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Landesanstalt für Bienenkunde

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Wirksamkeit, Nebenwirkung und Verteilung von Lithiumchlorid: ein neuer Wirkstoff zur Behandlung von Varroa destructor bei Honigbienenvölkern (Apis mellifera)
(2024) Rein, Carolin Vanessa; Rosenkranz, Peter
Honigbienen sind unverzichtbare Bestäuber sowohl für unser Ökosystem als auch für die Landwirtschaft. Die weltweit verbreitete ektoparasitische Milbe Varroa destructor stellt seit Jahrzehnten das größte Problem für Honigbienen und die Imkerei dar und gilt zweifellos als Hauptverursacher der periodischen Völkerverluste (Genersch et al. 2010; Le Conte et al. 2010). Derzeit gibt es kein zufriedenstellendes Behandlungsverfahren, das alle Anforderungen der Imker erfüllt. Im Jahr 2018 wurde mit Lithiumchlorid (LiCl) ein neuer Wirkstoff mit varroazider Wirkung entdeckt, welcher eine sehr gute Wirksamkeit auf Varroamilben mit einer guten Verträglichkeit für adulte Bienen verbindet (Ziegelmann et al. 2018). Aufgrund der systemischen Wirkungsweise kann er zudem sehr einfach angewendet werden und bietet dadurch großes Potential für eine effektive Varroabehandlung. Im Rahmen der Dissertation wurden verschiedene Applikationsformen von LiCl zur Behandlung von Bienenvölkern getestet und Daten zur Wirksamkeit, Nebenwirkung und Verteilung des Wirkstoffes erhoben. Die gesammelten Daten sollen dazu dienen, eine fundierte Bewertung der Chancen und Risiken für ein Zulassungsverfahren zu ermöglichen und erfolgsversprechende Strategien für die Zulassung zu entwickeln. Zu diesem Zweck wurden verschiedene Feld- und Laborexperimente durchgeführt. Unter praxisnahen Freilandbedingungen führt die Applikation von LiCl zu konzentrationsabhängigen Schäden und hohen Ausräumraten der Bienenbrut. Diese Brutschäden sind von der Dauer der Fütterung und dem Larvenalter abhängig (Rein et al. 2022). Bei einer Fütterung von 25 mM LiCl überleben nur knapp 40% der Bienenbrut. Analysen der Lithium-Konzentration verschiedener Larvenstadien zeigen, dass in zwei Tage alten Larven noch kein Lithium nachgewiesen werden kann. Die Analysen der Futtersaftdrüsen haben bestätigt, dass das Sekret der Ammenbienen, mit dem die Königin und die jungen Larvenstadien gefüttert werden, weitgehend frei von Lithium ist (Rein et al. 2024). Daher sollte eine LiCl-Applikation kein Risiko für die Königin, oder die jungen Larven darstellen. In älteren Larven steigt die Lithium-Konzentration dagegen signifikant an (Rein et al. 2022). Dies ist auf die stadienspezifische Futterumstellung von reinem Futtersaft auf gemischtes Larvenfutter zurückzuführen. Hierbei wird auch eingelagertes Futter aus den umliegenden Waben beigemengt, wodurch auch zuvor eingelagertes Lithium weitergegeben werden kann. Aufgrund der schädigenden Wirkung von LiCl auf die Bienenbrut wurde eine Applikation für brutfreie Völker entwickelt und die Wirksamkeit unter Feldbedingungen getestet. Die Kombination einer brutfreien Phase durch Sperren der Königin mit der LiCl-Behandlung erzielte durchschnittliche Wirkungsgrade von 78 – 98% (Rein et al. 2024). Die höchsten Wirkungsgrade wurden erst bei einer Fütterungsdauer von > 5 Tagen erreicht, was zu Schäden bei der neu angelegten Brut nach der Brutpause führte. In Käfigversuchen, in denen jeweils eine Biene mit einer parasitierenden Milbe mit LiCl gefüttert wurde, konnten dagegen bereits nach 48 Stunden Milbenmortalitäten von über 95% erreicht werden (Rein et al. 2024). Offenbar verzögerte sich im Volk der Wirkungseintritt aufgrund des sozialen Futteraustauschs (Trophallaxis) und