Chalkbrood is a disease of bee brood caused by a fungus, Ascosphaera apis, which was discovered in the United States in 1968. The larvae must ingest the spores of the fungus in order for the infection to occur. It only infects larvae that are three to four days old. There are no chemical treatments for this disease. Instead, it can be controlled by bee breeding and good management. The infected larvae are quickly covered by the white cotton-like mycelium (Fig. 1) of the fungus which eventually fills the entire cell (Fig. 2). The white/grey mass soon hardens, forming a hard, shrunken mummy which is easily removed from the cell (Fig. 3). The larva in the cell will resemble a chunk of chalk, hence, the name of the disease chalkbrood.
Bee stocks selected for hygienic behavior can be expected to minimize outbreaks of this disease. Hygienic queens are available from nationally-advertised queen breeders. See advertisements listed in American Bee Journal and Bee Culture. Another way to minimize the disease is to maintain a warm, dry hive interior. Hives that are drafty, damp, lying in low spots, or heavily overgrown with vegetation are susceptible to chalkbrood disease. Hives should lean forward slightly so that rainwater runs out the entrance instead of accumulating inside; this precaution is not necessary if you use screen hive floors for varroa control. If a hive interior is moist, prop the lid to air out the interior. Some operations have recurring problems with the disease that are not easily traced to season or management practices. This suggests a genetic susceptibility in the bee stock. Because old combs harbor spores of the disease, brood combs should be replaced at least every five years to improve brood production.