Chalkbrood, whereby the larvae becomes mouldy with white hyphae (vegetative body of fungi), then hardened to be similar to pieces of white chalk (hence the name). Caused by a fungus (Ascosphaera apis), this disease is mostly considered a "stress" disease, only occuring in weak, or in otherwise stressed colonies (pesticide kill, not enough workers, sudden cold spell in spring etc). The spores are ubiquitous but can only germinate in larvae when the core brood temperature is 2-3 degrees below normal (35 C or 95 F). There is no chemical treatment. Photo by Prof. M.V. Smith, University of Guelph.