In addition to the notifiable Foulbrood diseases, there are also other less serious honey bee brood disorders including Sacbrood, Chalkbrood, Baldbrood, laying workers and failing queens. These can of course in certain circumstances cause serious problems. It is important that beekeepers are able to identify these and to distinguish between them and other diseases.
Sac Brood Infected Larva
Cause: Sacbrood is a viral infection of the brood caused by the Iflavirus genus and occurs when a diseased larva fails to pupate after being sealed in its cell. Fluid then accumulates between the body of the larva and the unshed skin, forming a sac. It is a relatively common disease during the first half of the brood-rearing season and can often go unnoticed, affecting only a small percentage of the brood. It does not usually cause severe colony loss.
Symptoms: initially during an infection, the virus particles replicate in the developing larva, which appear to develop normally until after being capped over. Typical symptoms include:
- The infected larva then turns from its usual pearly white to a pale-yellow colour;
- The larva will eventually die and begin to dry out, turning a dark brown to black colour, giving rise to the characteristic ‘Chinese slippers’ or ‘gondola shaped’ scales;
- As the larvae die, the workers will uncap the cells to expose them, creating an uneven brood pattern with discoloured, sunken or perforated cappings scattered through the brood cells;
- The skin of the dead larva also changes into a tough plastic-like sac, which is filled with fluid. It is this stage of infection that gives the virus its name. The sac can be carefully removed by using a pair of tweezers.
Spread: Varroa destructor is able to vector Sacbrood virus and will spread it when feeding off of honey bee larvae. The virus can also spread through the intervention of the beekeeper by transferring material from infected colonies to a healthy colony. In addition, the virus can spread through the feeding behaviour of nurse bees and bees robbing infected colonies.
Treatment: re-queening the colony can help to alleviate the symptoms of sacbrood and controlling Varroa mite populations will help to control the spread of the virus.
Hardened white Chalk Brood 'mummies'
Cause: Chalkbrood is caused by the fungus Ascosphaera apis. When ingested by the larva it penetrates the gut wall to absorb nutrients. As the spores germinate and multiply, the larva eventually dies of starvation. After a few days of growth, the larva and fungus swells and fills the brood cell where it will eventually harden after a few days to its distinctive 'mummified' appearance. Here it adopts a mottled white and black colour, and each chalkbrood mummy will produce millions of infective spores which stick to the cells, hive components and adult bees.
Symptoms: typical symptoms will start to appear in early spring as the colony starts to build up its population. Conditions such as damp and cold weather will promote fungal spores. Symptoms of chalkbrood include:
- Initially the dead larvae will be covered with a white cotton wool-like growth and may swell to fill the cell taking on its shape;
- After a time these will dry out and shrink to give the characteristic ‘mummies’ that are chalk-like at first turning to a greyish black colour as the fungal fruiting bodies develop;
- Worker bees uncap the cells of dead larvae so the mummies will be clearly visible;
- Shrunken chalk-like mummies in the brood and in and around the hive entrance.
- As the condition worsens, infected hives will also show a pepper pot brood pattern;
- If mummies are still contained in capped cells, when a comb is shaken gently the mummies may be heard rattling in the cells.
Spread: A. apis is highly infectious and can be easily spread between hives through robbing and drifting of drones and worker bees. Spores can be transferred between apiaries on contaminated equipment and through the intervention of the beekeeper.
Treatment: Chalkbrood is not usually a serious disease among strong healthy colonies. However, in smaller colonies or those under stress (for example suffering heavy Varroa infestations) it can become a problem. The best method for keeping chalkbrood to a minimum is the maintenance of good strong stocks which appear better able to resist the fungus. Those colonies which are susceptible can be re-queened. Avoiding damp apiary sites will also help to minimise the effect of chalkbrood in colonies.
Bald brood cells: the non-linear pattern and raised cell edges indicate this example is not the result of wax moth damage.
Cause: the most usual cause of bald brood is wax moth larvae (both the lesser (Achroia grisella) and greater (Galleria mellonella)) tunnelling below the surface of the comb. The moth larvae tunnel under the brood cappings. The bees will tear down the cell cappings to clean out the cells leaving perforated and exposed cells with brood in and sometimes these partial cappings have a raised lip protruding from the comb surface. The condition also occurs over multiple cells in a linear pattern.
Symptoms: the developing pupae are usually sealed in their cells under wax cappings 8-9 days after laying. Bald brood may be seen as small patches of normal developing larvae with uncapped or partially capped cells. These uncapped larvae will usually emerge as fully developed adults, although a few malformed adults may result from contaminants becoming deposited on the developing larvae. Wax moth damage is also apparent as a white linear line among the biscuit coloured cappings.
Treatment: strong colonies of bees will reduce the effects of wax moth, and in the case of the genetic form of bald brood re-queening of the colony will usually resolve the problem.
Drone brood in worker cells
Cause: the domed cappings which protrude from the cell are known as drone brood. These are present throughout the summer months, however, when large amounts are present throughout the hive, particularly in worker cells then this is a sign that something may be wrong. There are two reasons why a colony may exhibit drone brood in worker cells. firstly the colony may have a failing queen which could be caused by her not having been mated properly and the sperm running out in the spermatheca.
The second reason is when the colony has been queenless for some time and female workers begin to develop functional ovaries. As a result, workers start laying in cells. All the eggs will be unfertilised and therefore will develop into drone brood. In addition, one may notice multiple amounts of eggs in individual cells, particularly around the cell walls as well as the bottom.
Treatment: it is usually older queens that become drone layers, but it may also be apparent in younger queens that did not mate successfully. The best option in this instance is to re-queen with a young prolific, recently mated queen.
Unlike colonies with a failing or defective queen, those with laying workers are very difficult to re-queen. In some instances you can shake out all the original frames containing laying workers in front of the hive and introduce frames of emerging brood with a new caged queen and the colony will sort itself out. However, if the condition is too far advance or the colony is in a state of decline because of the depleting number of regular worker bees, this method may not be practical.