The thermal treatment of mites is something which has intrigued me in the past, but not having thought about this for several months it came as a complete surprise to me this morning when I woke up with what I believe may possibly be a novel idea ... [need to be careful here, as very few ideas in beekeeping are truly original ]
The thermal treatment of mites is basically a very sound proposition - there have been several papers published demonstrating that this technique does indeed work, and very effectively - BUT - the big turn-off for me has been the several hours required, especially as only one hive at a time is usually treated - unless you happen to have very deep pockets in order to treat several hives simultaneously, as each thermal kit is expensive. There's also the issue of raising the temperature to a level above that which the bees normally maintain, and so during treatment you're working against the bees as they try to cool the area being heated.
I'm an advocate of Vapourised Oxalic Acid which targets phoretic mites very effectively - claims of a 97% kill rate have been made - but VOA cannot make contact with those mites protected behind sealed wax cappings. Indeed, I often wonder whether the remaining 3% are simply those mites which have emerged from behind their protective wax shields between the application and it's measurement ?
As we know, VOA is at it's most effective when applied in mid-Winter, during which time the colony is either broodless or has only token brood, and that in contrast a sequence of 4-5 treatments are required in mid-season precisely in order to catch newly-emerged phoretic mites.
The idea which occurred to me this morning was - "why not conduct BOTH treatments at the same time during mid-season ?". 'Mid-season' here meaning most probably late August - but if needs be could be performed at any time.
By BOTH treatments, what I mean is to make several colonies broodless, thus maintaining the susceptibility of phoretic mites to VOA as happens during mid-winter, whilst placing the removed brood combs into a purpose-made cabinet in order for them to undergo thermal treatment.
What I envisage is a highly insulated (and sealed) thermal cabinet, housing (say) five colonies-worth of brood frames, with the frames spaced apart and mounted within skeleton cassettes which are then lowered into the box. The box itself having a 500W or so heater with multiple fans to ensure a uniform heat distribution. The heater would be controlled by a precision PI controller, with an additional over-temperature thermostat fitted for safety. In addition there would be a facility to provide high humidity - somewhere around 70%. All of this is very DIY do-able, and at a very modest cost when compared with commercially-manufactured equipment.
Without the bees being present, and with a fully sealed cabinet, I would anticipate a fast rise-time to set-point, with the only delay being that time required for the heat of the air to penetrate the wax combs. 2 hrs should be adequate for a saturated heat penetration, and it might even be possible to reduce this somewhat. But whatever time period is actually required to 'cook' the mites, this could productively be spent VOA-ing those five hives, such that immediately after treatment the brood combs could be returned, and (say) another five hives then be treated in exactly the same way.
Obvious negatives are the workload involved in dismantling hives (with tall stacks a sequence of 4-5 doses of VOA would be far more appropriate), as well as the shaking-off of bees. Also, there would then exist within the apiary a piece of kit which only gets to be used once a year.
Moving on to positives, I find the idea of treating the whole apiary for mites during the season - to be completely 'done and dusted' as it were, in one single day - to be particularly compelling, with the additional single application of VOA during mid-winter being all that would then be required for a complete resolution of the mite issue - that is, until a much better long-term solution is found.
The building of a suitable Thermal Cabinet is very DIY-able and affordable, and the (say) two-hour period involved in thermal treatment would lend itself very nicely to the use of the somewhat slower Varrox-style vapouriser wands.
As already commented, I doubt that this idea will appeal to those with very tall hive stacks, where the workload of dismantling them would be prohibitive - but - for those with single box hives, perhaps with one or two supers, or those with Long Hives and their derivatives - this dual mite-treatment protocol could appear attractive.