Varroa destructor is a parasitic mite of the honeybee that causes thousands of colony losses worldwide. The parasite cycle is composed of a phoretic and a reproductive phase. During the former, mites stay on adult bees, mostly on nurses, to feed on hemolymph. During the latter, the parasites enter brood cells and reproduce. We investigated if the type of bees on which Varroa stays during the phoretic phase and if the duration of this stay influenced the reproductive success of the parasite and the damage caused to bees. For that purpose, we used an in vitro rearing method developed in our laboratory to assess egg laying rate and the presence and number of fully molted daughters. The expression level of two Varroa vitellogenin genes (VdVg1 and VdVg2), known to vary throughout reproduction, was also quantified. Results showed that the status of the bees or time spent during the phoretic phase impacts neither reproduction parameters nor the Varroa vitellogenin genes levels of expression. However, we correlated these parameters to the gene expression and demonstrated that daughters expressed the vitellogenin genes at lower levels than their mother. Regarding the damage to bees, the data indicated that a longer stay on adult bees during the phoretic phase resulted in more frequent physical deformity in newborn bees. We showed that those mites carry more viral loads of the Deformed Wing Virus and hence trigger more frequently overt infections. This study provides new perspectives towards a better understanding of the Varroa-honeybee interactions.

Varroadestructor is a real challenger for beekeepers and scientists: fragile out of the hive, tenacious inside a bee colony. From all the research done on the topic, we have learned that a better understanding of this organism in its relationship with the bee but also for itself is necessary. Its biology relies mostly on semiochemicals for reproduction, nutrition, or orientation. Many treatments have been developed over the years based on hard or soft acaricides or even on biocontrol techniques. To date, no real sustainable solution exists to reduce the pressure of the mite without creating resistances or harming honeybees. Consequently, the development of alternative disruptive tools against the parasitic life cycle remains open. It requires the combination of both laboratory and field results through a holistic approach based on health biomarkers. Here, we advocate for a more integrative vision of V. destructor research, where in vitro and field studies are more systematically compared and compiled. Therefore, after a brief state-of-the-art about the mite’s life cycle, we discuss what has been done and what can be done from the laboratory to the field against V. destructor through an integrative approach.

A considerable part of the knowledge about the honey bee parasite Varroa destructor emerged from rearing protocols in semi-natural or laboratory conditions, yet a durable protocol over several generations of mites is still lacking. The development of such multigenerational rearing relies on the emergence of a sufficient number of new fertile females in the first generation of V. destructor . The optimization of the parasite's reproductive success in laboratory conditions thus represents an important prerequisite. The number of foundress mites in a cell is known to impact the probability of male survival and thus the number of mated daughters. We therefore investigated the effect of the degree of bee larvae infestation under laboratory conditions. The results showed that the probability of finding at least one foundress alive at the end of the rearing was significantly higher in doubly infested cells. This leads to the improvement of the reproductive parameters and more specifically of the number of daughters per mite. In doubly infested cells with one dead foundress, the presence of a surviving female would in fact allow both its descendants and those of the dead mite to complete their development. The mated daughters from this system were used in a subsequent experiment to test their ability to complete their reproductive cycle in laboratory conditions, from the perspective of developing a multigenerational rearing. The reproduction and development of the offspring measured were similar to those of the first generation. However, many of the females from the second generation died before the completion of their first reproductive cycle. We suggest that these females are fertile but might lack the energy necessary to survive throughout reproduction. The results from our bioassay could constitute a basis for the development of a durable V. destructor laboratory rearing and for the improvement of our understanding of the parasite's reproductive cycle.

