A common fungal disease of honey bees
Chalk Brood is caused by a fungal organism called Ascosphaera apis. It is widely stated that Maassen "identified" it in 1913. The visible signs are present in most colonies to a greater or lesser degree and the level of infection can vary considerably throughout the year in the same colony. There has been a lot of research done and much has been written about the disease, but it is still a problem and is probably the most commonly seen bee disease in the U.K. and Ireland.
My experience and observation of the disease differs somewhat from the "official" sources. I think that says far more about the disease than the information and advice you may be given, as in my experience it can be very unpredictable, with the visual signs in a colony changing very quickly from what appears to be a very high infection, to the odd infected cell in only a few days. I will give my experiences, including anecdotes, but I must warn you they may conflict with others. Don't forget though, that there is a mass of information available, but how much is simply copied from elsewhere?
The infected larvae are usually seen in the capped stage, but it is common for unsealed larvae to show signs too, where they can lose their segmentation, shape and colour. They can look so much like EFB that on one occasion I was convinced I had EFB, so I called out the Bee Inspector who took one look at it and said "Yes EFB, I'm afraid". To the surprise of both of us the LFD (lateral flow device) test was negative, so we thought the test kit was faulty and used another one, that was also negative. The danger here of course is that someone who is trained to concentrate on nothing but disease and a very experienced beekeeper were both fooled. A less experienced beekeeper may think the next time they see the same symptons it is chalk brood, when it may be EFB.
The appearance of the infected larvae varies depending on the stages and as well as foul brood they can be confused with mouldy pollen by the inexperienced eye. They take on a white fluffy appearance, with the outside looking remarkably like the rind of camembert cheese, with a chalky look, hence the name. At this stage the dead larvae swell up and fill the cells, often taking on the hexagonal shape. There can be a yellow protrusion that is the larval mouthparts. They shrink and harden so they look "mummified" and change colour to grey and black. It is usually at this stage when the bees can easily remove the mummies from the cells and drop them on the floor of the hive. Turning the comb during inspection can allow these to partly fall out.
When sealed, the cappings can sink in a similar way to AFB. Sometimes the bees will thin the capping, so it has tiny perforations, with others they will simply make a hole through the capping, which will usually be off centre, unlike a cell that is in the last stages of sealing where the hole will be on centre. The bees uncap the cells and remove the infected contents. The more "hygienic" bees are much better at removing mummies than the less hygienic and there will be a lot of evidence on the floor of the hive and the alighting board. The less hygienic bees will be slower to remove the mummies, so it looks as if the level of infection is higher. The level of chalk brood in a colony can't be determined by what you see on the floor. That may just show the ability of the colony to clean out infected larvae.
I know some will only accept scientific evidence, but the scientists haven't come up with a solution and in any case much of the advice has limited success. For that reason I will tell you of some of my own observations and simple experiments that any beekeeper can do. The results often conflict with "standard advice", but that doesn't mean the advice is wrong, just that it may only work in some instances, or perhaps the disease reduced soon after a "treatment" was given. We are often told what to do to overcome chalk brood, but I think it's so complex it is difficult to understand and deal with. I can't remember dealing with a wild colony that had anything more than a very mild infection of chalk brood, certainly not as bad as I often see in a managed colony. That suggests to me that we are likely to aggravate the disease in some way, possibly stress, which is understandable considering the length of time some colonies are open during inspections.
I will give details of one of my colonies (called "Rampion") in 2013 that was a very cold wet spring, following a poor summer and winter. At inspections there was bad chalk brood and on 1st May I made a note to requeen the colony. It had built up very slowly. On 16th May the chalk brood was still bad with a covering on the floor, apart from one largely sealed comb that was very different from the rest. There wasn't the "pepperpot" appearance of other combs and only a handful of infected cells. The "obvious" explanation is that was a new comb, but it wasn't, having been in the colony the previous year. I checked back and we had 3-4 days of good weather soon after that comb was laid up by the queen, so my thinking was the brood on that comb was fed better. An inspection on 7th June, after a spell of fine weather when the bees were doing very well, showed there was virtually no chalk brood in any of the combs. In this instance I think it showed the chalk brood was possibly caused by poor nutrition and was not genetic. In other respects the queen was good and I could have culled her when there was no need to. In the spring of 2013 chalk brood was bad in many colonies, so it would have been unreasonable to cull queens for that reason.
We are often told not to raise queens from colonies that show chalk brood and logically that sounds right. At the Wisborough Green teaching apiary we had a queen that produced a high proportion of chalk brood, even when other colonies didn't. I deliberately kept her, so we could show members what bad chalk brood looked like. I know this policy won't appeal to some, but it is easier to teach beekeepers if you can show them. We had her for a couple of years and in other respects she was a very good queen, but the chalk brood got to the unacceptable point. We have a queen rearing section, so as we are a teaching apiary we raised queen cells from that colony to see what would happen. This resulted in 8 mated queens and to our surprise not one of them produced more than the odd cell of chalk brood.
On a number of occasions I have deliberately put a comb of bad chalk brood in the centre of a colony where there was little chalk brood and this has not spread the disease, nor has the subsequent brood in those combs been infected. I have taken queens producing bad chalk brood from one colony and introduced them to another colony and there has been no chalk brood.
When open mesh floors (OMFs) were advocated in the U.K. I thought bees would winter badly on them, so I wintered half on OMFs and half on solids, a total of 18 colonies. To my surprise in the spring the colonies on solid floors all had bad chalk brood and those on OMFSs were very light.
I know the above is only anecdotal evidence, but it has been gained by simple experiment and observation. It does show that much of what we are told may not always be correct. It is my view that beekeepers can reduce the levels of chalk brood in some instances.
In my experience the above will do far more good to control chalk brood than some of the advice given. I am opposed to any form of medication as it is only likely to mask the disease and not address any real causes. There is no doubt in my mind the simple steps suggested above will help to reduce the disease naturally.
Chalk brood is often treated as a minor disease, but I believe that gives the wrong impression. It is often heavier in the spring when colonies need to be building up and although it is unlikely to kill a colony, it can seriously delay its development.
One well known method of reducing chalk brood is to cut a banana lengthways, open it out and place on the top bars of the brood frames. I don't know what it does, but many people report it works. Perhaps it encourages the bees to be more hygienic, so they clear out the mummies, making it look as if the chalk brood has reduced, or it may be the decomposing banana is giving off a substance that is controlling the chalk brood. I have never tried this myself, but I will one day.
As ever, the reading and observation of a colony is important. Compare all the colonies in the same apiary and only requeen after you have assessed all the other possibilities. I'm sure many a good queen has been culled when something else caused the problem.