Seasonal occurrence of Lygus bugs on field crops in Finland Anna-Liisa Varis Department ofApplied Zoology, PO Box 27, FIN-00014 University of Helsinki, Finland A long term survey ofLygus populations (Heteroptera, Miridae) was carried out on eight arable crops in southern Finland. Sweep net samples were taken weekly during the growing season in 16 succes- sive years. The hibernated adults first appeared in May on overwintered crops: winter rye, red clover- timothy ley and winter turnip rape. Turnip rape and ley harbored the most Lygus adults until the end of June even if some of them started to move to spring cereals (wheat, oats and barley), potato and sugarbeet after the emergence of these plants. The numbers of adults on cereals remained small until the crops reached the heading stage. The peak occurred in the first half of August. The adult numbers on potato and sugarbeet were also highest in August. The total number of adults was highest on wheat. Nymphs were found on all the studied crops. They were first captured in the second half of June and reached a peak between mid-July and mid-August. Numbers were highest on wheat. The most common species on all crops was L. rugulipennis Poppius, constituting 92% of the adults. The other Lygus species were L. gemellatus (Herrich-Schaeffer), L. pratensis (L.) and L. punctatus (Zetter- stedt). A few specimens of L. wagneri Remane were also found. Key words: cultivated plants, growing seasons, Heteroptera, Lygus rugulipennis, Miridae ntroduction Lygus bugs occur on numerous crop and weed plants throughout the world (Graham et al. 1984). In many European countries the most 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 popula- tion of Lygus bugs is usually low, and their abun- dance varies considerably from year to year (Varis 1995). They may cause damage early in the summer when the plants are beginning their development. The species is univoltine in Fin- land and hibernates as adults. This study de- scribes the seasonal occurrence of Lygus bugs on eight 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 1955 to 1970) from eight © Agricultural and Food Science in Finland Manuscript received September 1997 409 V01.6(1997):409^H3. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Varis, A.-L. Lygus bugs onfield crops in Finland crops: barley, Hordeum vulgäre L,; oats, Avena saliva L.; spring wheat, Triticum aestivum L.; winter rye, Secale cereale L.; potato, Solanum tuberosum L.; sugarbeet, Beta vulgaris L. v. al- tissima Doell.; ley: red clover-timothy, Trifolium pratense L. - Phleum pratense 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, near Helsinki (60° 17’ N, 25°04' E). The fields were managed according to normal agricultural practices. Of the crops, the perennial ley was usually kept for 3-4 years. It was cut yearly at the full heading stage oftimothy at the end of June or in 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 and sugarbeet was harvested in October. Samples were collected weekly with a sweep net, diameter 34 cm, and they consisted of 30 double sweeps. Lygus nymphs could not be identified to species. Crops and details of sam- pling are the same as in Varis (1995), where the yearly variations and the possibilities of fore- casting the abundance of Lygus populations are discussed. More information about the climato- logical data, the use of insecticides, 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 started to 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 adults were captured at the beginning of June and from po- tato in the second half of June. There were some difficulties in getting sweep samples on newly- emerged crops in the spring without doing any harm to plants. For that reason samples were tak- en by sweeping the net above them as low as possible. - From June onwards all the studied crops were colonized. The overwintered adults reached their peak in May-June, the numbers being highest on turnip rape and ley. On the oth- er crops the numbers were rather small and no distinct peak in the occurrence of overwintered adults was observed. These Lygus species are highly mobile, and may easily move from one plant species to another. The first nymphs were caught in the second half of June, the peak occurring from mid-July to mid-August (Fig. 2). As earlier stated by Varis (1995) the mean temperature in June, when most eggs are being laid and hatching of nymphs starts, highly affects the total numbers of nymphs during the summer. The numbers ofnymphs were by far the highest on wheat and second highest on potato. The numbers were lowest on rye. All plants were oviposition hosts, based on the fact that nymphs were collected from all of them. Ackording to Bilewicz-Pawihska (1965) the movement of the main species L. rugulipennis from one crop to another was always observed at the adult stage. Most of the crops in this study have also earlier been identified with oviposi- tion or nymphal growth forL. rugulipennis (Varis 1972, Holopainen and Varis 1991). Nymphs were found until mid-September. The peak of new adults occurred at the end of July and at the beginning of August (Fig. 1). The numbers were considerably higher than those of the overwintered adults, and highest on wheat, where the bugs were concentrated on de- veloping grains. Also, other spring cereals and potato harbored considerable amounts of bugs. On these crops the bugs stayed until harvest, even if some of them started to move to their over- wintering habitats earlier. Although Lygus bugs are capable of living on a great number of plants, these late season crops are valuable hosts offer- ing plenty of nourishment and shelter for bugs before their hibernation. The total numbers of nymphs were consider- ably lower than those ofadults (Fig. 3). This may 410 AGRICULTURAL AND FOOD SCIENCE IN FINLAND partly be due