Seasonal occurrence of Lygus bugs on field crops in 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 successive years. The hibernated adults first appeared in May on overwintered crops: winter rye, red clovertimothy 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 (Zetterstedt). A few specimens of L. wagneri Remane were also found.

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 (Holopainen and Varis 1991). In Finland the population of Lygus bugs is usually low, and their abundance 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 Finland and hibernates as adults. This study describes the seasonal occurrence of Lygus bugs on eight arable crops in southern Finland.

Material and methods
Seasonal occurrence of Lygus species was determined from sweep net samples taken in 16 successive years (from 1955 to 1970) from eight 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. altissima 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 experimental farm of the Agricultural Research Centre, 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 August-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 ofsampling are the same as in Varis (1995), where the yearly variations and the possibilities of forecasting the abundance of Lygus populations are discussed. More information about the climatological 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 potato 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 taken 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 other 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 oviposition or nymphal growth forL. rugulipennis (Varis 1972, Holopainen andVaris 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 developing 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 overwintering habitats earlier. Although Lygus bugs are capable of living on a great number of plants, these late season crops are valuable hosts offering plenty of nourishment and shelter for bugs before their hibernation.
The total numbers of nymphs were considerably lower than those ofadults (Fig. 3). This may 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 accuracy with that of D-Vac and absolute sampling for determining Lygus hesperus Knight densities 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 individual 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  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.).
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 occurred 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 species 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 September, 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 abundant populations than others. Their occurrence is synchronized with the developmental stage of the crops and their abundance is dependent on temperature.