Peter Rosenkranz

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.

The German bee monitoring project: a long term study to understand periodically high winter losses of honey bee colonies

The Western honey bee, Apis mellifera, is the most important animal pollinator in agriculture worldwide providing more than 90% of the commercial pollination services. Due to the development in agriculture the demands for honey bee pollination are steadily increasing stressing the pollination capacity of the global managed honey bee population. Hence, the long-term decline of managed honey bee hives in Europe and North-America is of great concern and stimulated intensive research into the possible factors presumably causing honey bee colony collapse. We here present a four-year study involving more than 1200 bee colonies from about 120 apiaries which were monitored for the entire study period. Bee samples were collected twice a year to analyze various pathogenic factors including the ectoparasitic mite Varroa destructor, fungi (Nosema spec., Ascosphaera apis), the bacterium Paenibacillus larvae, and several viruses. Data on environmental factors, beekeeping management practice, and pesticides were also collected. All data were statistically analyzed in respect to the overwintering mortality of the colonies. We can demonstrate for several factors that they are significantly related to the observed winter losses of the monitored honey bee colonies: (i) high varroa infestation level, (ii) infection with deformed wing virus (DWV) and acute bee paralysis virus (ABPV) in autumn, (iii) queen age, and (iv) weakness of the colonies in autumn. No effects could be observed for Nosema spec. or pesticides. The implications of these findings will be discussed.ZusammenfassungDie Honigbiene Apis mellifera ist weltweit der wichtigste Bestäuber in der Landwirtschaft und nach aktuellen Schätzungen wird der globale Bedarf an kommerzieller Bestäubung weiter steigen. Dadurch stellt der seit Jahren zu beobachtende stetige Rückgang der Bienenvölker in Nord-Amerika und Europa ein ernsthaftes Problem für die Landwirtschaft dar. Für die Abnahme der Bienenvölker werden neben wirtschaftlichen Faktoren vor allem periodisch auftretende Völkerverluste verantwortlich gemacht, für die aber eine eindeutige Ursachenanalyse bisher fehlt.Zur Ursachenaufklärung von Winterverlusten führten wir von 2004 bis 2009 ein Monitoringprojekt durch, in dem mehr als 1200 Bienenvölker auf 125 über ganz Deutschland verteilten Bienenständen (Abb. 1) kontinuierlich beprobt und kontrolliert wurden. Die beteiligten „Monitoringimker“ stellten hierfür 10 ihrer Völker zur Verfügung und lieferten Daten zu Honigerträgen, Wanderungen und Ablegerbildung. Mitarbeiter der Bieneninstitute nahmen zweimal im Jahr Bienenproben für Krankheitsuntersuchungen (Nosema spec, Varroa destructor, 4 verschiedene Bienenviren) sowie Bienenbrotproben für Rückstandsuntersuchungen. Die Stärke der Bienenvölker wurde bei der Ein- und Auswinterung bestimmt; als „Überwinterungsverlust“ wurden Völker definiert, die tot waren bzw. nicht genug Bienen für eine erfolgreiche Frühjahrsentwicklung aufwiesen.Die Winterverluste schwankten zwischen 3,5 % und 15,2 % (Abb. 3) mit ungleicher Verteilung innerhalb der beteiligten Imker (Abb. 4). Für die Ursachenanalyse wurden die überlebenden mit den zusammengebrochenen Völkern verglichen. Dabei zeigten sich die größten und hochsignifikanten (P < 0,000001, U-Test) Unterschiede beim Varroabefall der Bienen im Oktober (Tab. III, Abb. 5). Ebenfalls hochsignifikante Unterschiede ergaben sich für die Bienenviren DWV (P < 0,00001) und APBV (P < 0,0039), nicht jedoch für KBV, SBV und den Nosemabefall (Tab. V). Erstaunlicherweise waren Völker mit jungen Königinnen signifikant seltener von Winterverlusten betroffen als mit älteren Königinnen (Tab. VI), während z. B. Beutenmaterial oder Rähmchenmaß keine Rolle spielten.Bei den insgesamt in drei Jahren auf Pestizidrückstände untersuchten 215 Bienenbrotproben wurden insgesamt über 50 Wirkstoffe (von 256) nachgewiesen, die meisten im Spurenbereich. Häufig wurden mehrere Wirkstoffe gefunden und nur etwas mehr als 20 % der Proben waren frei von messbaren Rückständen (Tab. VII). Neonikotinoide wurden nur in einer einzigen Probe nachgewiesen. Es konnte keine Korrelation von Rückstandswerten mit Winterverlusten festgestellt werden. Es gab auch keinen Zusammenhang zwischen der Überwinterung von Bienenvölkern und dem Umfang des zuvor eingetragenen Rapshonigs (Abb. 6).Unser Projekt zeigt, dass der Varroabefall im Herbst (zusammen mit den assoziierten Sekundärinfektionen) eine Hauptursache für Überwinterungsverluste darstellt. Eine konsequente Varroabehandlung und starke Bienenvölker mit jungen Königinnen sind daher die wichtigste Empfehlung, um Winterverlusten vorzubeugen. Ein zusätzlicher Einfluss der übrigen Faktoren kann nicht ausgeschlossen werden, hierfür sind aber modifizierte Versuchsansätze notwendig.

