Long term research on the infestation dynamics of beech scale (Cryptococcus fagisuga LIND.) and on the formation of necroses in a mixed stand of beech and valuable broad leaved trees

Abstract

In the Lower Saxonian billy region, Forest office Kattenbuhl, Forest district Brackenberg, the development of 55 to 73 year old beech trees after a mass propagation of beech scale (Cryptococcus fagisuga) in 1982/83 was followed for 15 years. In neighbouring mixed stands of beech and valuable broad leaved species from natural regeneration experimental twin plots (r = 15 m), each in valley-, side slope-, and plateau position were subjected to thinning of varied intensity at different time intervals. Irrespective of the treatment, scale infestation decreased from 1983 to 2001 on all plots. The relative share of infested trees as an average of the years 1985, 1988 and 1996 came to 15-20 % (fig 3). It remained similar in the individual classes of the 6 experimental plots with quantitative differences in the intensity of the attack by the scale (fig. 3). After 18 years the attack of the scale was stronger on the thinned plots, the necrosis causes by the scale remained similar in all cases (fig. 5). Maximum necrosis occured on the thinned side slope plot within the class of dominated trees, on the corresponding unthinned one within the class of dominating trees (fig. 4). The necroses caused by an unidentified pathogen, probably Nectria ditissima, was observed only on all unthinned plots, it decreased from valley to plateau (fig. 5). Regular thinning measures are most important in the control of infestations. The time and intensity of the procedure are strongly influenced by the water regime of the stands. Up to 5 years after a mass propagation of the scale a higher proportion of final crop trees changing their social position should be preserved. As well the monitoring and control of xylophagous insects should be intensified within this time span.