Viljakaskaan (Calligypona pellucida f.) aiheuttaman kaurantuhon vaikutus kauran viljelyalaan ja satoihin Suomessa

The damage to oats caused by Calligypona (Delphacodes) pellucida F. established in Finland, has previously been discussed in many publications (1—11, 13 and 14) The material of this study is composed of the investigations at the Department of Pest Investigation, of the informations given by farmers and of the agricultural statistics. KANERVO et al. (10) have previously described the symptoms of the damage to oats. The damaging of oats to various degrees seems to be due to the grouping of C. pellucida on certain oats plants and to the constancy of these groups on the same plants. The oats plants of an earlier developmental stage are damaged more seriously and the severity of damage makes a positive correlation to the number and sucking-time of C. pellucida. The symptoms vary depending on the habitat and the year. In 1957, for example, the colour of the oats damaged by C. pellucida was more slightly yellowish brown or red than in the previous year both in the field and in the isolation cage experiments. Accordingly the number of basal and lateral shoots was smaller and the yields better. The area of damage extended rapidly until the year 1954, as the figures 2 and 3 indicate. After the peak of the years 1954 and 1955 the extension seems to slacken. The distribution area and the severity of damage estimated are presented in figure 4. The damage borders on a healthy tract with little arable land in the north and it is slightest in the barren watershed of Suomenselkä with dispersed farms and in the sandstone tract of the river Kokemäki. Calligypona pellucida lives in the grass biotopes, as in cereals, grass, crops, meadows and swamps. The species increases mainly on cereals, and the grass, usually timothy, sown to cereals is the most important food plant of the nymphs. Theorethically it is to be concluded; 1. The more cereals the more favourable places for reproduction of C. pellucida. 2. The more crops layd on the cereals the better possibilities for the cicadas to emerge. The larger the ratio (H) between the area of the 1st year leys layd on the crops and the cultivated area of crops the more cicadas can be expected to occur per the area of crops and the more sever the damage will be. The ratio H in table 1 was calculated of the areas of some agricultural societies (cf. fig. 8). In drawing conclusions on the basis of the ratio H, the old leys and berms, too, must be taken with. The ratio between the areas of the 1st year leys and those of the older ones on the tracts in question is about 1 : 3.2. The observations made in 1956 indicate that the nymphs occur more abundantly in the areas with high ratio H. The calculations on the number of eggs done at the areas of the agricultural societies 1 and 3 (see figure 8) point to the same direction. The severity of damage shows also the same tendency (fig. 5). The example in table 2 indicates the percentage of destroying of the nymphs in the increasing biotopes of C. pellucida, treated in different ways at the area. About 45—60 per cent of the grain fields situated at the area of damage and about 60—75 per cent of the grain fields situated near this area became ploughed in autumn 1949 or in the next spring. The situation possibly continuing as this may on its part explain the territorial fluctuation in abundance of C. pellucida and the retarded extension of the area of damage and the moderating of damage towards the inland. The better oats yields in 1957 (compared with that in the previous year) is partly due to the decreasing abundance of C. pellucida. In spring 1956, on an average 927 nymphs/m2 were stated in 11 1st year timothy grasslands obtained in sucking samples (3 x 0.10 m2), in spring 1957 only 163 nymphs/m2 as an average of 13 observation places. The number of egg groups (on an average 10.9 eggs per a group) per 100 plants in 11 observation places was 42.5 in 1956 and 22.3 in 1957. The second reason is the more late swarming of C. pellucida in 1957 in regard to the developmental stage of oats. For the third the severity of damage is effected by some other factors, for the present less known (e.g. temperature and humidity) or unknown. The effect of damage on the yields and acreages of oats appears in the agricultural statistics drawn up by the agricultural societies (16). The area 1 entirely belongs to the areas of damage as