Abstract
The biological resistance of species of the genus Ulmus L. is interrelated with various biotic, abiotic and anthropic factors, in particular, abrupt changes in the hydrothermal regime, failure to comply with the rules of intermediate cutting management in the stands with these species, damage by leaf and stem insect pests that transmit and spread the infection, infectious agents, agents of undetermined etiology, etc.It has been discovered that the uneven distribution of rainfall over the years (the least rain in the research area fell in 2007, 2009 and 2015) induced deterioration of the sanitary state of elm woody plants, which caused them being affected by pathogens of mushroom and bacterial etiology and their settlement by xylophagous insects. On the basis of the analysis of changes in the population parameters in the phases of the development of the hotbeds of these pests it was shown that the studied plantations in the dynamics during the period from 2004 to 2006 were in a state of dispersion or the so-called crisis, but, starting from 2007, transformed into a state of the actual outbreak of the hotbed. Detected hotbeds of stem pests are characterized by high, medium and low degree of settlement of trees. At the same time, it was recorded that chronic hotbeds are characterized by a long period of existence, relatively low, although elevated (in comparison with healthy plantings), the number of insects and the size of the current dead trees; for episodic hotbeds, or hotbeds of mass reproduction, - a relatively short (3 to 5 years) period of development, a great number and size of the current dead trees. It has also been found that in both cases there are possible inverse and irreversible reactions of tree stands, although in the second scenario of the development of the population of pests, in the conditions of their mass reproduction, the full destruction of the forest plantation is most often observed. It was recorded that, near the hotbeds of mass reproduction, which are characterized by excessive population density, migration centers usually appear, where harmful insects migrate in search of new settlements.
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