Varroa destructor from the Laboratory to the Field: Control, Biocontrol and IPM Perspectives-A Review

Abstract

Varroa destructor 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.

Keywords: Varroa destructor; biocontrol; holistic approach; honeybees; integrated pest management.

Figure 1

Varroa destructor life cycle and its population dynamics throughout the year. On the left, the two phases of the ecto-parasite cycle. The black line is in case of single infestation, the dashed line is in addition of the black one, thus in case of co-infestation in a brood cell. Inside the brood cell, the foundress laid eggs which go through several steps from protonymph to deutonymph until becoming a new daughter and son. On the right, the pattern of infestation over time. The balance between the number of brood available and the number of ecto-parasites in the colony influence the genetic diversity. Varroa destructor mating biology leads to intense inbreeding with no exchange among lineages in the population while a single foundress invades a brood cell in spring. Co-infestation happens in drone brood cell early in the year. However, cross-mating events occur around summer and fall with two or more foundresses co-infesting the same cell resulting in more heterozygous loci [54]. Contrary to assumptions, the ecto-parasite is genetically diversified with a dynamic structure over time [48].

Figure 2

Intra colonial variability of V. destructor population. Genetics through several traits can influence the phenotype [168]. Mites differ from winter to summer with a dynamic morphotype ratio going from 20% of winter morphotype in summer to 20% of summer morphotype in winter [173,174]. In addition, V. destructor can be divided in four different states: dispersal ecto-parasite attached to honeybee’s body, comb mite freely moving inside the hive, reproductive acari confined in brood cell and non-reproductive mite confined in brood cell [30,172]. Virus loads add a third dimension where the mite can be free of viruses, virus-positive with replication or virus-positive without replication (asymptomatic). This combination of different states, morphotypes and virus loads produce a variety of acari which represents distinct targets with their own reachability.

Figure 3

Holistic approach to study V. destructor. This process includes in silico, in vitro, semi-field, field tests and back to modelling with new enriched data. Each arrow indicates enriched exchanges between scales.