einem damit verbundenen Verdünnungseffekt des Wirkstoffs. Jedoch ist dieser Futteraustausch notwendig, um den Wirkstoff gleichmäßig im Volk zu verteilen und alle Milben zu erreichen. Es besteht daher weiterer Optimierungsbedarf, um die für die Zulassung notwendigen Wirkungsgrade von über 90% zu erreichen, ohne die Bienenbrut zu schädigen. Anhand von quantitativen Lithium-Analysen über ICP-OES konnte bestätigt werden, dass der Wirkstoff im Volk gleichmäßig verteilt wird. Bereits einen Tag nach der LiCl-Applikation wurden im Schnitt 93 mg/kg Lithium in den Honigblasen der Bienen nachgewiesen, unabhängig von ihrer Position im Volk (Rein et al. 2024). Analysen der Bienen-Hämolymphe zeigen, dass bereits nach einer 12-stündigen Fütterung der Bienen im Käfig ein Gleichgewicht von 5 – 8 mg/kg Lithium erreicht wird, welches zum Tod der Milbe führt (Rein et al. 2024). Das Lithium sollte daher nach einer LiCl-Applikation möglichst schnell im gesamten Volk verteilt werden und ausreichend lange im Volk zirkulieren, um möglichst in allen Bienen die für Milben tödliche Konzentration zu erreichen. Diese Konzentration sollte für etwa 48 Stunden aufrechterhalten werden, um eine Milbenmortalität von über 95% zu erreichen. Die im Rahmen der vorliegenden Dissertation durchgeführten Versuche bieten eine umfangreiche Basis für die Entwicklung einer zulassungsrelevanten Applikation. Es konnte gezeigt werden, dass Lithium eine hohe akarizide Wirkung besitzt und lediglich noch einige Optimierungen notwendig sind, um eine bessere Verteilung des Wirkstoffes und damit stabile Wirkungsgrade von über 90% bei brutfreien Völkern zu erreichen. Für brütende Völker müssen jedoch alternative Behandlungsmöglichkeiten entwickelt werden, um den Kontakt der Brut mit dem Wirkstoff zu vermeiden.
Temporal increase of Varroa mites in trap frames used for drone brood removal during the honey bee season
(2022) Odemer, Richard; Odemer, Franziska; Liebig, Gerhard; de Craigher, Doris
Varroa mites are highly attracted to drone brood of honey bees (Apis mellifera), as it increases their chance of successful reproduction. Therefore, drone brood removal with trap frames is common practice among beekeepers in Europe and part of sustainable varroa control. However, it is considered labour‐intensive, and there are doubts about the effectiveness of this measure. At present, it is mostly unknown how many mites a drone frame can carry at different times of the season, and how many mites can be removed on average if this measure is performed frequently. Therefore, we sampled a total of 262 drone frames with varying proportion of capped cells (5–100%) from 18 different apiaries. Mites were washed out from brood collected from mid‐April to mid‐July based on a standard method to obtain comparable results. We found that a drone frame carried a median of 71.5 mites, and with the removal of four trap frames, about 286 mites can be removed per colony and season. In addition, mite counts were significantly higher in June and July than in April and May (Tukey‐HSD, P < 0.05). The number of mites and the proportion of capped cells, however, were not correlated (R2 < 0.01, P < 0.05). Our results suggest that drone brood removal is effective in reducing Varroa destructor numbers in colonies, supporting the findings of previous studies on the efficacy of this measure. Although mite counts varied, we believe that increasing sample size over different seasons and locations could elucidate infestation patterns in drone brood and ultimately improve drone brood removal as an integrated pest management tool for a wider audience of beekeepers.
