Varroa destructor rearing in laboratory conditions: importance of foundress survival in doubly infested cells and reproduction of laboratory-born females

Abstract

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.

A method for semi-field rearing of Varroa destructor (Acari: Varroidae) to obtain mites of controlled age and specific life cycle

“…Statistical analysis showed no significant differences between P, F1, F2, and F3 generations for these variables (p > 0.05, GLMM results, Supplementary Table S2; Figure 2). As expected, the mean mite survival (95%) registered in our semi-field rearing experiment is higher than the values reported in in vitro rearing systems (80.5% in Jack et al (2020), 78.5 and 50% for the first and the second mite generations, respectively, in Piou et al (2020)). Regarding reproductive parameters analyzed on the remaining 1164 healthy female mites, we observed that 927 (79.64%) were fertile and 237 (20.36%) were non-fertile (Supplementary Table S1).…”

Transmission of deformed wing virus between Varroa destructor foundresses, mite offspring and infested honey bees

“…During this reproductive phase, mite reproduction was low compared to previous studies [59,60]. For the most part, bee mortality associated with the virus explained the reduced reproduction of the mite.…”

“…Two brood frames from the uninfested indoor colony were taken to the laboratory and fifth instar larvae from unsealed cells were transferred into 0.33-ml gelatin capsules as described in [59,60]. The 40 V. destructor females retrieved at the end of the artificial infection experiment were introduced into the capsules containing healthy bee larvae.…”

“…Additional studies are needed to further assess the correlations and observations from the cohort sampled herein. In any case, this artificially induced high mortality is in fact counterproductive for the mite, because it does not allow the female to complete its reproductive phase [59,60]. More generally, the viral history of a mite, i.e.…”

“…The assessment of the accuracy of the viral copy numbers quantified in individual bee heads by DWV-A and DWV-B RT-qPCRs was based on the construction and interpretation of accuracy profiles [17]. Crushed bee-head samples were spiked with pC1 and pFab1 recombinant plasmids including DWV-A or DWV-B VP3 coding sequence, respectively (recombinant plasmids used also for standard curves according to Schurr et al [60] and processed for viral quantitation. Three plasmid load levels (10 6 , 10 8 and 10 10 copies/bee or head) were used to determine the standard deviation of reproducibility (SD R ), the tolerance interval (± 2 × SD R ), and the mean bias between the theoretical value and the mean of the obtained values of six results per load level.…”