DIGITALNA ARHIVA ŠUMARSKOG LISTA
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| ŠUMARSKI LIST 7-8/2014 str. 39 <-- 39 --> PDF |
Studies conveyed in nature revealed that adults of new generation occurred on mass from the beginning of the second decade of May, hence causing the increased number of adults on ash branches (Figure 1 and 2). Insect development calendar for S. fraxini in Branjevina forest is shown in Table 2. Immediately after the eclosion, the adults of new generation began with supplementary feeding for overwintering period. It was confirmed that the supplementary feeding lasted between 9 and 13 days. During supplementary feeding adults consumed on average 2.5 cm2 of the ash leaf area. Adults which immediately after eclosion were placed on the ash plants in pots, and covered with the net fed on leaf of the nursery plants for a short time, and then entered the dormancy without copulation and oviposition, which revealed that the ash weevil had one generation per year. 4. DISCUSSION Rasprava Activation of adults from the winter shelters was observed at the beginning of the second decade of March, i.e. in early April, and the great difference in the time of adult activation in the two observed years may be explained by the temperature differences. Average temperature in March of 2009 was 0,9°C lower compared to 2008. By comparing the beginning of swarming with the average daily temperatures it was observed that activation of adults started after 2 – 4 days with average daily temperatures about 10°C (Drekić 2011). Obtained results revealed that the temperature is one of the major environmental factors influencing the time of adults winter diapauses interruption. Differences in time of overwintered adult activation were also mentioned by other authors. Mikloš (1977) determined that overwintered adults occurred in late March or early April. Similarly, Lemperiere and Malphettes (1983) for France and Blando and Mineo (2004) in Sicily mentioned that adults occurred in late March. Tsankov et al. (1990) mentioned that in Bulgaria activation of adults occurred in early April, and according to Pojras (1993) in Moldavia the adults occurred in the second half of April. The difference between authors relating to the time of adult activation was probably the results of different ecological conditions and different mean daily and monthly temperatures prevailing in the years of investigation. This indicates necessity of monitoring of adults activation in order to apply the control measures in appropriate time. Oviposition began 12 to 22 days after female activation in spring. Marović (1963) claimed that oviposition began after only two days, which was quiet short period of supplementary feeding compared to our studies. Differences in terms of oviposition, which under our conditions started in early April and extended till early June, and according to Schwerdtfeger (1957), which in biological formula mentioned oviposition in May, might be the consequence of late activation of overwintered adults under climatic conditions in Germany, resulting in late oviposition and larva development which according to Schwerdtfeger (1957) and Scherf (1964) took place in May and June. Development of larvae in our country was observed from the second decade of April which was in accordance with Mikloš (1977), whose research was done under similar environmental conditions. In our studies the larvae occurred almost at the same time, although the occurrence of adults in 2008 was observed 20 days earlier, which revealed that oviposition coincided with flushing of the ash trees in order to supply feed for newly hatched larvae. Upon death, unlaid eggs were found in ovaries of 85% of females. Eggs remaining in ovaries could be linked to growing conditions in experiment, which were certainly less favorable than those prevailing in nature. It could be assumed that the number of ovposited eggs under natural conditions is greater compared to those under growing conditions. Data on the number of eggs were mentioned by Tsankov et al. (1990), who used dissection аnd find aprox. 40 well developed eggs in female ovaries. Lesser number of eggs per female compared to our study could be explained by the fact that female oviposit eggs during prolonged period of time, which coincided with the process of oogenesis, and only number of formed eggs found in ovaries could be confirmed by dissection. Mikloš (1954) disregards significance of additional summer feeding of young adults for overwintering, stating that it is insignificant compared to larvae consumption. Comparison of determined average leaf area of 3.3 cm2 consumed by larvae during their development with the average damaged area of 2.5 cm2 caused by adults feeding for overwintering revealed that the larvae nutrition was only 32% higher, and that damage caused by supplementary nutrition of adults for overwintering could not be neglected. These damages were even more significant because they occurred later during vegetation, and that late defoliation of deciduous trees has more negative influence on tree vitality than that occurring in early spring (Androić et al. 1981). By studying literature it was observed that there were big differences among authors in terms of the number of generation. Some authors (Mikloš 1954, 1977, Scherf 1964, Androić et al. 1981, Tsankov et al. 1990, Pojras 1993, Blando and Mineo 2004) stated that ash weevil had only one generation per year, which was also confirmed by our studies, but some others mentioned two, and even more generations per year (Nüsslin 1913, Nüsslin and Rhumbler 1927, Lemperiere and Malphettes 1983, Nageleisen 1992). Various quotations regarding the number of generations were probably not the consequence of various conditions under which the studies were carried out, for Blando and Mineo |