Auslöser und Ausprägung des Varroa Sensitiven Hygiene (VSH) Verhaltens im Zusammenhang mit der Reproduktion der Varroamilbe Varroa destructor
(2024) Fölsch, Lina; Hasselmann, Martin
The ectoparasite Varroa destructor causes massive economic damages to the Western honey bee (Apis mellifera) and thus threatens the health of honey bee colonies worldwide. When left untreated, varroa populations can lead to high winter colony losses (Genersch et al. 2010; Traynor et al. 2020; Le Conte et al. 2010). Beekeepers must thus routinely treat against varroa to keep their colonies healthy. A sustainable solution to overcome the varroa problem is selection for varroa resistant honey bees. In addition to studies on natural selection against varroa, there is much interest in breeding varroa resistant honey bees. During varroa resistant selection the focus is often on different selection criteria, such as mite non-reproduction (MNR) and varroa sensitive hygiene (VSH). The selection criteria MNR describes the proportion of non-reproductive mites in a colony (Virag et al. 2022). VSH encompasses the specialized removal behavior of mite infested brood by worker bees (Dietemann et al. 2013; Harbo und Harris 2005). This brood removal interrupts varroa reproduction, which leads to reduced varroa infestation in honey bee colonies. The experiments in thesis were conducted as part of the SETBie project. The SETBie (Selection and Establishment of Varroa-tolerant Bee Colonies) project was a four-year selection project whose objectives included breeding a varroa-resistant honey bee in Baden-Württemberg, Germany. Several institutions and beekeeper organizations cooperated in this project. This dissertation focuses on the factors that trigger VSH behaviour, in addition to documenting the reproductive success of the mite (MNR) in selected colonies. Initial experiments investigated the relationship between VSH and the reproductive success of female Varroa (MNR). A widely held hypothesis was that the reproduction of the mite and the associated nymphal stages in the brood cells triggered the removal behaviour of the workers. However, this hypothesis was disproved by placing mites in capped brood cells and comparing the removal behaviour of reproducing mites with that of mites whose reproduction had been blocked by a special procedure (publication 1). Both groups of mites were removed at a rate of about 40% and did not differ significantly from each other. Therefore, reproduction of the mite could be excluded as a trigger for removal behavior of the workers. This result was supported by simultaneous analysis of VSH behaviour and the proportion of non-reproductive mites (MNR) in a large number of pre-selected colonies. It was also assumed that the increased number of non-reproductive mites (MNR) within a colony is directly connected to VSH behaviour, because the workers would primarily remove the reproductive mites, leaving only non-reproductive mites behind. In addition to providing evidence that mite reproduction is not the trigger of removal behaviour, our analysis of the data collected during the SETBie project showed no correlation between MNR and VSH values (publication 3). The MNR value of a colony therefore does not allow any conclusions to be drawn about the VSH value, which should be taken into account in recommendations for breeding practices. Furthermore, an experiment was conducted to rule out the possibility that removal behaviour effects the normal reproduction of the mite in the next cycle (publication 3). The need for other triggering factors for removal behaviour to occur was clearly demonstrated in this first set of experiments. Varroa mites manipulated to remove specific factors and inanimate objects were placed in capped brood cells to determine what influences removal behaviour. We found that an object (bead) alone does not induce removal behaviour (publication 2). Dead and odour-reduced mites were removed more frequently than the bead and control, but not anywhere near as frequently as live mites. We can thus assume that, in addition to the odour of the mite, the movement of the mite in particular has a signalling effect that triggers this removal behaviour. Other as yet unknown factors such as the reaction of the larvae to the parasitism are likely to play an important role in the removal behaviour of workers. The SETBie project took place over a period of four years, which allowed us to analyse colonies over several generations. Although MNR and VSH are in principle selectable traits, our analysis has shown that they are difficult to improve over multiple generations (publication 4). Colonies with low MNR values could often be improved by crossing them with a drone (artificial insemination) from a colony with a high MNR value. However, it also became clear that it is difficult to maintain high MNR values. Methodological difficulties in targeted selection of MNR/VSH also became noticeable during the collaborative project, which involved the cooperation of numerous breeders. Despite prior infestation of the selected colonies with 180 mites, it was often not possible to collect enough mites to make a valid statement about the reproductive capability of the mite. Even after opening many cells, the minimum number of 10 single infested cells could not be found, which means that MNR values could not be determined for about 63 % of the evaluated colonies due to the presence of too few mites. The selection and breeding work involved in this project is extremely labour-intensive and therefore expensive and requires a high level of expertise among the breeders. It was shown that MNR and VSH are in principle selectable and thus theoretically a Varroa-resistant honey bee line can be bred. However, for future projects, methodological problems need to be resolved or the evaluation methods of the MNR and VSH traits need to be simplified. An optimal solution would be the identification of molecular markers to support selection via genetic heritability. However, this requires further fundamental knowledge about the Varroa mite, the mechanisms that trigger the behaviour of the currently used selection traits, and more precise phenotypic definition of the resistant behaviours.