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.

Varroa destructor : research avenues towards sustainable control

Summary Pollination by honey bees plays a key role in the functioning of ecosystems and optimisation of agricultural yields. Severe honey bee colony losses worldwide have raised concerns about the sustainability of these pollination services. In many cases, bee mortality appears to be the product of many interacting factors, but there is a growing consensus that the ectoparasitic mite Varroa destructor plays the role of the major predisposing liability. We argue that the fight against this mite should be a priority for future honey bee health research. We highlight the lack of efficient control methods currently available against the parasite and discuss the need for new approaches. Gaps in our knowledge of the biology and epidemiology of the mite are identified and a research road map towards sustainable control is drawn. Innovative and challenging approaches are suggested in order to stimulate research efforts and ensure that honey bees will be able to sustainably fulfil their role in the ecosystem.

Caste determination is a sequential process: effect of larval age at grafting on ovariole number, hind leg size and cephalic volatiles in the honey bee (Apis mellifera carnica)

SUMMARYArtificial queen rearing with worker larvae grafted at different developmental stages resulted in gradual effects on ovary size (number of ovarioles per ovary), as well as hind leg and wax gland structures in adults. A significant decrease in ovariole number was observed when third instar larvae were grafted. Basitarsus shape was affected when fourth instar larvae were grafted. Queen—worker intermediates developed when early-fifth instar worker larvae were transferred. As newly emerged adults, spectra of cephalic volatiles of queens and workers are still very similar, and do not yet exhibit the caste-specific elements of the mandibular glands. At one day after emergence, most of the dominant compounds in these spectra are represented at higher levels in workers.

Differential hygienic behaviour towards Varroa jacobsoni in capped worker brood of Apis cerana depends on alien scent adhering to the mites

SUMMARYFive Apis cerana colonies in Newton hives in Allahabad, northern India, were used in the experiments. Female Varroa jacobsoni, in the reproductive phase, were transferred from drone brood into worker brood in the same colony (1), or into worker brood in another colony (2). Some introductions (3) involving dead mites washed with ethanol and pentane (and therefore odour-free) were also carried out. Mites collected from a single Apis mellifera colony were introduced into some A. cerana brood (4), and some ‘sham inoculations’, involving the opening and closing of brood cells without the introduction of a mite, were made (5). After 5 days, in (1) and (5) only about 10% of manipulated cells were found empty. In (1) and (3), in addition, about 40% of cells had been opened, the mites removed, and the cells, still containing pupae, resealed. About 50% of manipulated cells were untouched. The percentage of empty cells was much higher in (2), and even higher in (4). The results indicate that differences in hy...

The reproductive program of female Varroa destructor mites is triggered by its host, Apis mellifera.

