... the relative humidity (RH) levels at the top of the hive immediately above the frames and below the crown board are not controlled by the external weather ... but by the action of the bees.
No, not really, not as such.
The bees quite critically control the brood nest temperature, and they promote ventilation if the humidity in the rest of the hive is too high for nectar evaporation (if that is happening).
Thus the temperature above the frames is not being
controlled by the bees. Yes, its influenced by their actions, but it is not being controlled by them.
Go back to the brood nest. In the UK (almost all the time) that is going to be the hottest part of the hive - and insulated by bee bodies. Because it is hotter, the humidity there is lower.
To get the humidity in the brood nest above the 80% minimum used in the 1997 paper, the humidity in the (cooler) rest of the hive would have to be even higher - but the bees won't stand for that. Because the excessive humidity prevents nectar evaporation (and promotes moulds) the bees natural response is to ventilate.
As the 1997 paper tries in passing to explain, it is
only when the external temperature AND humidity are simultaneously high that the bees "hive climate control" can be defeated.
RH decreases with increasing temperature. Since ambient tempera- ture in cold and temperate climates is generally clearly below brood nest temperature, RH within the brood nest is comparably low even when ambient RH is high. RH in the brood nest of A. mellifera colonies is usually about 40% [14], and most likely lev- els above 70% hardly occur in tempe- rate and cold climates even under ex- treme conditions [15]. In Mediterra- nean climates during summer tem- peratures close to brood nest tempera- ture are frequent, but RH is usually low. Only in tropical climates are both temperature and RH frequently high.
The 1997 paper used incubators to achieve those conditions BECAUSE the bees would not allow such conditions in a real hive - they would fight against them, not least by increasing the rate of air-change in the hive.
Trying to generate those conditions in a hive with temperate conditions outside would simply result in your forcing the bees into wasting more of their energies on aircon.
The only way (outside of an incubator) that you will get sustained
brood nest humidity above 80% is when you have tropically high temperature and humidity outside the hive - so that the bees aircon effort does nothing - when they choose not to waste their energy.
The 1997 paper explains why varroa are more of a problem in hot/temperate climates than in really hot (and humid) tropical ones.
It does not see 80% humidity in the brood nest as being practically achievable when air change with the outside is possible.
Sure, you could seal them in and steam cook them, but what will happen is that a large number of bees will vacate the hive (possibly even abscond) and nectar evaporation will stop, existing nectar work in progress will ferment, and bees will get sick.
High ambient temperatures combined with high RH do not allow honey bees to control conditions sig- nificantly and the bees partially evac- uate the nest, clustering at the nest en- trance [17]. Under tropical conditions RH values within the brood nest are therefore most likely frequently equal to ambient RH.
And you'd have to keep this up for months (varroa can live outside brood cells through the winter) to get varroa numbers to decline.
The paper explains a previously puzzling observation; it does not indicate a novel control method for use in temperate climates. Unfortunate but true.
