Die Nachwirkung einer Bayvarolbehandlung auf später in die Bienenvölker eingebrachte Varroamilben

Abstract - The after-effect of Bayvarol treatment in honey bee colonies on Varroa mites introduced later on
The system of Varroa control by means of Bayvarol and Apistan is based on a constant distribution of active agent within the whole colony during the treatment period. Moosbeckhofer and Kohlich (1990) and Moosbeckhofer (1991) have first shown that residues with toxic effects on mites remain on bees and on combs after treatments with Apistan and Bayvarol. Such after-effects also have to be considered in experiments using systematic inoculation of test colonies with Varroa mites in order to determine differences in Varroa tolerance of bees. The present study aims at the determination of the necessary waiting period between Bayvarol treatment and a new experimental inoculation. Within one apiary, 3 groups of 6 colonies each were subsequently treated over 3 wk with 8 strips of Bayvarol (I: 21 May-11 June; II: 4 June-25 June; III: 18 June-9 July). In a fourth group (control) mites were removed by means of the trapping-comb-technique (Maul et al, 1988). On 23 July, all colonies were inoculated with weighed samples of infested bees, containing an average of 431 Varroa mites each. Mite mortality was determined weekly by means of protected inserts. Light and dark mites were counted separately. After another Bayvarol treatment from 14.08.-04.09., the total of residual mites was determined for all colonies. The results show clearly a dependance of the spontaneous mite mortality from the lengths of the waiting period (numbers of dark/light mites in table I). Group I with the longest waiting period has the lowest spontaneous mite mortality and the highest number of residual mites 3 wk after inoculation. The numerical increase in the Varroa population [residual mites - (inoculated mites - spontaneously killed mites)] (fig 1) is correlated with the waiting period and is surprisingly low in the control group compared to group I. This indicates an influence of the trapping comb treatment on the subsequent Varroa reproduction, which so far has not been recorded. Finally, the percentage of light mites (immature daughters), an indicator of Varroa reproduction, is shown in figure 2. The percentage of light mites from the spontaneously killed mites is higher and from the residual mites is lower for groups with a longer waiting period. It is concluded from the results that after Bayvarol treatment under the tested conditions a 6-wk-waiting period is necessary before new mites can be safely inoculated. In this experiment 98.7% of the total treatment effect was achieved within 14 d. The phenomenon of a reduced Varroa reproduction after the application of trapping-comb treatment is assumed to be connected with the different age structure of the bee population in this group.