Stonebrood and Honey Bees
Among the many diseases that can affect honey bees, stonebrood is one of the most difficult to treat. This fungal disease can be caused by any of three fungi: Aspergillus fumigatus, Aspergillus flavus, or Aspergillus niger. All three are common soil inhabitants, and can also be pathogenic to other insects, birds, and mammals. Humans, too, can suffer from respiratory damage through contact with these viruses. Aspergillus fumigatus is widespread and is typically found in decaying organic matter and self-heating environments such as compost heaps. In fact, the fungus plays an essential role in carbon and nitrogen recycling. Colonies of this fungus produce minuscule gray-green spores, 2-3 microns in length, that can readily become airborne and enter hives. Because the spores are ubiquitous, most humans around the world may inhale hundreds of them every day. Healthy individuals are able to eliminate the spores via their immune systems, but people with an immunodeficiency condition, such as organ transplant recipients or people with AIDS or leukemia, are at risk of falling ill as a result of the spores. Aspergillus flavus is a common mold that can cause damage to stored grains, particularly corn and peanuts. Studies on pistachio trees have shown that treating the trees with a certain yeast can inhibit the growth of Aspergillus flavus — the yeast successfully competes with A. flavus for space and nutrients. A. flavus is allergenic and can cause various infections in humans; many strains also produce aflatoxin, a carcinogenic that is acutely toxic. The mold also flourishes in water-damaged carpets. Aspergillus niger is also extremely common; it attacks certain fruits and vegetables such as onions, grapes, and peanuts, causing a disease called “black mold.” It is ubiquitous in the soil and is also found in indoor environments; it has even been found on the walls of ancient Egyptian tombs. A. niger spores are less dangerous to humans than related fungi, but can still cause serious respiratory ailments if inhaled in large amounts. Peat dust is often rich with A. niger spores, and horticultural workers who handle peat on a daily basis can be particularly susceptible. The spores can also enter the ear canal, causing fungal ear infections. In bees, the spores are difficult to detect in the early stages of infection, and the three primary varieties of fungus that can cause stonebrood have spores of different colors. A. fumigus spores are gray-green; A. flavus, the most common, yellow-green; and A. niger, black. The spores attack bee larvae and pupae, penetrating to a larva’s gut and hatching. The fungus then grows rapidly, forming a collar-like ring near the larva’s head. This powdery growth, a greenish color (with A. flavus), can be readily seen with the naked eye. At this stage, little can be done to save a larva; once a larva has died, it turns black in color and becomes solid in texture; the name of the disease, “stonebrood,” derives from this mummification. Worker bees themselves clean out the infected brood; if the hive is strong, it may well be able to contain the infection and recover. But the best way to avoid stonebrood is to effectively manage your hive, and maintain a strong hive. Fungal spores that cause stonebrood are most readily spread by beekeepers themselves, in transferring contaminated equipment and materials between hives and colonies. If a colony is weakened by stonebrood, it becomes subject to robbing by other bees, who then transfer the fungus to other colonies as well. And swarming bees can carry the spores to new sites, where the disease may be perpetuated within their own colony and spread to other colonies. There are no known medical treatments for stonebrood, but vigilant inspection of your hives can help you detect the disease early and help contain it. Examine brood frames regularly; stonebrood mummies are easily detected and may simply drop out of their cells on inspection. If a colony has died out because of stonebrood, be sure to seal it to prevent entry by bees from a healthy colony. The spring and autumn are the most vulnerable times of year for the proliferation of these fungal spores, so pay particular attention during these times. Whenever you transfer combs between colonies, inspect them very closely; systematically replace old brood combs, melting down the old combs. Always verify the origin and condition of any new bees or equipment you introduce into a hive; sterilize any secondhand equipment beforehand. And report any evidence of stonebrood to your local beekeeping association; other members may have additional information. Stonebrood is not a particularly serious honey bee disease, but it can kill off a weakened hive. If you take the proper procedures to maintain healthy hives, you can in all likelihood avoid serious problems with stonebrood.
