Abstract
Interactions between parasites and environmental factors have been implicated in the loss of managed Western honey bee (=HB, Apis mellifera) colonies. Although laboratory data suggest that cold temperature may limit the spread of Nosema ceranae, an invasive species and now ubiquitous endoparasite of Western HBs, the impact of weather conditions on the distribution of this microsporidian in the field is poorly understood. Here, we conducted a survey for Nosema spp. using 18 Swiss apiaries (four colonies per apiary) over a period of up to 18 months. Samples consisting of 60 workers were collected monthly from each colony to estimate Nosema spp. intensity, i.e., the number of spores in positive samples using microscopy. Ambient apiary temperature was measured daily to estimate the proportion of days enabling HB flight (>10 °C at midday). The results show that Nosema spp. intensities were negatively correlated with the proportion of days enabling HB flight, thereby suggesting a significant and unexpected positive impact of cold ambient temperature on intensities, probably via regulation of defecation opportunities for infected hosts.
Highlights
Parasites have been identified as one threat to the health of managed Western honey bee colonies, Apis mellifera [1,2,3], which have faced increased winter losses in recent years in the northern hemisphere [4,5]
The Western honey bee can act as the host of multiple parasites [1], ranging from ectoparasites such as Varroa destructor [14] and Tropilaelaps mercedesae [15] to endoparasites, such as various bee viruses [1,14,16,17] and microsporidia Nosema spp. [16,18]
Because brood requires a constant temperature of 34.5 ◦ C, and because active heating is costly in terms of energy that requires foraging opportunities, the colony is usually broodless during winter [19]
Summary
Parasites have been identified as one threat to the health of managed Western honey bee colonies, Apis mellifera [1,2,3], which have faced increased winter losses in recent years in the northern hemisphere [4,5]. Such host–parasite systems can be influenced by numerous biotic and abiotic factors (e.g., genetics, host immunity, temperature and climate, respectively, [6,7,8,9]). By influencing the annual life cycle of the honey bee colony, temperature can have
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