Reproducing Varroa females begin oviposition on a host larva by laying an unfertilized (male) egg, followed by fertilized (female) offspring. Using transfer experiments, we examined whether the sequence of sexes in the brood cell is triggered by a host stimulus. When reproducing Varroa females were transferred from white-eyed pupae (worker brood) into freshly capped worker brood cells, 77% (n = 22 fertile mites after the transfer) began a new reproductive cycle by laying a male egg. The proportion of brood cells with male offspring was similar to naturally infested brood cells. Varroa females transferred into brood cells with young pupae reproduced, but only 6% (n = 16 fertile mites after the transfer) produced male offspring. This was significantly different from male production in naturally reproducing Varroa females and those transferred into freshly capped brood cells. We conclude that a host stimulus present in freshly capped brood cells triggers both the start of reproduction and the sequence of sexes.

Thermoregulation in the nest of the Neotropical Stingless bee Scaptotrigona postiça and a hypothesis on the evolution of temperature homeostasis in highly Eusocial bees

In the Brazilian stingless bee, Scaptotrigona postica, social thermoregulation was studied. Intranidal temperature was recorded in different parts of the nest and related to changes in ambient temperature. In addition, cooling and overheating experiments were carried out using colonies in laboratory observation hives. The brood chamber is the warmest part of the nest, well insulated by a multi‐layered involucrum. In the brood nest 32 ± 3° C were measured. In case of low aerial temperature the bees are capable of heating up the brood combs by mass incubation. In addition, cold spots around the brood chamber are insulated by covering with cerumen, and the entrance is more or less closed. Overheating of the brood is reduced by accelerated and aligned fanning and withdrawal of all the bees from the combs. Any direct cooling by evaporation of water could not be observed and probably is not needed in natural nests of Sc. postica which are built in trunk cavities of old trees in the neotropical rain forest. The ...

Activation and interruption of the reproduction of Varroa destructor is triggered by host signals (Apis mellifera)

The reproductive cycle of the parasitic mite Varroa destructor is closely linked to the development of the honey bee host larvae. Using a within colony approach we introduced phoretic Varroa females into brood cells of different age in order to analyze the capacity of certain stages of the honey bee larva to either activate or interrupt the reproduction of Varroa females. Only larvae within 18 h (worker) and 36 h (drones), respectively, after cell capping were able to stimulate the mites oogenesis. Therewith we could specify for the first time the short time window where honey bee larvae provide the signals for the activation of the Varroa reproduction. Stage specific volatiles of the larval cuticle are at least part of these activation signals. This is confirmed by the successful stimulation of presumably non-reproducing mites to oviposition by the application of a larval extract into the sealed brood cells. According to preliminary quantitative GC-MS analysis we suggest certain fatty acid ethyl esters as candidate compounds. If Varroa females that have just started with egg formation are transferred to brood cells containing host larvae of an elder stage two-thirds of these mites stopped their oogenesis. This confirms the presence of an additional signal in the host larvae allowing the reproducing mites to adjust their own reproductive cycle to the ontogenetic development of the host. From an adaptive point of view that sort of a stop signal enables the female mite to save resources for a next reproductive cycle if the own egg development is not sufficiently synchronized with the development of the host. The results presented here offer the opportunity to analyze exactly those host stages that have the capacity to activate or interrupt the Varroa reproduction in order to identify the crucial host signals.

Three QTL in the honey bee Apis mellifera L. suppress reproduction of the parasitic mite Varroa destructor

Varroa destructor is a highly virulent ectoparasitic mite of the honey bee Apis mellifera and a major cause of colony losses for global apiculture. Typically, chemical treatment is essential to control the parasite population in the honey bee colony. Nevertheless a few honey bee populations survive mite infestation without any treatment. We used one such Varroa mite tolerant honey bee lineage from the island of Gotland, Sweden, to identify quantitative trait loci (QTL) controlling reduced mite reproduction. We crossed a queen from this tolerant population with drones from susceptible colonies to rear hybrid queens. Two hybrid queens were used to produce a mapping population of haploid drones. We discriminated drone pupae with and without mite reproduction, and screened the genome for potential QTL using a total of 216 heterozygous microsatellite markers in a bulk segregant analysis. Subsequently, we fine mapped three candidate target regions on chromosomes 4, 7, and 9. Although the individual effect of these three QTL was found to be relatively small, the set of all three had significant impact on suppression of V. destructor reproduction by epistasis. Although it is in principle possible to use these loci for marker-assisted selection, the strong epistatic effects between the three loci complicate selective breeding programs with the Gotland Varroa tolerant honey bee stock.