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2023
(2024) Traynor, Kirsten
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2022
(2023) Traynor, Kirsten
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2021
(2022) Rosenkranz, Peter
Zierpflanzen als Nahrungsquelle und Bewertung der Blütenmerkmale für die Attraktivität der Bestäuberinsekten im urbanen Raum
(2022) Marquardt, Melanie; Rosenkranz, Peter
Currently, an increasing extension of urban areas can be observed worldwide, which implies a concurrent loss in natural habitats. If the current biodiversity shall be preserved, efforts must be reinforced in order to provide alternative habitats for the flora and fauna in urban areas. Regarding the assessment of the ecological value of the habitat ‘city’, scientific studies came to varying conclusions. However, all of them emphasize the importance of green urban areas in promoting the urban biodiversity. It has been frequently pointed out that sufficient and suitable foraging resources are an important and basic requirement for the survival of flower-visiting insects. However, it has hardly been investigated whether ornamental plants are suitable foraging resources for insect pollinators. While this is still highly contentious, there is growing evidence that ornamental plants could contribute to the provision of pollen and nectar. But apparently, the attractivity of different ornamental plants varies widely and furthermore, not all pollinator groups can profit equally from the mostly exotic ornamental plants. At present, scientific data for all ornamental genera or even species are not available. So, the first aim of this study was the comparison of the attractivity of certain ornamental plants, in particular those with a high market value. In order to conduct such trials, already existing acquisition methods have been assessed and refined. A further and so far rarely considered focus of this thesis is the analysis of impact factors that might affect the composition and abundance of urban pollinators. In order to identify the pollinator friendliness of ornamental plants, field tests in urban areas and semi-field tests in flight tents were conducted during the years 2017 – 2019. In the first trail, raised flower beds with an identical set of ornamental plants were installed at 13 different locations in the city area of Stuttgart. During the summer months of the years 2017 and 2018, all flower beds were visited in weekly intervals. Over a time period of 20 minutes, the number of foraging insects – divided into different groups of insect pollinators – was recorded. In total, 10,565 nectar and/or pollen foraging insects were counted. First of all, this confirms that our selection of ornamental plants was used as a foraging resource by pollinating insects. The attractivity of the tested ornamental plants, however, varied to a considerable degree among the plant species and the number of counts ranged from 1.2 flower visits in 20 minutes on Bracteantha bracteata (strawflower) to 5.3 flower visits on Bidens (beggar-ticks). It is noteworthy, that the attractivity also varied within the cultivars of the same species, partly even to a greater extent than between species. Interestingly, not only the abundance but also the composition of pollinators varied among the different test plants. Furthermore, the applied statistical models indicate significant impacts of the study year and the location on the results. This highlights the need of a continuously testing of all ornamental plants in regard to insect friendliness, for which the described methods were found to be very appropriate (publication I). Pollinating insects often use characteristic floral traits of the plants for their decision to visit the respective flower. These floral traits are often genera-, species- or even cultivar-specific and have been well studied in the native plants. In contrast, very little is known about the role of floral traits in ornamental plants. This includes e. g. the petal colour, several floral morphomet-rics or the floral scent of different plants. The impact of these traits on attractivity for insects was analysed in semi-field experiments using Calibrachoa cultivars and Bombus terrestris as model pollinator. Similar to the first part of the thesis, the attractivity of the different cultivars varied significantly. While the floral scent explained the observed differences in attractivity only to a small extend, it could be shown unequivocally that the petal colour constitutes a significant factor in the attractivity on B. terrestris. For a better understanding on the impact of certain floral traits for pollinating insects, however, there is further research required (publication II).
