Chalkbrood, a fungus disease attacking honey bee larvae, has been in the U.S. since the mid- to late 1960s. During its 30 or so year stay it has caused problems ranging from barely noticeable to major epidemics in some areas, in some years. This year bordered on the epidemic in many areas, especially the northern two-thirds of the country.
Like most fungi, chalkbrood produces spores for reproduction. Spores are transferred to previously uninfested colonies by drifting bees, using contaminated tools, or contaminated comb transfer. Once spores are in a colony, they come in contact with 3-4 day-old larvae by being fed to them, or clinging to cell walls. After being sealed in the cells, if the larvae are chilled in the next two days, even briefly, the spores germinate and begin growing when the temperature increases again. They germinate in the gut, or on the larvae’s surface, and begin feeding (surface germinating spores work their way into the larvae’s gut). Once growing the fungus feeds on the same food as the larvae and out-competes them. The larvae starve to death.
When the larvae die the fungus continues to grow, causing the larvae’s very white corpse to swell and fill the cell completely. When all the food reserves are consumed, larvae dry and shrink and become hard, white, chalklike “mummies.” Severe infections result in many larvae dying and drying in their capped cells and when a frame is shaken a rattling can be heard.
Chilling the larvae occurs when there are not enough bees to maintain brood nest temperature, especially during erratic spring weather. Swarming can lead to too few bees also.
If the fungus has enough food and good growing conditions it will produce fruiting bodies (which in time produce reproductive spores) that are gray, dark gray or black. Spores produced on mummies are picked up by cleaning bees and moved throughout the colony, or are left on the cell wall. Either way, when conditions are right they germinate and reinfect more larvae. Spores can remain viable for at least 15 years.
There is no chemical control for chalkbrood. Destroying infected combs, running equipment through an ETO chamber, and fostering genetic lines of bees with both hygienic behaviors (uncapping cells and removing diseased larvae) all help. Requeening with stock shown to be resistant to the disease may help. Requeening also breaks the brood cycle enabling the colony to clean up diseased cells.
Generally, reducing or avoiding stress on a colony (weather, internal moisture accumulation, adult bee population, temperature fluctuations, swarming, other diseases and pests), keeping the pathogen load low (removing infected frames), and maintaining strong colonies with good nutritional levels will keep chalkbrood at bay.
Chalkbrood, out of control, can, when coupled with other problems kill a colony. Don’t take this disease for granted and act accordingly to prevent it from occurring or to decrease its levels in your colonies.