to the sampling method, because the nymphs often stay in the lower stratum of vegetation and thus avoid sweeping. Schotzko and O'Keeffe (1986) compared sweep net accu- racy with that of D-Vac and absolute sampling for determining Lygus hesperus Knight densi- ties in lentils and found that the estimates were similar, although sweep net sampling consistent- ly gave a lower estimate of nymph numbers. Racz and Bernath (1993) found the sweeping method to be more successful than an inspection of in- dividual plants in ten years of surveys of maize stands. It is possible that the insecticide treatments on sugarbeet and turnip rape in the spring (Varis 1995) may have had some effect on the catches Fig. I. Numbers of Lygus adults collected with a sweep net in 1955-1970 from eight crops in southern Finland. Numbers aver- aged over different years. Fig. 2. Numbers of Lygus nymphs collected with a sweep net in 1955-1970 from eight crops in southern Finland. Numbers aver- aged over different years. 411 Vol. 6 (1997): 409-413. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Varis, A.-L. Lygus bugs onfield crops in Finland on these crops. Because of the high mobility of the bugs the effect was, however, most likely of short duration. Insecticides were not used at all on other crops, the only exception being the year 1959, when spring cereals were sprayed against bird cherry-oat aphid, Rhopalosiphum padi (L.). Because L. rugulipennis constituted 92% of the adults the diagrams mainly reflect the abun- dance of this species. The other Lygus species were L. gemellatus (Herrich-Schaeffer) (4.8%), L. pratensis (L.) (2.2%), and L. punctatus (Zetterstedt) (0.8%). A few specimens of L. wag- neri Remane were also found. L. gemellatus occurred on all the crops. The first adults were caught from rye and turnip rape at the 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 numbers on ley derived from one year, 1962, when this species exceptionally comprised 42% of total numbers. In that year the stand was more weedy than in the other years, which may have affected the numbers. On the other crops the numbers were highest on wheat. The peak oc- curred in August. The numbers of L. pratensis were highest on turnip rape, with the peak occurring at the end of June. Most adults from cereals were caught in August. The last adults were found in cereals and ley in the first half of September. The spe- cies occurred on all crops. The first L. punctatus adults were found in the first half of June on ley and turnip rape and the latest on potato in Sep- tember, Most adults were obtained in the second half of August. The species was sampled from all the crops 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 are able to develop populations on all the studied crops. However, some of them harbor much more abun- dant populations than others. Their occurrence is synchronized with the developmental stage of the crops and their abundance is dependent on temperature. Acknowledgements. Field data for this study was collected while the author was working at the Agricultural Research Centre. The samples were taken by Silja Mäkelä. References Bilewicz, T. 1958. Numerical occurrence of Lygus pubes- cens (Reut.) and Lygus pratensis (L.) on some plants commonly cultivated in Poland. Ekologia Polska B 4: 299-303. Bilewicz-Pawiriska, T. 1965. Ecological analysis of Het- eroptera communities in cultivated fields. Ekologia Polska A 13: 593-639. Boness, M. 1963. Biologisch-ökologische Untersuchun- Fig. 3. Numbers ofLygus adults and nymphs collected with a sweep net in 1955-1970 from eight crops (wheat, oats, barley, rye, ley, sugarbeet, rape, potato) in southern Fin- land. Numbers averaged over different years and crops. 412 AGRICULTURAL AND FOOD SCIENCE IN FINLAND gen an Exo/ygus Wagner (Heteroptera, Miridae). Zeitschrift fur Wissenschaftliche Zoologie 168: 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. Department of Agriculture, Agricultural Research Service. Bibliographies and Literature of Agriculture 30: 1-205. Holopainen, J. & Varis, A.-L. 1991. Host plants of the European tarnished 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) spe- cies in Hungarian maize stands (1976-1985), as functions of climatic conditions. Journal of Applied Entomology 115: 511-518. Schotzko, D.J. & O'Keeffe, L.E. 1986. Comparison of sweepnet, D-Vac, and absolute sampling for Lygus hesperus (Heteroptera: Miridae) in lentils. Journal of Economic Entomology 79: 224-228. Varis, A.-L. 1959. Einige Wanzen der GruppeLygusprat- ensis L. (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 species to sugar beet. Annates Agriculturae Fenniae 11: 1-56. - 1995. Species composition, abundance, and fore- casting of Lygus bugs (Heteroptera: Miridae on 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 olivat kaura, ohra, ruis, vehnä, pe- runa, sokerijuurikas, timotei-apilanurmi ja syysryp- si. Kasvustot sijaitsivat Maatalouden tutkimuskeskuk- sen viljelyksillä Tikkurilassa. Talvehtineet ludeaikuiset ilmestyivät viljelyksil- le toukokuussa, ensin talvehtineisiin kasvustoihin: ru- kiiseen, nurmeen ja rypsiin. Kesäkuun loppuun saak- ka luteita oli eniten rypsissä ja nurmessa, joskin ne kevätviljojen orastuttua ja sokerijuurikkaan tultua tai- melle alkoivat muuttaa myös näille. Viljoissa lutei- den määrät pysyivät vähäisinä kasvien tähkälletuloon asti. Esiintymishuippu saavutettiin elokuun alkupuo- liskolla. Myöskin perunassa ja sokerijuurikkaassa lu- demäärät olivat suurimmillaan elokuussa. Kaikissa tutkituissa kasvustoissa oli ludeaikuis- ten lisäksi myös näiden toukkia. Toukkien esiintymi- nen alkoi kesäkuun loppupuolella ja oli huipussaan heinäkuun puolivälistä elokuun puoliväliin. Sekä ai- kuisia että toukkia oli kasvukauden aikana eniten vehnäkasvustoissa. 413 Vol. 6 (1997): 409^)13. AGRICULTURAL AND FOOD SCIENCE IN FINLAND