Mortalität der Blattwespen-Arten Euura laeta und E. mucronata infolge physiologischer Ursachen und durch Abwehr-Überwallungswachstum der Wirtspflanzen-Gewebe (Symphyta: Tenthredinidae)

Abstract

In 1965 - 1975, a gradation of the sawfly species Euura laeta Zaddach & Brischke 1883 and E. mucronata Hartig 1837 occurred in an osier plantation of Salix viminalis L. at Skalicka near Hranice na Morave (Prerov district). The population densities of these pest species, however, were controlled by various parasitoids, particularly by Eurytoma salicis Walker 1834 and E. aciculata Ratzeburg 1848 (Hymenoptera : Chalcididae), but also by the weevil species Curculio crux Fabricius 1776 (Coleoptera : Curculionidae) and by predacious birds. Their natural mortality furthermore resulted from physiological reasons including the wound squeezing (closing) activities of the host-plant tissues. On an average, 8.8% of the population of E. laeta, and 17,1% of E. mucronata were afflicted due to being enclosed and thus buried in galls. Eggs account for up to 40%, and the first instar larvae for 25% of the total amount of dead individuals observed. The mortality of larvae gradually decreases from the first to the sixth instar, and in the latter amounts to less than 5%. With increasing length of shoots, the mortality increases slightly in E. laeta, but significantly decreases in E. mucronata. Size and form of galls are particularly influenced by the presence of alive in-habitants. In osier plantations, 1-year-old growing and only slightly lignified shoots of Salix vimi-nalis provide optimum conditions for the development of sawflies, probably due to their hypertrophic dimensions, anatomical and morphological structures and physiological conditions. This might also be the reason for frequent sawfly gradations in osier plantations. In wild growing and thus untrimmed, or in artificially planted willows outside osier plantations, the development of sawflies is more seriously affected by insect parasitoids, as in these cases, they are not reduced with earlier harvest.