Background Varroa destructor is the major ectoparasite of the western honey bee (Apis mellifera). Through both its parasitic life-cycle and its role as a vector of viral pathogens, it can cause major damage to honey bee colonies. The deformed wing virus (DWV) is the most common virus transmitted by this ectoparasite, and the mite is correlated to increased viral prevalence and viral loads in infested colonies. DWV variants A and B (DWV-A and DWV-B, respectively) are the two major DWV variants, and they differ both in their virulence and transmission dynamics. Methods We studied the transmission of DWV between bees, parasitic mites and their offspring by quantifying DWV loads in bees and mites collected in in vitro and in situ environments. In vitro, we artificially transmitted DWV-A to mites and quantified both DWV-A and DWV-B in mites and bees. In situ, we measured the natural presence of DWV-B in bees, mites and mites’ offspring. Results Bee and mite viral loads were correlated, and mites carrying both variants were associated with higher mortality of the infected host. Mite infestation increased the DWV-B loads and decreased the DWV-A loads in our laboratory conditions. In situ, viral quantification in the mite offspring showed that, after an initially non-infected egg stage, the DWV-B loads were more closely correlated with the foundress (mother) mites than with the bee hosts. Conclusions The association between mites and DWV-B was highlighted in this study. The parasitic history of a mite directly impacts its DWV infection potential during the rest of its life-cycle (in terms of variant and viral loads). Regarding the mite’s progeny, we hypothesize that the route of contamination is likely through the feeding site rather than by vertical transmission, although further studies are needed to confirm this hypothesis. Graphical Abstract

As the main source of lipids and proteins in honey bees, pollen is a major nutrient provider involved in development and health and has been studied for tolerance stimulation against pathogens and parasites. In the case of Varroa destructor Anderson & Trueman (Acari, Mesostigmata: Varroidae) parasitization, the lack of a complete laboratory system to rear both the bee larva and the acarian parasite limited the studies concerning larval nutrition effects on the bee tolerance and resistance against varroatosis. Due to the development of this complete rearing protocol, we managed to feed young honey bee larvae with pollen supplemented solutions and to study the effect on their later development under parasitism conditions. In our experimental conditions, pollen influences neither the deformity rate, nor the survival of bees both parasitized and unparasitized. However, pollen extract supplementation seems to significantly impact the weight of the spinning bee larvae without having an effect on the physiological weight loss during pupation, so the differences found at the larval stage remain the same as at emergence. Varroa has a deleterious effect on bee pupae and led to a steady increase of the physiological weight loss experienced during metamorphosis. Interestingly, this ponderal loss associated with Varroa parasitization seems to be reduced in the polyfloral pollen supplementation condition. Altogether, this work is to our knowledge the first to study in laboratory conditions the impact of larval nutrition on the tolerance to parasitism. A diverse pollen diet may be beneficial to the bees' tolerance against V. destructor parasitism.

Inquiline ants are highly specialised social parasites. They usually do not produce their own worker caste but instead use the worker force of the host ant colony to ensure the rearing of their sexual progeny. Several barriers are expected to severely limit their migration, and the mechanism allowing them to disperse remains largely enigmatic. Here, we tested two hypotheses to account for the low level of infestation of inquiline parasites, in populations of the parasite ant Plagiolepis xene and its host P. pygmaea: (1) the establishment of a new P. xene colony is such a rare event that a single colonisation should be expected per population, and (2) once a P. xene colony is established in one location, it has very little chance to succeed in infecting a neighbour genetically unrelated colony. We sampled nests from both species along four separate transects, and genotyped host and parasite individuals at eight polymorphic microsatellite loci. Our genetic data contradict both hypotheses: multiple colonisation events were recorded in all four transects sampled and, in at least one case, P. xene has successfully migrated from one host colony of P. pygmaea to a spatially close unrelated nest. This shows that the dispersion capacity of the social parasite is sufficiently effective to ensure its long-term survival.

Varroa destructor (Anderson and Trueman) is known as a major pest of Apis mellifera L, especially in the Northern Hemisphere where its effects can be deleterious. As an obligate parasite, this mite relies entirely on its host to reproduce and complete its cycle. Studies focusing on isolated organs are needed to better comprehend this organism. To conduct such targeted molecular or physiological studies, the dissection of V. destructor mites is crucial as it allows the extraction of specific organs. Here, we propose a technical article showing detailed steps of females V. destructor dissection, illustrated with pictures and videos. These illustrated guidelines will represent a helpful tool to go further in V. destructor research.

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