Common walnut has gained recognition in Poland as a tree decorating the space around us because of its habit and impressive leaves. More and more frequent use of walnut in the city landscape induced a more thorough analysis of the factor causing a significant decrease in the decorative values of this plant. The aim of this paper was to trace the numerical strength dynamics of Ch. juglandicola occupying the Juglans regia L. trees, referring to the weather conditions system and to determine the harmfulness degree of the above- mentioned plant louse species. Studies were conducted on walnut trees (Juglans regia L.) in Lublin in the years 2006–2008. Three stands in housing estate areas were selected for observation: (A) –a park estate of single-family houses (B) –in the University Campus area, and near the street (C) –at the crossroads of busy streets near the petrol station. The terms of occurrence of this plant louse, as well as the dynamics of its numerical strength were determined against the background of weather conditions. It was established that Chromaphis juglandicola (Kalt.) occurs on the Juglans regia L. trees growing in Lublin town plantings. It was reported that these plant lice occurred most numerously in the year 2007 on the housing estate post. Ch. juglandicola were observed at the bottom side of the leaf blade in dispersion. The terms of spring appearance and autumn disappearance of plant lice were significantly affected by the course of weather conditions. Their development was enhanced by warm spring with not very intense precipitation. However the air temperature above 30ºC and storm-like down pouring rain in summer limited the numerical strength of Ch. juglandicola.

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