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

Abstract

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.

Essential Oils for a Sustainable Control of Honeybee Varroosis

“…The extreme pharmacological practicality of EOs when administered by fumigation techniques was also demonstrated by Bava et al (2022) [137]. The authors found that fennel EO vapors were toxic to Varroa, while bees began to experience toxic effects only when subjected to doses ten times higher than those of Varroa [137]. Regarding this, it is also important to cite the study conducted by Hoppe (1990) [138].…”

“…The effectiveness and low risk to honeybees of fumigation as an administration technique for Acantholippia seriphioides and Schinus molle EOs to control V. destructor in a laboratory setting (16% and 8% mortality rate for honeybees, respectively, for Acantholippia seriphioides and Schinus molle), compared to the use of the complete exposure method (87% and 42% mortality rate for honeybees, respectively), has been proven [136]. The extreme pharmacological practicality of EOs when administered by fumigation techniques was also demonstrated by Bava et al (2022) [137]. The authors found that fennel EO vapors were toxic to Varroa, while bees began to experience toxic effects only when subjected to doses ten times higher than those of Varroa [137].…”

“…The former method is definitely the one that causes less traumatization of the V. destructor mites. The second is also a good harvesting method, as verified by Bava et al in 2022 [137]. Powdered sugar-based methods, on the other hand, is objectionable because it subjects mites to traumatization that could affect toxicity tests.…”

Prevalence of Varroa destructor in Honeybee (Apis mellifera) Farms and Varroosis Control Practices in Southern Italy

“…Considering this evidence, veterinarians or beekeeping technicians should recommend the application of GBPs. For example, the treatments should be administered in combination with bio-techniques, such as queen caging, and the use of drugs that are unlikely to lead to the development of resistance, such as those based on essential oils, should be preferred [ 18 , 53 , 54 , 55 , 56 ].…”

Phytochemical Composition and Pharmacological Efficacy Evaluation of Calamintha nepeta, Calamintha sylvatica, Lavandula austroapennina and Mentha piperita Essential Oils for the Control of Honeybee (Apis mellifera) Varroosis

“…Being composed of several molecules that have different cellular targets, treated pest populations are unlikely to develop drug resistance [ 38 ]. Moreover, being natural compounds, they are easily degradable in the environment and do not accumulate in the food matrices of the hive [ 39 , 40 , 41 ]. EOs from the Lamiaceae family have proven to be particularly effective in controlling V. destructor [ 33 , 42 , 43 , 44 ].…”