Feeding larval honey bees (Apis mellifera) dead Nosema spores improves their ability to resist Nosema infection as adults

Nosema ceranae (Microsporidia) is an important honey bee pathogen that is thought to only affect adult bees and which currently has a single effective treatment, the antibiotic fumagillin. However, this treatment may be losing its efficacy. Previously, we showed that N. ceranae can infect Apis mellifera larvae. These data suggested that a sufficiently high dose triggers a larval immune response that conveys some protection. We therefore tested if feeding larvae dead N. ceranae spores can protect bees subsequently exposed to Nosema as adults. In addition, probiotics fed to larvae can activate larval immune genes. However, it was not known if probiotics could actually help bees resist infection. We therefore tested the prophylactic effect of two larval feeding treatments: autoclaved Nosema spores and probiotics. Probiotics were not beneficial: they significantly reduced larval and adult longevity and did not help bees subsequently resist adult infection (measured as the number of midgut Nosema spores). However, Nosema treatment reduced adult infection levels by 57% without significantly altering larval or adult longevity as compared to controls. Nosema treatment also had no significant effect on adult morphology, though it slightly increased bee mass at adult emergence. These data provide the first evidence that honey bee disease resistance can be boosted by larval exposure to a dead pathogen, a promising approach that deserves further study