Abstract
Varroatosis, caused by the Varroa destructor mite, is currently the most dangerous parasitic disease threatening the survival of honey bees worldwide. Its adverse effect on the welfare and health of honey bees requires the regular use of specific acaricides. This condition has led to a growing development of resistance phenomena towards the most frequently used drugs. In addition, another important aspect that should not be understated, is the toxicity and persistence of chemicals in the environment. Therefore, the identification of viable and environmentally friendly alternatives is urgently needed. In this scenario, essential oils are promising candidates. The aim of this study was to assess the contact toxicity, the fumigation efficacy and the repellent effect of Origanum heracleoticum L. essential oil (EO) against V. destructor mite. In the contact tests, each experimental replicate consisted of 15 viable adult female mites divided as follows: 5 treated with EO diluted in HPLC grade acetone, 5 treated with acetone alone (as negative control) and 5 treated with Amitraz diluted in acetone (as positive control). The EO was tested at concentrations of 0.125, 0.25, 0.5, 1 and 2 mg/mL. For each experimental replicate, mortality was manually assessed after one hour. The efficacy of EO fumigation was evaluated through prolonged exposure at different time intervals. After each exposure, the 5 mites constituting an experimental replicate were transferred to a Petri dish containing a honey bee larva and mortality was assessed after 48 h. The repellent action was investigated by implementing a directional choice test in a mandatory route. During the repellency tests the behavior of the mite (90 min after its introduction in the mandatory route) was not influenced by the EO. In contact tests, EO showed the best efficacy at 2 and 1 mg/mL concentrations, neutralizing (dead + inactivated) 90.9% and 80% of the mites, respectively. In fumigation tests, the mean mortality rate of V. destructor at maximum exposure time (90 min) was 60% and 84% at 24 and 48 h, respectively. Overall, these results demonstrate a significant efficacy of O. heracleoticum EO against V. destructor, suggesting a possible alternative use in the control of varroatosis in honey bee farms in order to improve Apis mellifera welfare and health and, consequently, the hive productions.
Highlights
To date, varroatosis is the main parasitic disease of honey bees, with a great impact on the welfare and health of bees and on their productivity and performance
The essential oil from the fresh aerial parts of O. heracleoticum was extracted by steam distillation for 2 h with a Clevenger-type apparatus obtaining an extraction yield of 0.8% w/w
Essential oils can be a valid alternative to synthetic acaricides for the control of varroatosis in beekeeping [21,22,24,67,68]
Summary
Varroatosis is the main parasitic disease of honey bees, with a great impact on the welfare and health of bees and on their productivity and performance This disease is caused by the Varroa destructor mite, which was assumed to be Varroa jacobsoni before the last century [1,2]. Since its first infestation against Apis mellifera, the mite has spread rapidly all over the world, profoundly changing the approach to beekeeping [2] This ectoparasite, in addition to exerting an activity of depletion of essential nutrients for the proper physiological maintenance of the honey bee, is vector of many viruses during its meal [1]. The risk associated to a reduced survival of honey bee colonies is mainly related to the viral vector action of the parasite This peculiarity means that V. destructor is often indicated as the main cause of the collapse of the hives [3–5]
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