[Experiences with varroatosis control (field cases)].
The prevalence of Varroa jacobsoni in 20 bee-farms with an average of 371 swarms in the administrative districts of Tübingen and Stuttgart was investigated between 1983 and 1987. In summer as well as after each treatment the debris was examined regularly. On 19 bee-farms the population dynamics of the mites showed a similar pattern. During the first year after the varroatosis diagnosis we found an average of 30-50 mites after autumn treatment. This number increased to 300-600 varroa mites during the second and to several thousand during the third and fourth year. Evidently, the late autumn/early winter treatment of the more heavily infected swarms (during the third or fourth year after diagnosis) with Perizin or Folbex-VA-neu was usually insufficient to prevent damage or even a complete breakdown. In such swarms formic acid in form of the Illertisser mite plate must be applied as early as August or at the latest in September (after the end of the honey season). On one of the 20 bee-farms the development of the varroa mite infection differed from the other 19 farms. In this case the mite infection developed very slowly. The husbandry and environmental conditions could not explain this phenomenon. We think that this warrants further investigations.