Bettina Ziegelmann

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.

Standard methods for varroa research

Summary Very rapidly after Varroa destructor invaded apiaries of Apis mellifera, the devastating effect of this mite prompted an active research effort to understand and control this parasite. Over a few decades, varroa has spread to most countries exploiting A. mellifera. As a consequence, a large number of teams have worked with this organism, developing a diversity of research methods. Often different approaches have been followed to achieve the same goal. The diversity of methods made the results difficult to compare, thus hindering our understanding of this parasite. In this paper, we provide easy to use protocols for the collection, identification, diagnosis, rearing, breeding, marking and measurement of infestation rates and fertility of V. destructor. We also describe experimental protocols to study orientation and feeding of the mite, to infest colonies or cells and measure the mites susceptibility to acaricides. Where relevant, we describe which mite should be used for bioassays since their behaviour is influenced by their physiological state. We also give a method to determine the damage threshold above which varroa damages colonies. This tool is fundamental to be able to implement integrated control concepts. We have described pros and cons for all methods for the user to know which method to use under which circumstances. These methods could be embraced as standards by the community when designing and performing research on V. destructor.

The mating behavior of Varroa destructor is triggered by a female sex pheromone. Part 2: Identification and dose-dependent effects of components of the Varroa sex pheromone

Reproduction of female Varroa destructor happens within the sealed brood cell of the honeybee host. The mating represents the last step of the reproductive cycle and is usually performed between the mature male offspring and one or more daughter mites. By offering solvent extracts of freshly molted females to male Varroa mites in our mating bioassay, we could clearly confirm the presence of a volatile female sex pheromone. After separation of the extract into a polar and non-polar fraction, only the polar fraction elicited the typical mating behavior of male mites. GC-MS analysis of the active fraction revealed a pattern of three fatty acids as the main components and the respective ethyl esters. We could prove that all these substances stimulated the male mating behavior, and we present results on the dose-dependent reactions of the males toward these compounds. The identification of a Varroa sex pheromone might enable new options for a biological control of the parasite.

Male mites (Varroa destructor) perceive the female sex pheromone with the sensory pit organ on the front leg tarsi

Varroa destructor males are attracted by a volatile sex pheromone of female mites. We assume that this pheromone is perceived by the sensory pit organ on the front leg tarsi. To test this hypothesis, the front legs of the males were varnished with nail polish. The behavior of the thus treated males toward attractive female mites was analyzed in our mating bioassay and compared to untreated control males and to males with varnished idiosoma. The control males with the varnished idiosoma revealed the same distinct copulation behavior as untreated males whereas the males with the varnished front legs did not show copulation attempts any more. Hence, the sensory pit organ is responsible for the perception of female signals that elicit the copulation behavior. Additionally, a first scanning electron microscopy (SEM) analysis is presented to characterize the male sensory pit organ.

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.

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

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

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.