwas more weedy than in the other years, which may have affected the numbers. On the other crops the numberswere highest on wheat. The peak oc- curred in August.

The numbers of L. pratensis werehigheston turnip rape, with the peak occurring atthe end of June. Most adults from cereals were caught in August. The last adultswere found in cereals and ley in the first half of September. The species occurredon all crops. The first L.^punctatus adultswerefound in the first half of Juneonley and turnip rape and the lateston potato in Sep- tember,Most adultswereobtained in the second half of August. The species was sampled from all thecrops except rye.

The numbers of bugs during the different growing seasons varied considerably and were highly affected by weather conditions (Varis 1995).In_warm springs they_were first captured considerably earlier than in cold springs.

The results show that Lygus bugs ^areable to develop populations _on all the studied crops.

However, someof them harbor muchmoreabun- dant populations than others. Their occurrence is synchronized with the developmentalstage of the crops and their abundance is dependent on temperature.

Acknowledgements.Field data for thisstudy^wascollected while the authorwas^working^at^theAgriculturalResearch Centre.The samplesweretaken bySiljaMäkelä.

References

Bilewicz, T.^1958.Numericaloccurrence ofLygus pubes- cens(Reut.) and Lygus pratensis(L.)on some^plants commonlycultivated inPoland. Ekologia PolskaB4:

299-303.

Bilewicz-Pawiriska, T. 1965.Ecological analysisof Het- eroptera communitiesin cultivated fields. Ekologia Polska A 13: 593-639.

Boness,M. 1963.Biologisch-ökologischeUntersuchun- Fig. 3.Numbers ofLygus adults andnymphscollected with

asweep netin 1955-1970fromeightcrops (wheat, oats, barley, ^rye, ley, sugarbeet,^rape, potato) insouthernFin- land. Numbers averaged^overdifferent years and crops.

(5)

gen an Exo/ygus Wagner (Heteroptera, Miridae).

Zeitschrift furWissenschaftlicheZoologie168: 375- 420.

Graham, H.M., Negm, A.A.^&Ertle, L.R. 1984.World- wide literature of the Lygus complex (Hemiptera:

Miridae), 1900-1980.U.S. Departmentof Agriculture, AgriculturalResearch Service. Bibliographies and Literature^ofAgriculture30: 1-205.

Holopainen, J. ^& Varis, A.-L. 1991. Host plants of the Europeantarnished plant bug Lygus rugulipennis Poppius(Het., Miridae). Journal of Applied Entomol- ogy 111: 484-498.

Racz, V.^&Bernath, I. 1993:Dominance conditions and population dynamics of Lygus (Het., Miridae) species in Hungarian maize stands (1976-1985), as functions of climatic conditions. Journal of Applied

Entomology115: 511-518.

Schotzko, D.J. ^&O'Keeffe, L.E. 1986.Comparison of sweepnet, D-Vac, and absolute sampling for Lygus hesperus (Heteroptera:Miridae)inlentils. Journal of Economic Entomology79: 224-228.

Varis, A.-L. 1959.^EinigeWanzen der GruppeLygusprat- ensisL.(Hem., Miridae) als Schädlinge der Zucker- rube. Publications of the Finnish State Agricultural Research Board MS: 132-138.

- 1972.The biology of Lygus rugulipennisPopp. (Het., Miridae) and the damage caused by this speciesto sugarbeet. AnnatesAgriculturaeFenniae 11: 1-56.

- 1995. Species composition, abundance, and fore- castingof Lygus bugs (Heteroptera: Miridaeon^field crops in Finland. Journal of Economic Entomology 88: 855-858.

SELOSTUS

Niittyluteiden esiintyminen viljelykasveissa kasvukauden eri aikoina

Anna-Liisa Varis Helsingin yliopisto

Tutkimuksessa selvitettiin niittyluteiden esiintymis- runsautta kahdeksalta viljelykasvilta kuutenatoista kasvukautena viikottain otettujen haavintanäytteiden avulla.Koekasvit^olivatkaura, ohra, ruis, vehnä,pe- runa,sokerijuurikas, timotei-apilanurmi jasyysryp- si. KasvustotsijaitsivatMaatalouden tutkimuskeskuk- senviljelyksilläTikkurilassa.

Talvehtineet ludeaikuisetilmestyivät viljelyksil- letoukokuussa,ensin talvehtineisiin kasvustoihin: ru- kiiseen, nurmeen ja rypsiin. Kesäkuun loppuun saak- ka luteita oli enitenrypsissä janurmessa,joskinne kevätviljojen orastuttuaja sokerijuurikkaantultua tai-

melle alkoivat muuttaa myös ^näille. Viljoissa lutei- denmäärät pysyivät vähäisinä kasvien tähkälletuloon

asti.Esiintymishuippu saavutettiin elokuun alkupuo- liskolla.Myöskinperunassaja sokerijuurikkaassa lu- demäärät olivat suurimmillaan elokuussa.

Kaikissa tutkituissa kasvustoissa oli ludeaikuis- tenlisäksi myös näiden toukkia. Toukkien esiintymi- nenalkoi kesäkuun loppupuolella jaolihuipussaan heinäkuunpuolivälistäelokuun puoliväliin. Sekäai- kuisia että toukkia oli kasvukauden aikana eniten vehnäkasvustoissa.