Two Faces of the Screened Bottom Boards—An Ambiguous Influence on the Honey Bee Winter Colony Loss Rate

We conducted a citizen science survey on the winter honey bee colony losses in Poland from 2017/18 to 2019/20 to determine the influence of the use of screened bottom boards on the winter colony losses due to various causes. A total of 1035 beekeepers with 40,003 colonies reported valid data. The overall winter colony loss rate ranged from 10.7% to 13.9%, and in every year, the overall winter colony loss rate was higher than 10% (which is considered as acceptable in Poland). The study reveals that the use of screened bottom boards was associated with reduced overall loss rate. However, the nature of this relationship was not the same in terms of all types of colony losses: while the use of screened bottom boards was associated with a reduced mortality rate (management-related colony loss rate due to dead colonies) in which the empty hives were observed (colony depopulation syndrome, CDS), it was associated with an increased mortality rate in which the lack of food was observed (starvation). Given that in our study the role of CDS in the overall colony loss rate was 2.5-fold higher than the role of starvation, the final influence of the use of screened bottom boards on the overall colony loss rate turned out to be beneficial. Given the well-known beneficial role of screened bottom boards in varroosis control, they are highly recommended in beekeeping practices in Poland.