Author Correction: Lithium chloride effectively kills the honey bee parasite Varroa destructor by a systemic mode of action.

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Spermatozoa production in male Varroa destructor and its impact on reproduction in worker brood of Apis mellifera.

Reproduction in Varroa destructor exclusively takes place within the sealed honey bee brood cell and is, therefore, limited by the duration of the postcapping period. Oogenesis, ontogenetic development and mating must be optimized to ensure the production of as many mated daughter mites as possible. One adult male mite has to mate with up to five sister mites and transfer 30-40 spermatozoa to each female. We analyzed the production and transfer of male spermatozoa during a reproductive cycle by counting all spermatozoa in the genital tracts of the male and daughter mites in 80 worker brood cells at defined times after cell capping. We could show that spermatozoa production in male mites is an ongoing process throughout their adult lifetime starting after the adult molt. The spermatozoa are transferred to the females in an early non-capacitated stage and require further maturation within the female's genital tract. Our study points out that a Varroa male has at any time in the brood cell enough spermatozoa to inseminate all daughter mites but does not waste energy in producing a big surplus. In total one male produced, on average, 125 spermatozoa during a reproductive cycle in worker brood which is sufficient for successful matings with at least three daughter mites. Spermiogenesis in Varroa males represents therefore a further adaptation to the limited time available for reproduction.

Lithium chloride effectively kills the honey bee parasite Varroa destructor by a systemic mode of action.

Honey bees are increasingly important in the pollination of crops and wild plants. Recent reports of the weakening and periodical high losses of managed honey bee colonies have alarmed beekeeper, farmers and scientists. Infestations with the ectoparasitic mite Varroa destructor in combination with its associated viruses have been identified as a crucial driver of these health problems. Although yearly treatments are required to prevent collapses of honey bee colonies, the number of effective acaricides is small and no new active compounds have been registered in the past 25 years. RNAi-based methods were proposed recently as a promising new tool. However, the application of these methods according to published protocols has led to a surprising discovery. Here, we show that the lithium chloride that was used to precipitate RNA and other lithium compounds is highly effective at killing Varroa mites when fed to host bees at low millimolar concentrations. Experiments with caged bees and brood-free artificial swarms consisting of a queen and several thousand bees clearly demonstrate the potential of lithium as miticidal agent with good tolerability in worker bees providing a promising basis for the development of an effective and easy-to-apply control method for mite treatment.

Spermatozoa capacitation in female Varroa destructor and its influence on the timing and success of female reproduction.

Mating of Varroa destructor takes place inside the sealed honey bee brood cell. During copulation, male mites transfer the spermatozoa into the genital openings of the females. Before the fertilization of female germ cells, the transferred spermatozoa have to pass through a final maturation process inside the genital tract of the female, the so-called capacitation. We here describe for the first time the morphological changes and chronological sequence of spermatozoa capacitation within female V. destructor. We have defined seven distinct stages of spermatozoa during the process of capacitation and have shown that it takes about 5 days from mating to the occurrence of spermatozoa ready for fertilization. This might explain the results of an additional experiment where we could show that freshly mated daughter mites need a phoretic phase on bees before their first reproduction cycle. The transfer of non-capacitated spermatozoa from male V. destructor and the resulting long capacitation period within the female mites seems to be a consequence of an adaptive pressure for the male mites to inseminate several daughter mites within the short time span inside the sealed honey bee brood cell.

Biology and control of Varroa destructor.

The ectoparasitic honey bee mite Varroa destructor was originally confined to the Eastern honey bee Apis cerana. After a shift to the new host Apis mellifera during the first half of the last century, the parasite dispersed world wide and is currently considered the major threat for apiculture. The damage caused by Varroosis is thought to be a crucial driver for the periodical colony losses in Europe and the USA and regular Varroa treatments are essential in these countries. Therefore, Varroa research not only deals with a fascinating host-parasite relationship but also has a responsibility to find sustainable solutions for the beekeeping. This review provides a survey of the current knowledge in the main fields of Varroa research including the biology of the mite, damage to the host, host tolerance, tolerance breeding and Varroa treatment. We first present a general view on the functional morphology and on the biology of the Varroa mite with special emphasis on host-parasite interactions during reproduction of the female mite. The pathology section describes host damage at the individual and colony level including the problem of transmission of secondary infections by the mite. Knowledge of both the biology and the pathology of Varroa mites is essential for understanding possible tolerance mechanisms in the honey bee host. We comment on the few examples of natural tolerance in A. mellifera and evaluate recent approaches to the selection of Varroa tolerant honey bees. Finally, an extensive listing and critical evaluation of chemical and biological methods of Varroa treatments is given. This compilation of present-day knowledge on Varroa honey bee interactions emphasizes that we are still far from a solution for Varroa infestation and that, therefore, further research on mite biology, tolerance breeding, and Varroa treatment is urgently needed.