Charakterisierung der Qualität von Blütenpollen in unterschiedlichen Regionen Baden-Württembergs
(2022) Friedle, Carolin Gertrud Maria; Hasselmann, Martin
Honey bees (Apis mellifera) collect nectar and pollen from plants to feed their brood. Pollen provides a wide range of nutrients, such as proteins and lipids, but also carbohydrates, vitamins and enzymes. Because of these ingredients, pollen is also attractive to humans and is used as a dietary supplement. However, honey bees collect pollen not only from wild plants, but also from flowering crops grown in agriculture. Accordingly, contamination from plant protection products can be found in bee pollen and bee bread. In order to get a deeper insight into the occurrence and distribution of pesticide residues during an entire season, a total of 102 daily pollen samples were collected from April to July 2018 using pollen traps in an orchard in southern Germany. Almost 90% of the pollen samples showed detectable levels of pesticide residues. A total of 29 pesticides were detected in the samples, with more than half being fungicides, followed by insecticides and herbicides. Maximum concentrations of up to 4500 ng/g could be measured at the end of April. Samples collected in early May and late June also showed high levels of pesticides. A general risk management was performed to assess the risk of the detected pesticide concentrations for honey bees. The microbial quality of bee pollen is highly dependent on its botanical and geographic origin, as well as climatic conditions and post-harvest processing steps by the beekeeper. If no processing steps such as freezing or drying follow after harvest, the growth of microorganisms can be promoted and the pollen quality can be influenced by negative side effects such as fermentation or the production of mycotoxins. Bacterial and fungal colonies can be determined both by culture-dependent methods such as colony counting on plates and by culture-independent methods such as 16-rRNA amplicon sequencing. Following the hypothesis that storage conditions influence the composition of microorganisms in bee pollen, freshly harvested bee pollen was stored for seven days in June 2018 and 2019 under defined conditions (cold, room temperature, warm) and analyzed by sequencing 16S and 18S PCR amplicons. The bacterial community varied slightly between the sites studied and showed no significant difference between the storage conditions. The fungal community showed significant differences both between the studied sites and between the different storage conditions. The dominant fungal genera in the pollen samples were Cladosporium, Aspergillus and Zygosaccharomyces. While Cladosporium was most dominant in freshly collected pollen and the percentage decreased during storage, Aspergillus and Zygosaccharomyces showed a significant increase especially under warm storage conditions. Other contaminants naturally produced by plants can also have negative impacts on human health. Pyrrolizidine alkaloids belong to a group of phytochemicals, of which more than 600 structures are known in around 3% of all flowering plants worldwide. PA are known to be able to cause both acute poisoning and chronic damage or cancer in animals and humans. In July 2019, pollen was collected at 57 locations in Baden-Württemberg and analyzed for 42 different PAs and their N-oxides in order to expand knowledge about PA contamination in pollen and to be able to estimate the risk of the concentrations. A total of 22 different PAs were detected in over 90% of all samples examined. Only 5% of the PA were obtained as PA from plants of Senecio sp. identified, while 95% of PAs with a botanical background are from Echium sp. and Eupatorium sp. could be identified. The maximum total concentration of PA per sample was determined to be 48,400 ng/g. According to the risk values calculated by the BfR, however, 42% of the samples represented an increased risk to human health.
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2020
(2021) Rosenkranz, Peter
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2019
(2020) Rosenkranz, Peter
The production of melezitose in honeydew and its impact on honey bees (Apis mellifera L.)
