In the early spring, chalkbrood disease is often prevalent since it is considered to be a stress-related disease. It is a fungal brood disease caused by the spore-forming fungus, Ascospharea apis, with worker, drone and queen larvae all being susceptible. Dead larvae are chalky white in color and usually covered with filaments (mycelia) that have a fluffy, cotton-like appearance. Initially, the dead larvae are swollen to the size of the cell. Later, they dry into hard shrunken chalk-like lumps. 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 two different strains (+ and -) of mycelia present and in contact with each other.
Diseased larvae can be found throughout the brood-rearing season, but are most prevalent in the spring when the brood nest is rapidly expanding. The disease usually disappears or is reduced as the air temperature increases in the summer. The affected larvae are usually found on the outer fringes of the brood nest where sufficient nurse bees are unavailable to maintain brood nest temperature. Brood cells can either be sealed or unsealed. Young pupae or recently sealed larvae are most susceptible. House bees often puncture or remove cappings. Diseased larvae are stretched out in their cells in an upright condition. Infected larvae are removed by nurse bees 2-3 days after symptoms first appear. Dead larvae (mummies) are often found in front of the hive, on the landing board, or in your pollen trap. In strong colonies most of these mummies will be discarded outside of the hive, thus reducing the possibility of reinfection from the corpses of those that have died from the disease.
Spores of the fungus are ingested with the 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 mycelia (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 through the gut wall and invade the larval tissues until the entire larva is overcome. This generally requires 2 to 3 days.
Spores remain viable for years. Therefore, infected equipment, especially brood combs are a reservoir for further infection. Chalkbrood normally does not destroy a colony. However, it can prevent normal population build-up and surplus honey production when the disease is serious. Research has shown that the spores are easily passed from bee to bee. Therefore, drifting and robbing bees are potential vectors of the disease. Both workers and queens taken from infected colonies can transmit the disease to healthy colonies. Also colonies fed pollen collected from infected colonies will contract the disease.
Chalkbrood infections are not always visible in the brood nest. Therefore, beekeepers who collect pollen to sell or feed to their bees should check the pollen and pollen traps from each colony for the presence of whole or parts of mummies. No treatment is presently available for control. Strong healthy colonies receiving optimal nutrition are less likely to have chalkbrood than weaker ones. Variation is susceptibility of bee colonies to chalkbrood is an important factor in expression of the disease. In some cases, chalkbrood can be reduced by requeening colonies with a young queen. Elimination of susceptible strains can often be accomplished with routine requeening, which has the added benefit of producing a break in the brood cycle to allow the bees to remove mummies and clean the cells. Moisture accumulation and poor ventilation in hives should be prevented because dampness encourages the development of chalkbrood.
Clarence Collison, Emeritus Professor