Phytochemical Profile of Foeniculum vulgare Subsp. piperitum Essential Oils and Evaluation of Acaricidal Efficacy against Varroa destructor in Apis mellifera by In Vitro and Semi-Field Fumigation Tests

Simple Summary Essential oils (EOs) are promising tools for controlling Varroa destructor mites. The extreme volatility that characterizes these compounds makes it possible to achieve acaricide action without the parasites coming into contact with the drug. Despite this particularity, relatively few studies have analyzed the effectiveness of fumigation as opposed to contact efficacy. In this study, the efficacy of the EO of Foeniculum vulgare sbps. piperitum (epigeal part) and its fractions (leaves, achenes and flowers) was evaluated through in vitro and semi-field experiments. Fennel EO and its fractions showed moderate efficacy using techniques that only allowed evaporation. In particular, in in vitro tests, at the highest concentration and exposure time, the whole plant and the leaf, achene and flower fractions returned acaricidal efficacies of 56% (flowers), 52% (achenes), 64% (leaves) and 68% (whole plant). In the semi-field trials, the EO of the whole plant at the highest concentration caused the death of 53% of the mites exposed to the vapours. Varroatosis is an important parasitic disease of Apis mellifera caused by the mite Varroa destructor (V. destructor). The parasite is able to transmit numerous pathogens to honeybees which can lead to colony collapse. In recent years, the effectiveness of authorized drug products has decreased due to increasing resistance phenomena. Therefore, the search for alternatives to commercially available drugs is mandatory. In this context, essential oils (EOs) prove to be a promising choice to be studied for their known acaricide properties. In this research work, the acaricide activity of EO vapours isolated from the epigeal part (whole plant) of fennel (Foeniculum vulgare sbps. piperitum) and its three fractions (leaves, achenes and flowers) against V. destructor was evaluated. The effectiveness of fumigation was studied using two methods. The first involved prolonged exposure of mites to oil vapour for variable times. After exposure, the five mites in each replicate were placed in a Petri dish with an Apis mellifera larva. Mortality, due to chronic toxicity phenomena, was assessed after 48 h. The second method aimed to translate the results obtained from the in vitro test into a semi-field experiment. Therefore, two-level cages were set up. In the lower compartment of the cage, a material releasing oil vapours was placed; in the upper compartment, Varroa-infested honeybees were set. The results of the first method showed that the increase in mortality was directly proportional to exposure time and concentration. The whole plant returned 68% mortality at the highest concentration (2 mg/mL) and highest exposure time (48 h control), while the leaves, achenes and flowers returned 64%, 52% and 56% mortality, respectively. In the semi-field experiment, a concentration up to 20 times higher than the one used in the in vitro study was required for the whole plant to achieve a similar mite drop of >50%. The results of the study show that in vitro tests should only be used for preliminary screening of EO activity. In vitro tests should be followed by semi-field tests, which are essential to identify the threshold of toxicity to bees and the effective dose to be used in field studies.