Landesanstalt für Bienenkunde

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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.
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.

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