SIGNIFICANCE OF BARK BEETLES (COL, SCOLYTIDAE) IN THE SPREAD OF THE DUTCH ELM DISEASE IN THE AREA OF EUSKIRCHEN

In the area of Euskirchen the bark beetles Scolytus scolytus F., S. multistriatus Marsh, S. pygmaeus F., Pteleobius vittatus Eichhoff and P. kraatzi Eichhoff were found to be vectors of the Dutch elm disease [caused by Ceratocytis ulmi (Buis.)]. In stands of older elm trees S. scolytus was dominant, whereas in younger tree stands, S. multistratus was found most frequently S. pygmaeus, P. vittatus, and P. kraatzi wereonly of minor importance. Daily temperatures below 16.degree. C delayed development and influenced the behavior of S. scolytus and S. mulitstriatus. The higher insolation on the southern sides of the trees accelerated development. Elm trees that were infested with feeding bark beetles showed the 1st symptoms after a few days. The disease can be transmitted by the feeding of 1 bark beetle. Progress of the disease was observed. In the beginning of the investigations, only younger tree stands were healthy; 81.8% of elm trees younger than 20 yr were healthy, whereas 91% older than 60 yr had died already. Therefore, it appeared that old elm trees were preferred by bark beetles. In 1983 only a high increase in the infestation of younger elm trees was noticed.