(2021) Seeburger, Victoria; Hasselmann, Martin
Honeydew honey is a honey type which is of high economic importance in Europe. Phloem sap feeding insects of the order Hemiptera (true bugs) excrete honeydew, the key component of honeydew honey. Beekeepers move their hives between forest regions so that their bees can process the honeydew into honey. In case of high osmolality in the phloem sap of the hemipterans’ host trees, they counteract osmotic pressure by osmoregulation and produce oligosaccharides such as melezitose. Melezitose-rich honeydew honey is a major issue for beekeepers; it crystallises and obstructs the combs, leading to an economical loss. Nevertheless, precise analyses of the conditions of the occurrence of melezitose have not been realised. Furthermore, it is not known which impacts the trisaccharide has on honey bee health and the honeydew flow disease documented in beekeepers’ journals can have one explanation in the nutrition on melezitose. In order to determine influence factors for the emergence of melezitose, more than 600 honeydew droplets from defined honeydew producer species were collected under different environmental conditions (hemipteran species (host tree specific), natural area, air temperature, relative humidity, altitude, time of the year and of the day) between 2016 and 2019. The sugar spectra were analysed by high performance anion exchange chromatography with pulsed amperometric detection. To obtain the impact of melezitose on honey bee health, additional feeding experiments with daily evaluation of food uptake, gut-body weight ratio and mortality have been realised between 2017 and 2019. Additionally, comprehensive 16S rRNA Illumina sequencing of the gut microbial community has been performed. Remarkable differences could be found in the amount of melezitose between honeydew samples collected from different honeydew producer species and according to different environmental conditions. Air temperature increases and decreases in relative humidity increased the melezitose production in honeydew by the observed seven hemipteran species. Both, scale insect species on Picea abies and aphid species on Abies alba produced significantly less honeydew containing melezitose than aphid species on Picea abies. Additionally, honeydew with increased melezitose content was significantly more frequent collected in natural areas with limited water reservoir capacities, at higher altitudes and years with low precipitation. All results lead to the conclusion that hemipteran species produce more melezitose when the host trees have less access to water, increasing the osmolality of the phloem sap and indirectly enhancing the osmoregulation with producing melezitose by hemipteran species. Bees fed with melezitose showed increased food uptake and higher gut-body weight ratio than the control groups. Furthermore, melezitose feeding caused disease symptoms such as swollen abdomen, abdomen tipping and impaired movement and a significantly higher mortality than in control groups. Gut microbiota analyses indicated a shift of the bacterial species Lactobacillus Firm-4 and Lactobacillus kunkeei in favour of Lactobacillus Firm-5 in melezitose fed bees. This PhD project provides the important knowledge about the indicators that point out an enhanced melezitose production. This is a valuable contribution to design a warning system for beekeepers that will help to prevent harmful nutrition for honey bees or crystallised honey in the future by timely removal of bee colonies from local regions at risk. Additionally, feeding experiments point out the high effort that is required for the degradation process of the large-molecule melezitose. This effort might lead to a higher uptake of food, heavier guts, shorter lifespan and a higher susceptibility to intestinal diseases. Finally, an evidence was presented that the lactic acid bacteria of the gut microbiota are significantly involved in the digestion of melezitose.
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2018
(2019) Landesanstalt für Bienenkunde; Rosenkranz, Peter
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2017
(2018) Landesanstalt für Bienenkunde; Rosenkranz, Peter
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2016
(2017) Landesanstalt für Bienenkunde; Rosenkranz, Peter
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2015
(2016) Landesanstalt für Bienenkunde; Rosenkranz, Peter
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2014
(2015) Landesanstalt für Bienenkunde; Rosenkranz, Peter
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2013
(2014) Landesanstalt für Bienenkunde; Rosenkranz, Peter
Der Einfluss von Wirtsfaktoren der Honigbiene (Apis mellifera L.) auf den Reproduktionserfolg der parasitischen Milbe Varroa destructor (Anderson & Trueman) auf die Auswirkungen einer horizontalen Verbreitung des Parasiten auf den Befall der Bienenvölker
(2014) Frey, Eva; Bessei, Werner
The honey bee colony is faced with a huge number of pathogens, including bee viruses, bacteria, fungi and mites. Among these pathogens, the ectoparasitic mite Varroa destructor is considered the most important parasite of the honey bee worldwide. This mite was discovered at the beginning of the last century in South East Asia within colonies of the original host, the Eastern honey bee Apis cerana. From the middle of the last century the mite has been spread worldwide by transports of infested A. mellifera colonies with dramatic consequences for both, feral and managed honey bee colonies. In the meantime this parasite has become the most serious economic problem for global beekeeping. In temperate climates nearly all honey bee colonies are infested and without yearly Varroa treatments these colonies would collapse within a few years. This confirms that a stable host parasite relationship has not been established yet. Therefore the control of V. destructor still represents the main challenge for beekeeping. The main reason for host damages is the dramatic increase of the Varroa population during the season. Our honey bee colonies are