Abstract
In common
practice, when articles are written about bee living conditions in a hive, and
especially about temperatures, they typically refer to the air temperature
inside the hive. However, how this affects brood temperature in a hive full of
comb, bees, and honey is generally overlooked. In the Sunny Hive, this is a
vital parameter, determining the survival of bees, brood, and the queen when
the hive interior heats up to high temperatures, which can kill Varroa mite egg
masses. Our experiment on monitoring the temperature inside the brood comb showed
that the brood temperature follows the air temperature in the hive by 3-4
degrees, and stabilizes, most likely due to the bees' active efforts to lower
it.
Keywords
Mites,
Varroa, Solar Hive, solar heating, brood
ypically,
the temperature in Sunny Hive, even in typical temperate climate during summer,
doesn't reach critical levels and is only 1-2 degrees higher than the typical
temperature in the bee nest. Only on certain hot and sunny days does the
temperature in the hive rise to 38-40°C or more. During such periods, the key
conditions for mite death—high temperature and low relative humidity—should be
achieved. But is this achieved? How does it affect the brood, the bees, and the
queens? Our experiments provide answers to the part of these questions.
First, the
queens. Queens can withstand high temperatures as long as they are short-term
and don't reach the bees' survival threshold of 45-50°C. In our experiment,
when the air inside the hives heated to 43-45°C, the queens in two small
colonies died, in one became infertile.
Second, the
eggs. Eggs also disappear from the nest at such high temperatures. Italian
beekeepers believe that queens stop laying eggs, but more likely, the eggs
cannot withstand the high temperatures and dryness and die, and the bees remove
them. It's also possible, of course, that queens sharply reduce egg production
after heat stress.
Thirdly,
the bees. Bees generally don't suffer from high temperatures of 42-43°C
(104-111°F), especially those that can leave the hive and create clusters in
the shade. However, it can happen that bees in small colonies die completely at
such temperatures, as the hive has zones of above-average temperatures
approaching 50-60°C (122-140°F). In these cases, young honeycombs also can melt
and collapse. But only young combs, which lack cocoons and the wax foundation
of which is too soft. Large colonies with a full complement of combs containing
brood and honey, as a rule, don't suffer.
Fourthly,
the brood. The brood situation has been unclear until now. Experiments showed
that heating the air around the brood comb to 43°C (111°F) did not harm the
brood. Normal bees eventually emerged from such combs. However, the actual
temperature in the comb during this period is unclear. After all, a temperature
of 38°C (102°F) is considered the threshold for normal brood development.
Materials
and Methods
For this reason, we conducted another experiment to monitor the brood comb temperature in a single-body ten-frame hive of a typical bee colony with three frames of brood and seven frames of food. The sensors were positioned on the comb as shown in Figure 1.
Figure 1.
Location of temperature sensors on a comb and their temperature indicators
arranged in the corresponding order. The sixth sensor shows the air temperature
between the brood combs.
Five
sensors were placed inside a brood comb so that the metal body of the sensor
was completely deepened into the comb. Half of the sensor was on one side of
the comb, the other on the other side. Only the wire remained exposed. Sensors
1 and 2 were located on the south side of the comb, 3 in the center, and 4 and
5 on the north side opposite the hive entrance. One sensor was positioned in
the air between the brood frames, approximately in the middle of the comb, well
away from the south wall and the hot air flow from the solar heater. This means
that the temperature of the hot air coming from the solar heater down the south
wall of the hive could be significantly higher. We only measure an average air
temperature between the brood frames.
Measurements
were taken at 0.5-1 hour intervals over two hot and, most importantly, sunny
days with temperatures ranging from 26-27°C (80-82°F) and 28-29°C (82-82°F).
This difference is quite significant, as the air temperature inside the hive
approaches the critical limit of +40°C and above. It was important to clarify
several questions, three of which are key:
• To what
temperature does the brood actually warm up, and what is the difference between
the air and brood temperatures, if any?
• Is there
a delay, and for how long, in the brood reaching its maximum temperature when
the hive is heated in the sun from morning to evening?
• Are bees
able to withstand overheating of the hive and interfere with its efficient
operation, while still preserving the lives of
brood and queen?
Results
The results of temperature monitoring on the first day are shown in Figure 2.
