A forest as a complex of nature of the defined properties performs a series of economic and non — economic functions (wood production, side forest produce, preservation of biological equilibrium in the environment, effects of tree stands on a local climate, soil and water relations, landscape shaping). As a natural complex, it is not very stable and it is influenced by numerous biotic, abiotic and anthropogenic factors.
It appears, from the practice of the forest protection within the administrative borders of the Regional Administration of State Forests in Olsztyn, that insects — from the group of primary pests — that feed on living or seemingly healthy trees weakened by the action of one or several factors, usually abiotic ones. The appearences of some of them force to undertake the control measures.
This paper deals with the of keeping the numbers of three butterflies, i.e. Lymantria monacha L., Panolis flammea Schiff., and Tortrix viridana L., under the level called for many years as „an iron ration".
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