Sacbrood
Sacbrood is an infectious disease that affects the brood of honey bees. It occurs mostly 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 sacbrood but many are weakened to an extent where they succumb to other threats.
Cause
Sacbrood is an infection caused by Sacbrood virus (SBV).
Life cycle
Remains of larvae that have recently died are yellow and highly infectious.
Larval remains more than 2 months old are brown and dry, and not infectious.
Affected animals
- European honey bees (Apis mellifera)
Clinical signs
The presence of dead or dying brood is the first symptom. The brood dies soon after being capped but before changing to pupae. You may find dead larvae in cells that are fully capped or 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. They then turn slightly yellow with a dark head and become brown after a few days, eventually turning almost black. The 'Chinese slipper' or 'canoe' shape of dead larvae is characteristic of this disease.
During decay, the outer skin of the dead larva toughens. The name sacbrood refers to the appearance of the watery and granular remains enclosed in the tough skin. You can use a pair of tweezers to remove the larva intact from the cell.
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.
Bees working darling pea or blue gum generally develop symptoms like sacbrood disease. These symptoms usually disappear when bees are removed from these plants.
Sacbrood disease can also be confused with American foulbrood disease. However, if contents of a cell infected with sacbrood disease are stirred with a match or twig, they will not rope out.
How it is spread
Sacbrood is 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.
Sacbrood is spread from hive to hive through:
- exchange of contaminated equipment or bees
- natural causes such as bees robbing or drifting from hive to hive.
Risk period
Sacbrood disease may appear at any time of the year but more often during the brood rearing season (September to February).
Control
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, replace 2 to 4 of the oldest brood combs with foundation.
Old black combs are less efficient for brood rearing and more likely to harbour disease organisms.
Re-queen annually using queen bees from a reputable supplier.
Mild infections
- If more than 5% of brood is infected, consider replacing the queen with 1 bred from a hive showing no sign of the disease.
- Remove combs with less than 20% of the brood infected from the brood chamber and place in the honey super.
- Replace the combs with foundation if the bees are strong enough and conditions permit.
- Check hives with mild infections every few weeks for any sign of the disease worsening.
Severe infections
Remove combs with more than 20% of brood infected from the hive and either:
- melt down—use a solar wax melter to melt down combs on a small scale.
- place in storage for 2 months—protect combs in storage from wax moth.
After removing old and known contaminated combs, the hive should be:
- re-queened
- packed down
- stimulated by feeding sugar syrup or moved to a honey flow.
Control measures will not remove the problem immediately but should reduce the sacbrood problem if these practices are used over time.