Sacbrood is an infectious disease that affects the brood of honey bees. It mostly occurs as a mild infection, which only kills a few larvae, but it can be more severe. Few hives die out as a direct result of it, but many are weakened to an extent where they succumb to other threats.
. Cause of disease
It is an infection caused by a virus. Remains of larvae that have recently died are yellow in colour and are highly infectious. Larval remains more than two months old, are brown and dry, and not infectious.
The presence of dead or dying brood is the first symptom. The brood dies soon after being capped but before changing to pupae. Dead larvae may be found in cells that are fully capped or in cells subsequently uncapped. Affected capped cells often contain a small puncture, usually about the size of a pinhead.
When first affected, larvae are dull and grey in colour. They then turn slightly yellow with a dark head and become brown after a few days, eventually turning almost black. The ´Chinese clipper´ or ´canoe´ shape of dead larvae is characteristic of this disease. During the period of decay, the outer skin of the dead larva toughens. The larva is easily removed intact from the cell with a pair of tweezers. The name refers to the appearance of the watery and granular remains enclosed in the tough skin. If a large amount of brood becomes affected, the colony is weakened. Bees appear reluctant to remove dead material from the cells and the queen is forced to lay elsewhere in the hive.
It is often believed to be spread by feeding young larvae contaminated pollen, nectar or water. Nurse bees become infected with the virus while cleaning out cells containing diseased larvae. The spread of sacbrood from hive to hive is attributed to the exchange of contaminated equipment or bees, or natural causes such as bees robbing or drifting from hive to hive.
Sacbrood disease may appear at any time of the year but more often during the brood rearing season (September to February). Colonies showing symptoms of sacbrood disease will often recover if moved to pollen and nectar flow conditions. Evidence suggests that some lines of bees are resistant to the disease.
Each year, two to four of the oldest brood combs should be replaced with foundation. Old black combs are less efficient for brood rearing and are more likely to harbour disease organisms. Re-queen annually using queen bees from a reputable supplier.
. Mild infections
Re-queening infected colonies is a recognized form of control. It is not practicable to re-queen every colony that displays only a few larvae infected with sacbrood. However, once more than 5% of brood is infected, you should consider replacing the queen with one bred from a hive showing no sign of the disease.
Combs with less than 20% of the brood infected should be removed from the brood chamber and placed in the honey super. Replace the combs with foundation if the bees are strong enough and conditions permit. Hives with mild infections should be checked every few weeks for any sign of the disease worsening.
. Severe infections
Combs with more than 20 % of brood infected should be removed from the hive and either melted down or placed in storage for two months. Combs in storage need to be protected from wax moth. A solar wax melter is suitable for melting down combs on a small scale. After removing old and known contaminated combs, the hive should be re-queened, packed down, and stimulated by feeding sugar syrup or moved to a honey flow.
Control measures will not ensure immediate removal of the problem from apiaries but if brought into general management practice over a period of time, the sacbrood problem should be reduced. Antibiotic treatment will not control this disease.