Abstract
Varroa destructor is the most important ectoparasitic mite of honey bees that has a negative impact on bee health and honey production. The control programs are mainly based on the use of synthetic acaricides that are often administered indiscriminately. All this has led to drug resistance that now represent a great concern for honey bee farming. The research for alternative products/methods for mites’ control is now mandatory. The aim of this study was to test whether Citrus spp. essential oils could diminish the growth of the V. destructor mite. In Calabria (southern Italy), plants of the Citrus genus are very common and grow both spontaneously and cultured. The essential oils used in this study were extracted from bergamot (Citrus bergamia), grapefruit (Citrus paradisi), lemon (Citrus limon), orange (Citrus sinensis), and mandarin (Citrus reticulata) by hydrodistillation. Every EO was in vitro tested against V. destructor. Each experimental replicate was performed using 35 viable adult female mites (5 for each EO) collected the same day from the same apiary and included negative controls (5 individuals exposed to acetone only) and positive controls (5 individuals exposed to Amitraz diluted in acetone). The essential oils (Eos) were diluted (0.5 mg/mL, 1 mg/mL, and 2 mg/mL) in HPLC grade acetone to obtain the working solution to be tested (50 µL/tube). Mite mortality was manually assessed after 1 h exposure under controlled conditions. The essential oils that showed the best effectiveness at 0.5 mg/mL were bergamot, which neutralized (dead + inactivated) 80% (p ≤ 0.001) of the parasites; grapefruit, which neutralized 70% (p ≤ 0.001); and lemon, which neutralized 69% of them. Interestingly, the positive control (Amitraz) at the same concentration neutralized 60% of the parasites. These results demonstrate that Calabrian bergamot, grapefruit, and lemon Eos consistently reduced V. destructor viability and open the possibility for their utilization to control this parasite in honey bee farming.
Highlights
Bees are essential for the regulation of ecosystems, and they are among the most economically important pollinators for crops and wild plants [1,2,3]
Essential oils (EOs)) were detected, with a maximum of 18 compounds detected for orange oil
Alphapinene, beta-myrcene, limonene, and linalool were detected in all 5 analyzed essential oils, and the most effective bergamot oil showed the highest concentration of linalyl acetate
Summary
Bees are essential for the regulation of ecosystems, and they are among the most economically important pollinators for crops and wild plants [1,2,3]. It is well known that honey bee farming generates income and job opportunities in rural areas and allows human beings to obtain valuable products with relevant nutritional and nutraceutical properties [6,7,8]. The causes are not yet completely known, but they are certainly linked to different human-related variables/factors, such as intensive agriculture and the use of pesticides [10,11] This led to consequences such as starvation and malnutrition of honey bees due to the diminished biodiversity and to the invasion of species such as the Asian hornet (Vespa velutina), the small hive beetle (Aethina tumida), and the parasitic mite Varroa destructor [11,12,13]. Its presence in farms is negatively affecting profitability, favoring the incidence of pathogens (bacterial and viral), and reducing the number of non-reared hives [15,16,17]
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