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ŠUMARSKI LIST 1-2/2013 str. 59     <-- 59 -->        PDF

generally prolonged. Other mechanisms to manage the inadequate content of nitrogen consist in the presence of symbiotic organisms in the digestive system or casual carnivory (Mattson 1980). With respect to the natural decline of nitrogen in the assimilatory organs of trees during the growing season it is possible to suppose that spring species of leaf-eating or sucking species profit from the higher level of nitrogen unlike species with summer feeding where the natural level of nitrogen is lower.
Comparing the mortality and duration of the development of caterpillars and the weight of pupae of L. dispar on birch, beech, maple and oak the highest mortality, the longest development of caterpillars and the lowest weight of pupae were noted in maple while the caterpillars developed best in birch (Barbosa, Greenblatt 1979). It could be related to the different level of nitrogen in leaves of trees – birch 25–40 mg.g–1, maple 16–23 mg.g–1 (Bergmann 1988).
Decreasing the level of nitrogen in artificial food from 24.4 to 12.6 mg.g–1 did not show any affect on the mortality of caterpillars of the 4th instar of L. dispar (Lindroth et al. 1991) due to the higher tolerance of older caterpillars to changes of the food quality. Subsequently, it was proved that the mortality of L. dispar caterpillars fed on food with the low content of nitrogen (15 mg.g–1) was 2.4-times higher than in caterpillars fed on food with the high content of nitrogen (37 mg.g–1) (Lindroth et al. 1997).
In rearing on birch, we determined that this difference was even six fold. Karowe, Martin (1989) came to similar conclusions at caterpillars of a moth Spodoptera eridania Stoll, which showed 4 generations per year in Florida (Mitchell, Tumlinson 1994). On the other hand, at a sawfly Neodiprion swainei Midd., the feeding of which culminates in August and at the beginning of September (McLeod 1970), the mortality of caterpillars with the content of nitrogen in needles of Pinus banksiana Lamb. increased (Smirnoff, Bernier 1973). Also caterpillars of C. pusaria (summer and late summer occurrence) responded to the higher level of nitrogen in leaves of birch by increased mortality (Kula et al., in print).
Slightly increased content of nitrogen (27.38–33.91 mg.g–1) shortened the duration of development of males of L. dispar by 30 days and that of females by 18 days as compared