# Varroa Control by Means of a Hyperthermic Device
## 1 Introduction  
_Varroa destructor_ can be considered the major pest of honey bees worldwide [1, 2]; in addition to the direct damage it causes to adult bees and brood, it is responsible for the spread of numerous viruses and is one of the main causes of colony loss [3]. A correlation has been observed between the presence of _V. destructor_ and infections with deformed wing virus and acute paralysis virus, which are often the cause of colony collapse [4]. Moreover, colony losses, directly or indirectly due to _V. destructor_ infestation, are generally more severe in autumn and winter [5]. Despite the fact that many control methods have been proposed over the years, varroa control is particularly difficult and costly for beekeepers. The use of chemical acaricides has been supplemented by the use of products with a low environmental impact, in particular essential oils and organic acids. Oxalic acid and thymol have demonstrated good efficacy, although they show a strong variability of action, suggesting the use of an integrated pest management (IPM) approach. It is now generally accepted that in order to limit the damage caused by varroa, such an approach should be adopted to minimize the use of acarticles [6, 7]; despite this, in current practice, beekeepers still tend to rely mainly on chemical control measures. Several studies have shown that heat can damage or kill _V. destructor_[8]. The practice of using heat for varroa control is called thermotherapy or hyperthermia and involves heating the whole bee-hive, the bees or the sealed brood alone, depending on how the hyperthermia is applied, to a temperature that is harmless to the bees but fatal to varroa. This is possible because honey bees and varroa have different sensitivities to heat. Juvenile forms of honey bee, such as larvae, can tolerate up to 42-43 \({}^{\circ}\)C while adults can tolerate up to 48 \({}^{\circ}\)C; therefore, if they are exposed for a sufficiently long time to temperatures above 40 \({}^{\circ}\)C, they are not harmed, unlike the mites, which are killed. The time required to kill or seriously damage most of the mites is inversely proportional to temperature; times up to eight hours were applied in various experiments without hampering adult bee and brood survival [8, 9], even extending worker lifespan [10]. In the 1980s, the first devices capable of applying hyperthermia were developed. These were thermal boxes (Hanko thermocube, thermo bell and thermobox) that utilized different methods of heating adult bees to temperatures above 45 \({}^{\circ}\)C in order to kill varroa mites in the phoretic phase. These devices could only be used for diagnostic purposes and also had obvious limitations in their use since the entire hive had to be moved inside the heat boxes. Subsequently, devices able to apply heat to the brood were designed, thus succeeding in targeting the parasite during the reproductive phase. During the brood rearing season, most varroa mites are in the brood cells to reproduce, so heat treatment of the brood keeps infestation levels low [11]. Adult bees in the combs react to heat with intense ventilation in order to cool the nest when brood temperature exceeds 36 \({}^{\circ}\)C. This suggested the removal of adult bees from the combs to avoid significant bee losses. In 1991, the first thermo-solar hive in history was developed, which was followed by the recent Rasnov thermo-solar hive [11]; the results obtained with this solution have shown high effectiveness, though it has a major limitation as it can only be used in the hottest period of the year, with temperatures above 20 \({}^{\circ}\)C. In order to apply thermotherapy under cooler conditions, the Varroa Terminator was developed; it is an electronic device consisting of two electrically heated boards that are placed above and below the hive. Once the device is switched on, the temperature inside the hive rises rapidly and the mites, as a result of falling off the bees, die on contact with the hot surface of the lower panel [12].