well as some parts the areas 2, 3, 4 and 5 (see figures 4 and 8). The yields of oats per hectare appear in figure 6 and the deviation of annual yields from the average yields of a period of eight years in table 3. The yield of oats per hectare at the area 1, was about 600 kgs in 1954, about 800 kgs in 1955 and about 700 kgs in 1956 smaller than the average yield of the whole country. The differences between the actual yield and the theoretical yield at the area of the agricultural society mentioned before appear in table 4. The yields in 1957 were rather good. No great exceptions between the yields of the area of damage and the yields of the area of the other agricultural societies were not to be stated. The slight yields in the three first mentioned areas of the agricultural societies are due either to the unfavourable weather conditions (in 1952) or to the damage caused by C. pellucida (in 1955) or by the combined influence of both factors mentioned above (in 1954 and 1956). The numbers in table 5a present the sale of the oats in a cooperative concern at Vähäkyrö (Vähänkyrön osuuskauppa). The number show the financial losses caused by the damage. The slight yields caused by C. pellucida resulted to the reduction of the acreage of oats (fig. 7.) The acreage of oats at the area of the agricultural society, in 1956 was only 1/6 from the acreage in 1954. The oats cultivation was wholly given up in the book-keeping farms of the same area (table 6). A good conception of what the use of the acreage is like at the areas of damage and which are the plants cultivated when the area of oats decreased is given by the book-keeping farms. They were grouped in three groups according to their location at the area of damage (I, II and IV). In addition to these there were for comparison two farm-groups III and V where no damage occurred (cf. figure 8) . The tables 6—10 clear up the proportion of the different cereals from the arable land. Giving up or lessering the oats cultivation the farmers proceeded to cultivate barley either directly or through the mixed crop of cereals. Accordingly they extended the area of bare fallow. The good yields of oats in 1957 and 1958 obviously will effect on that the use of arable land will return to the former state also at the areas of damage in case the damage will not occur severely in the following few years. There are no informations on the occurrence of damage in Finland before the year 1948.Suomen läntisellä rannikkoseudulla viime vuosina esiintyneen Calligypona (Delphacodes) pellucidan aiheuttaman kaurantuhon syraptomit saattavat eri vuosina vaihdella. V. 1957 oli tuhoutuneiden kaurojen väritys heikosti kellanruskea ja versominen vähäinen sekä sato parempi edelliseen vuoteen verrattuna. Tuhoeläintutkimuslaitoksen tutkimusten sekä viljelijöiltä saatujen tietojen mukaan on uusien kaurantuhopaikkojen määrä vuoden 1955 jälkeen vähentynyt ja tuhon voimakkuudessa on vuoden 1955 jälkeen tapahtunut lievenemistä niin, että tuho oli vv. 1957 ja 1958 vaikeasti havaittavaa. Tuhon voimakkuus vaihtelee esiintymisalueellaan samana vuonna melkoisesti, johtuen ilmeisesti viljakaskaan lisääntymis-, säilymis- ja aikuistumispaikkojen määrästä eli etenkin suojaviljaan perustettujen 1.-vuoden heinänurmien osuudesta koko viljakasvialasta. Vuodesta 1955 lähtien tapahtunut tuhon voimakkuuden väheneminen johtunee 1) viljakaskaan populatiotiheydessä tapahtuneesta laskusta ja 2) tuhon ilmenemiseen vaikuttavista ulkoisista tekijöistä mm. kosteudesta ja lämmöstä. Kaurantuhon satoa alentava ja pellon käyttöä muuttava vaikutus ilmenee Pohjanmaan maanviljelysseurojen sekä ko. alueen kirjanpitotilojen sato- ja viljelyalatilastoissa. Österbottens svenska lantbrukssällskap'in alueella oli kauranviljelyala v. 1956 vain 1/6 v. 1954 alasta ja saman alueen kirjanpitotiloilla on kauran viljely lopetettu kokonaan. Viljelijät ovat kauranviljelyn lopetettuaan tai sitä vähennettyään siirtyneet joko seosviljan kautta tai suoraan ohran viljelyyn sekä laajentaneet täyskesantoalaansa. Vuosina 1957 ja -58 saadut hyvät kaurasadot vaikuttanevat kauranviljelyalan palaamiseen lähes ennalleen, ellei voimakasta tuhoa esiinny lähi vuosina. Kaurantuhon esiintymisessä on havaittavissa selvä yleistyminen vuodesta 1948 vuoteen 1955, jonka jälkeen tuho vuoteen 1958 mennessä näyttää melkoisesti lieventyneen ja lähes kadonneen. Kaurantuhon aikaisemmasta esiintymisestä Suomessa vuotta 1948 edeltäneeltä ajalta ei ole tietoja