Spores of the fungus are ingested with the honey bee larval food. Larvae are most susceptible if they ingest spores when they are 3 to 4 days old and then are chilled briefly 2 days later, immediately after they are sealed in their cells to pupate. The spores germinate in the hind gut of the bee larva, but mycelial (vegetative) growth is arrested until the larva is sealed in its cell. At this stage, the larva is about 6 or 7 days old. The mycelial elements break out through the gut wall and invade the larval tissues until the entire larva is overcome. This generally requires from 2 to 3 days. Dead larvae are chalky white and usually covered with filaments (mycelia) that have a fluffy, cotton-like appearance. These mummified larvae may be mottled with brown or black spots, especially on the ventral sides, due to the presence of spore cysts or fruiting bodies of the fungus. Larvae that have been dead for a long time may become completely black as these fruiting bodies fully mature. Spores form only when there are 2 different strains (+ and -) of mycelia present and in contact with each other. – Clarence Collison, Mississippi State University