obviously unable to control this population dynamic of the parasite. The increase of the mite population is influenced by the reproductive rate of Varroa females within individual brood cells, by host-parasite-interactions on the colony level and by interactions among honey bee colonies on the population level. The dissertation at hand presents experimental approaches and results at all three levels. On the individual level we were able to demonstrate that age-dependent signals of the honey bee larvae not only activate the oogenesis of the Varroa females but even trigger the further course of mite reproduction. Our studies on the activation of the Varroa reproduction revealed that exclusively larvae within 18 h (worker) and 36 h (drones), respectively, after cell capping were able to stimulate the mite’s oogenesis. Furthermore, we were able to confirm for the first time the presence of a signal in the host larvae allowing the reproducing mites to adjust their own reproductive cycle to the ontogenetic development of the host. Under certain conditions such host signals can even stop an oogenesis of the female mite that has already been started. 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. My results indicate that age specific volatiles of the larval cuticle are involved in the regulation of mite reproduction. According to preliminary quantitative GC–MS analysis we suggest certain fatty acid ethyl esters as candidate compounds. These host signals – either involved in the activation or in the interruption of the Varroa reproduction – offer possibilities to influence the reproductive success of Varroa females and might therefore be used for biological control in the future. Within an EU cooperation project we could additionally demonstrate that the so called temporary infertility of Varroa females is significantly correlated with three QTL of the host larvae. This confirms a genetic basis for host resistance factors that inhibit the mite reproduction. For this study we made use of the fact that we had access to a honey bee population at the island of Gotland, Sweden that has survived mite infestation without any treatment for more than 10 years. We crossed a queen from this tolerant population with drones from susceptible colonies to rear hybrid queens and produced a mapping population of haploid drones from these hybrids. Because honey bees have a haplodiploid sex determination, the haploid drones provide an extremely simple and highly efficient model system for genetic studies. Subsequently, we 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 the suppression of V. destructor reproduction by epistasis. The detection of this epistatic interaction was only possible because we used the simple genetic make-up of haploid drones. For studies on Varroa resistance on the colony level and for selection programs the interactions among the colonies of the local honey bee population have to be considered. In two experimental approaches I was able to prove that the invasion of Varroa mites from neighboring colonies – often called “reinvasion” – significantly influences the population dynamic of the parasite within the colony. First, we quantified the number of mites invading individual colonies in relation to the invasion pressure (= number and distance of infested colonies). For this approach we made use of an isolated military training area near Münsingen at the Swabian Alb not accessible to other beekeepers. We established ten “mite receiver colonies” continuously treated against V. destructor and placed them at distances of 1m to 1.5 km from four heavily infested “mite donor colonies”. In the donor colonies, we estimated the population of bees, brood, and V. destructor at three week intervals. The invasion of mites into the receiver colonies was recorded every 7-12 days. During the measurement period of about two months, between 85 and 444 mites per colony were introduced into the receiver colonies. Surprisingly, there were no significant differences in the invasion rates in relation to the distance between donor and receiver colonies. The second approach was performed under more realistic field conditions of two experimental apiaries established in regions with high and low bee densities, respectively. Additionally, in this experiment we analyzed the multiplication of the invaded mites. Thereby we confirmed that horizontal transmission plus the reproduction of the invaded Varroa mites can cause an exponential increase of the mite population that may exceed the damage threshold within three months. We were further able to show that the invasion rates – and therefore the final infestation – differ significantly according to the number of honey bee colonies in the neighborhood of the apiary: At the site with a high bee density, the average invasion rate per colony over the entire three and a half months period was 462 mites per colony compared to only 126 mites per colony at the site with a low bee density. As a consequence, the colonies of the apiary at the high bee density site revealed an average final infestation in November of 2,082 mites per colony compared to 340 mites per colony at the low bee density site. The highly infested colonies lost about three times more bees compared to the lower infested colonies – obviously a result of Virus infections transmitted by Varroa mites. With my different approaches I was able to add further elements of knowledge for a better understanding of how host factors and ambient conditions influence the Varroa reproduction within individual brood cells and the population dynamic within a honey bee colony. A better knowledge of these host parasite interactions is essential for the selection of mite resistant colonies and further more important for the development of concepts for an effective Varroa treatment.
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2012
(2013) Landesanstalt für Bienenkunde; Rosenkranz, Peter
Bericht der Landesanstalt für Bienenkunde der Universität Hohenheim für das Jahr 2011
(2012) Landesanstalt für Bienenkunde; Rosenkranz, Peter