The principle of the AV bottom board hinges on the hypothesis that some varroa mites are alive when they fall naturally off of adult bees. We wanted to verify the validity of this hypothesis. By sampling six hives for natural mortality during a period of 24 hours, we observed that 16% of fallen varroa mites were alive (figure 11). In his study, C. Webster (4) concluded that the percentage of live fallen varroa mites varied from 39% to 50%. The proportion of living fallen varroa mites seems to vary according to different conditions. The confirmation that a part of the falling varroa mites are still alive validates the principle behind the use of the AV bottom board and explains the positive results obtained during the 2001 trials.
 

The thermal factor and the anti-varroa bottom board

    The important difference in the global results obtained in 2000 (29.2% more varroa mites) and 2001 (37% less varroa mites) for sub group AV suggest a confirmation of the negative thermal influence assumed in the 2000 trials. In 2000, all of the anti-varroa bottom boards were operated with the bottom opened while in 2001, with the exception of the YBO group, the bottom boards were operated with the bottom closed. To our knowledge, this is the only operational factor that was systematically different between the 2000 and 2001 trials. The results strongly suggest a connection between this factor and the negative results obtained with the use of anti-varroa bottom boards during the 2000 trials. We can legitimately assume that the brood cluster temperature was lowered with the use of the opened anti-varroa bottom board. Numerous references can be found in scientific literature confirming that lower temperature conditions enhance the development of varroa populations. Ingemar Fries (12) states: “(…) mite population seems to grow faster in cooler climates than in warmer areas (…) it has been suggested that climatic factors are decisive in determining the mite population growth although the mechanism remains unclear.” We can believe that a longer period of time in the capped brood stage resulting from a lower temperature favors an increase in the reproductive rate of the varroa mite’s population. An increase of time in the capped brood stage enables the young female varroa mites to reach maturity before the bee emerges from its cell. Kraus and Velthuis (14) found that artificially reducing the brood temperature of colonies had the effect of doubling the mite population in comparison with control groups. Their laboratory tests allowed them to determine that 33 C was the optimal temperature for varroa mite reproduction. Kraus and Velthuis (14) suggest that beekeepers adopt practices that aid colonies in maintaining brood temperature at 35 C. The results obtained by Kraus and Velthuis were not available when planning for the 2000 trials as they were published in October of the same year. Reference to the influence of temperature on the rhythm of natural varroa drop can also be found in recent scientific literature. Thomas C. Webster (4) found that this drop is correlated to the average outdoor daytime temperature. J.T. Ambrose (13) also found (2001) that when infested adult bees were exposed to variable temperatures in laboratory conditions, the percentage of varroa mites falling from the bees increased with the elevation of the ambient temperature. Here again we can deduce that the brood chamber temperature should not be lowered.