# Varroa Control by Means of a Hyperthermic Device
## 4 Discussion  
Hyperthermia applied using the Bee Ethic system at three-week intervals from late spring to autumn kept _V. destructor_ population at tolerable levels, whereas alarming increases in mite number were observed in untreated controls. Even if hyperthermia is a rather old approach to varroa control [9, 11], most of the tests were carried out in the laboratory [9, 10, 20] and several reports of apiary trials are of anecdotal nature [11, 13, 14]. The  
Figure 10: Yearly development of _Varroa destructor_ infestation in colonies subjected to nine heat treatments under different hypotheses of re-infestation and dripped oxalic acid treatment: (**a**) no re-infestation and a single acid oxalic treatment at the end of July; (**b**) 30 mites per month from June to September (120 mites in total) and a single oxalic acid treatment at the end of July; (**c**) 150 mites per month from June to September (600 mites in total) and a single oxalic acid treatment at the end of July; (**d**) 150 mites per month from June to September (600 mites in total) and one oxalic acid treatment per month from the end of July to the end of October (4 treatments). Adult mites alive on 31 December: (**a**) 0; (**b**) 18; (**c**) 133; (**d**) 12.  
experimental conditions adopted in our trials (42 \({}^{\circ}\)C for 90 min) were intermediate among those adopted in other studies and proved to be effective in varroa control for most of the season. In late summer and early autumn, sudden increases in _V. destructor_ infestation rate were often observed and hyperthermia alone was no longer able to control mites to tolerable levels. It is difficult to decide if such increases were due to a lesser effectiveness of thermic treatments or to re-infestation, but the latter appears as the more sensible explanation since re-infestation is widespread and of relevant occurrence [21]. In any case, to turn to an IPM approach coupling hyperthermia with rather soft chemical treatments would be sufficient to keep mite infestation under control. It is worth mentioning that throughout the entire three-year testing period, average daily mite falls were lower in heat-treated colonies. The results obtained from sealed brood inspections demonstrate that the main effect of heat treatments applied with the Bee Ethic system is on the reproductive phase of _V. destructor_, with the mortality of the juvenile forms ranging from 76% to 98%. On the contrary, the impact on adult mites was less positive as the maximum mortality recorded was only 45%. Adult and juvenile mite mortality in sealed brood cells has been reported as rather low [22] and some preliminary observations confirmed that this was also the case in our experimental hives. Such results are in accordance with the lethal temperatures verified in laboratory tests for _V. destructor_ adults and juveniles [20]. Since the aim of our investigations was to ascertain the fate of adult and juvenile mites within sealed brood cells, a random sampling method was adopted and therefore our data do not permit for verification of whether hyperthermia has any effect on mite reproduction. Such an effect appears rather likely and further investigations should be carried out to clarify it. The simulations with BEEHAVE produced trends in colony development that closely resembled those experimentally observed in apiary trials. As far as mite population dynamics are concerned, apiary data were subject to the obvious limitations that derive from the difficulty in quantifying re-infestation and the necessity of acaricide treatments in summer to prevent colony collapse before winter. Therefore, the mathematical model proved to be quite useful in comparing different re-infestation phenomena and alternative chemical control strategies. Since viral infections are often associated with high levels of _V. destructor_ and can be even more dangerous for honey bees than mites [3], model simulations were also useful for generating trends of healthy and virus-infected mites during the year.