Abstract
Nosema spp. fungal gut parasites are among myriad possible explanations for contemporary increased mortality of western honey bees (Apis mellifera, hereafter honey bee) in many regions of the world. Invasive Nosema ceranae is particularly worrisome because some evidence suggests it has greater virulence than its congener N. apis. N. ceranae appears to have recently switched hosts from Asian honey bees (Apis cerana) and now has a nearly global distribution in honey bees, apparently displacing N. apis. We examined parasite reproduction and effects of N. apis, N. ceranae, and mixed Nosema infections on honey bee hosts in laboratory experiments. Both infection intensity and honey bee mortality were significantly greater for N. ceranae than for N. apis or mixed infections; mixed infection resulted in mortality similar to N. apis parasitism and reduced spore intensity, possibly due to inter-specific competition. This is the first long-term laboratory study to demonstrate lethal consequences of N. apis and N. ceranae and mixed Nosema parasitism in honey bees, and suggests that differences in reproduction and intra-host competition may explain apparent heterogeneous exclusion of the historic parasite by the invasive species.
Highlights
Western honey bees (Apis mellifera, hereafter honey bees) are among the most vital and versatile pollinators, contributing to production of 39 of the world’s 57 most important crops [1]
Our experiment demonstrated that Nosema infection significantly increased honey bee worker mortality but had no influence on food consumption
Spore intensity and mortality was significantly greater for N. ceranae-infected individuals compared to those infected by N. apis
Summary
Western honey bees (Apis mellifera, hereafter honey bees) are among the most vital and versatile pollinators, contributing to production of 39 of the world’s 57 most important crops [1]. In Spain, N. ceranae is typically associated with reduced colony survivorship [20], whereas in other parts of Europe [21] and in North America [22,23,24,25], its virulence is debated Possible explanations for this variation include parasite or host genetics [15,26,27,28], climate [29,30], nutrition [18], or interactions with other stressors such as environmental contaminants or other parasites [31,32,33,34,35]. Biological mechanisms underlying relationships among stressors of honey bees are not well understood, it is likely that exploitative competition for limited resources, as well as host stress resulting from tissue pathology and immune suppression, play important roles [14,31,33], and could lead to numerical (i.e., intensity) or functional (i.e., realised niche) responses by parasites that are either symmetrical (both species experience equal responses) or asymmetrical [36]
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