Dose-dependent effects of antibiotic intake on Bombus Terrestris (Linnaeus, 1758) dietary intake, survival and parasite infection prevalence.

Diseases may contribute to the widespread declines seen in many bee species. The gut bacteria of bees may serve as one defence against disease, by preventing pathogen colonisation. However, exposure to antibiotics on forage or in the hive may disrupt bee gut bacteria and remove this protective effect. A number of studies show that high antibiotic doses reduce bee health but the effects of field-realistic antibiotic doses remain unclear. Here, we test how Bombus terrestris (Linnaeus, 1758) is affected by multiple field-realistic concentrations of the antibiotic oxytetracycline, which is sometimes used to protect flowering crops from bacterial infections. We measured survival, feeding behaviour and the likelihood of developing infection with the gut parasitic trypanosome Crithidia bombi Lipa & Triggiani, 1988 following oral inoculation with a range of antibiotic doses. Rising antibiotic concentrations were associated with reduced survival and food consumption, and an increased likelihood of becoming infected with C. bombi. These effects were seen at antibiotic concentrations that are applied to crops and so may be encountered by foraging bees in the field. These results support the hypothesis that field-realistic antibiotic doses have lethal and sub-lethal effects on B. terrestris and highlight the importance of improving our understanding of how field-realistic antibiotic doses affect pollinators.

Long-term effects of antibiotic treatments on honeybee colony fitness: A modelling approach.

Gut microbiome disequilibrium is increasingly implicated in host fitness reductions, including for the economically important and disease-challenged western honey bee Apis mellifera. In laboratory experiments, the antibiotic tetracycline, which is used to prevent American Foulbrood Disease in countries including the US, elevates honey bee mortality by disturbing the microbiome. It is unclear, however, how elevated individual mortality affects colony-level fitness.We used an agent-based model (BEEHAVE) and empirical data to assess colony-level effects of antibiotic-induced worker bee mortality, by measuring colony size. We investigated the relationship between the duration that the antibiotic-induced mortality probability is imposed for and colony size.We found that when simulating antibiotic-induced mortality of worker bees from just 60 days per year, up to a permanent effect, the colony is reduced such that tetracycline treatment would not meet the European Food Safety Authority's (EFSA) honey bee protection goals. When antibiotic mortality was imposed for the hypothetical minimal exposure time, which assumes that antibiotics only impact the bee's fitness during the recommended treatment period of 15 days in both spring and autumn, the colony fitness reduction was only marginally under the EFSA's threshold. Synthesis and Applications. Modelling colony-level impacts of antibiotic treatment shows that individual honey bee worker mortality can lead to colony mortality. To assess the full impact, the persistence of antibiotic-induced mortality in honey bees must be determined experimentally, in vivo. We caution that as the domestication of new insect species increases, maintaining healthy gut microbiomes is of paramount importance to insect health and commercial productivity. The recommendation from this work is to limit prophylactic use of antibiotics and to not exceed recommended treatment strategies for domesticated insects. This is especially important for highly social insects as excess antibiotic use will likely decrease colony growth and an increase in colony mortality.