Figure 2. Temperature graph in five zones of brood frame and the air between the frames, which warms them up due to heat transfer at an outside temperature of +26-27C
The results
showed that with an outside temperature of 26-27°C and clear skies, the
temperature inside the hive reaches 39-40°C, while the brood comb temperature
generally remains in the range of 36-37°C. Only in the southern part of the
hive does it exceed 37°C, and then mostly in the lower part, where brood is
typically sparse. The brood is in a comfortable temperature zone. The bees
ventilate the hive without gathering in a cluster on the wall of the hive around
the entrance. The graph shows that ventilation allows the bees to maintain a
stable temperature in most of the brood comb, with a 3-4°C difference relative
to the temperature in the interframe space.
The
monitoring results on the second day are shown in Figure 3.
Figure 3. Temperature graph in five zones of the brood frame and the air between the frames, which warms them up due to heat transfer at an outside temperature of +28-29C
Results
showed that with an outside temperature of 28-29°C and clear skies, the air
temperature inside the hive reaches 41-42°C, while the brood comb temperature
generally ranges near and above 37°C. In the southern part of the hive, it
reaches and exceeds 38°C, especially in the lower part, where hot air enters.
Conditions for brood are approaching a critical limit. Bees actively ventilate
the hive, partially crowding outside the hive. This temperature is not critical
for the brood and queen unless it lasts for several days at a time. However, if
it persists for a long time, some eggs may die, and the queen may cease laying.
In small colonies, bees and the queen may die, as they are unable to provide
adequate thermoregulation in the nest. Therefore, we recommend partially
closing the heating elements of the hive, which ensure its heating at low
external temperatures.
Discussion
In both
cases, Varroa mite eggs suffer from overheating. Based on the work of Akimov
and Piletskaya, we can draw conclusions about the approximate temperature
ranges for the death of mite eggs at an average relative humidity of 40%-60%
(Fig. 4).
• +35-36°C
- up to 50%
• +36-37°C
- up to 70%
• +37-38°C
- up to 95%
• +38°C -
up to 100%
In our case, we can talk about three temperature zones with varying effectiveness (Fig. 4). In the first, on the south side of the brood frames, all mite eggs will die, then up to 70%, and further north, toward the entrance, up to 50%. On average, up to 70% of all Varroa mite eggs in sealed brood will die after these temperatures are reached. Furthermore, increasing the hive temperature to 40-42°C will lead to a decrease in humidity to 15-20%, which in turn will affect the female mites present on the bees in the hive. This will further reduce the mite infestation in bee colonies.
Figure 4. Temperature and humidity ranges and
their lethality levels for Varroa mite egg laying, as well as an approximate
distribution of temperature zones in a brood frame in a Sunny Hive
In summary,
answering the main question posed before the experiment, we can say that:
• The brood
warms to temperatures 3-4 degrees Celsius lower than the average air
temperature inside the hive. This temperature is maintained for a long time
(3-4 hours in the experiment). This may be due to active thermoregulation by
the bees, coupled with natural heat loss of the hive. This difference ensures
brood survival at hive temperatures of up to +41-42°C.
• When the
hive warms up, surprisingly, there is almost no delay in the brood warming
relative to the air in the hive. A delay of about half an hour is possible. Subsequently,
during the established thermal regime, with air temperature fluctuations in the
hive within 1-2 degrees, the brood temperature remains fairly stable.
Convection in the hive is virtually absent. Zonal heating of the brood combs is
observed. • Analysis of the graphs shows that, when a certain air temperature
is reached in the hive, the brood temperature stabilizes with a decrease of 3-4
degrees. It is unlikely that this is due to heat loss within the hive.
Therefore, it can be concluded that this is the result of the bees' activity.
On the one hand, this has a negative effect, as the hive must be heated more
intensely to reach a temperature at which Varroa mite eggs in the brood will
die. This creates dangerous conditions for the bees themselves, and especially
for the queen, who cannot leave the hive like the bees if it overheats. On the
other hand, in the case of critical, uncontrolled hive overheating and the
absence of a beekeeper, this can at least save the part of brood from death.
However, this is only possible in complete colonies. In colonies with a small
number of frames and bees, this is not possible, and the colonies may die
entirely.
Conclusion
Thus, it
can be concluded that on hot, sunny days, the Solar Hive performs its
function—suppressing the growth of Varroa mites by regularly ovsrheating their
eggs to lethal temperatures. This means that the Sunny Hive demonstrates its
true effectiveness in combating Varroa mites in honeybee colonies.
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