Browsing by Subject "Nosema"

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    Effects of chronic pesticide and pathogen exposure on honey bee (Apis mellifera L.) health at the colony level
    (2018) Odemer, Richard; Bessei, Werner
    During the last decade the increasing number of honey bee colony losses has become a major concern of beekeepers and scientists worldwide. Extensive research and cooperation projects have been established to unravel this phenomenon. Among parasites, pathogens and environmental factors, the use of agrochemicals, most notably the class of neonicotinoid insecticides, are suspected to be a key factor for this collapse. Current approaches not only focus on colony collapse but also on the weakening of honey bees by the exposure to sublethal concentrations of such pesticides. Recently, the EFSA temporarily banned three neonicotinoids including clothianidin, imidacloprid and thiamethoxam, for the use in crops attractive to pollinators. Thiacloprid however, likewise a neonicotinoid insecticide, is still tolerated for agricultural use because it is considered less toxic to bees. Nevertheless, some publications indicate sublethal effects of this agent leading to impairments of the colony. A general problem for the study of such sublethal effects is that they often are measurable in individual bees without eliciting clear impact at the colony level. In addition, such effects might only have a consequence in combination with other stressors like pathogens. This thesis presents two new methodical approaches combining the controlled application of stressors to individual bees with an evaluation of the effects under field realistic conditions of free flying colonies. In all approaches, the bees were treated with a combination of different pesticides and/or a combination of pesticides and a pathogen in order to evaluate synergistic interactions. As pathogen, Nosema ceranae, a novel intracellular gut parasite introduced from Asia, was used. This parasite is considered to contribute to “CCD”-like symptoms (“colony collapse disorder”), particularly in Spain. In Retschnig et al. (2015), observation hives at two study sites (Hohenheim and Bern) were used to clarify possible synergistic effects when honey bees are exposed to pesticides of two different substance classes (thiacloprid and tau-fluvalinate), both in combination with an infection of N. ceranae. Mortality, flight activity and social behaviour of individually marked and treated worker bees were monitored. At the Hohenheim site, no impact from any of the treatments could be confirmed except a slightly higher flight activity of the Nosema treated bees. At the Bern site however, the pesticide treatments elicited a significant reduction of worker bee lifespan, whereas the Nosema infection resulted in higher ratios of motionless periods. Importantly and in contrast to several laboratory studies, in neither of the two sites an interaction among the pesticides and the pathogen could be confirmed. The inconsistency of our results suggests that the effects of both, sublethal application of pesticides and infection with N. ceranae were rather weak and that interaction among them may have been overemphasized. To extend this first approach in small observation colonies, Odemer & Rosenkranz (2018) focused on performance parameters such as colony development and overwintering in honey bee colonies, using the same pesticides as in the observation hives. Here, neither the single exposure to thiacloprid or tau-fluvalinate nor their combination had negative effects on the colony performance. However, the chronic application of the tau-fluvalinate significantly reduced the infestation with Varroa mites. In Odemer et al. (2018), a neonicotinoid (clothianidin) with an extraordinary high toxicity to bees was applied alone and in combination with N. ceranae and N. apis. A novel approach was developed with individually marked bees that were infected after hatching with a certain number of Nosema spores and introduced into mini-hives. In order to simulate worst case field conditions, the pesticide was then applied chronically in sublethal concentrations over the whole lifespan of the bees. Again in contrast to previous laboratory studies, no effect of the clothianidin treatment on mortality or flight activity could be observed. However, the lifespan of Nosema infected bees was significantly reduced compared to non-infected bees, but in agreement with the observation hive experiment, the combination of pesticide and pathogen did not reveal any synergistic effect. The results of the three experiments of this thesis indicate that (i) individual honey bees are less impaired by neonicotinoids if kept within the social environment of the colony and that (ii) sublethal concentrations of neonicotinoids in the field are not the main driver for colony losses. These statements refer exclusively to the honey bee colony as a eusocial superorganism that obviously is more resilient to pesticide exposure through mechanisms of “social buffering”.